Articoli di riviste: "GHz-bursts"

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White, Stephen M., Masumi Shimojo, Kazumasa Iwai, Timothy S. Bastian, Gregory D. Fleishman, Dale E. Gary, Jasmina Magdalenic e Angelos Vourlidas. "Electron Cyclotron Maser Emission and the Brightest Solar Radio Bursts". Astrophysical Journal 969, n. 1 (21 giugno 2024): 3. http://dx.doi.org/10.3847/1538-4357/ad4640.

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Abstract This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 1010 Jy have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well-studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: Gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher-frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion, it is found that for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets.

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Жданов, Дмитрий, Dmitriy Zhdanov, Сергей Лесовой, Sergey Lesovoi, Сусанна Тохчукова e Susanna Tokhchukova. "Sources of type III solar microwave bursts". Solnechno-Zemnaya Fizika 2, n. 2 (17 giugno 2016): 12–21. http://dx.doi.org/10.12737/17341.

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Microwave fine structures allow us to study plasma evolution in an energy release region. The Siberian Solar Radio Telescope (SSRT) is a unique instrument designed to examine fine structures at 5.7 GHz. A complex analysis of data from RATAN-600, 4–8 GHz spectropolarimeter, and SSRT, simultaneously with extreme UV data, made it possible to localize sources of III type microwave drift bursts in August 10, 2011 event within the entire frequency band of burst occurrences, as well as to determine the most probable region of primary energy release. To localize sources of III type bursts from RATAN-600 data, an original method for data processing has been worked out. At 5.7 GHz, the source of bursts was determined along two coordinates whereas at 4.5, 4.7, 4.9, 5.1, 5.3, 5.5 and 6.0 GHz, their locations were identified along one coordinate. The size of the burst source at 5.1 GHz was found to be maximum as compared to source sizes at other frequencies.

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Жданов, Дмитрий, Dmitriy Zhdanov, Сергей Лесовой, Sergey Lesovoi, Сусанна Тохчукова e Susanna Tokhchukova. "Sources of type III solar microwave bursts". Solar-Terrestrial Physics 2, n. 2 (10 agosto 2016): 15–27. http://dx.doi.org/10.12737/20996.

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Microwave fine structures allow us to study plasma evolution in an energy release region. The Siberian Solar Radio Telescope (SSRT) is a unique instrument designed to examine fine structures at 5.7 GHz. A complex analysis of data from RATAN-600, 4–8 GHz spectropolarimeter, and SSRT, simultaneously with EUV data, made it possible to localize sources of III type microwave bursts in August 10, 2011 event within the entire frequency band of burst occurrence, as well as to determine the most probable region of primary energy release. To localize sources of III type bursts from RATAN-600 data, an original method for data processing has been worked out. At 5.7 GHz, the source of bursts was determined along two coordinates, whereas at 4.5, 4.7, 4.9, 5.1, 5.3, 5.5, and 6.0 GHz, their locations were identified along one coordinate. The size of the burst source at 5.1 GHz was found to be maximum as compared to those at other frequencies.

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Palliyaguru, Nipuni T., Devansh Agarwal, Golnoosh Golpayegani, Ryan Lynch, Duncan R. Lorimer, Benjamin Nguyen, Alessandra Corsi e Sarah Burke-Spolaor. "A targeted search for repeating fast radio bursts associated with gamma-ray bursts". Monthly Notices of the Royal Astronomical Society 501, n. 1 (29 ottobre 2020): 541–47. http://dx.doi.org/10.1093/mnras/staa3352.

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ABSTRACT The origin of fast radio bursts (FRBs) still remains a mystery, even with the increased number of discoveries in the last 3 yr. Growing evidence suggests that some FRBs may originate from magnetars. Large, single-dish telescopes such as Arecibo Observatory (AO) and Green Bank Telescope (GBT) have the sensitivity to detect FRB 121102-like bursts at gigaparsec distances. Here, we present searches using AO and GBT that aimed to find potential radio bursts at 11 sites of past gamma-ray bursts that show evidence for the birth of a magnetar. We also performed a search towards GW170817, which has a merger remnant whose nature remains uncertain. We place $10\sigma$ fluence upper limits of ≈0.036 Jy ms at 1.4 GHz and ≈0.063 Jy ms at 4.5 GHz for the AO data and fluence upper limits of ≈0.085 Jy ms at 1.4 GHz and ≈0.098 Jy ms at 1.9 GHz for the GBT data, for a maximum pulse width of ≈42 ms. The AO observations had sufficient sensitivity to detect any FRB of similar luminosity to the one recently detected from the Galactic magnetar SGR 1935+2154. Assuming a Schechter function for the luminosity function of FRBs, we find that our non-detections favour a steep power-law index (α ≲ −1.1) and a large cut-off luminosity (L0 ≳ 1041 erg s−1).

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Sawant, H. S., R. R. Rosa, J. R. Cecatto e N. Gopalswamy. "Solar Simple Bursts Observed with High Spectral Resolution in the 18-23 GHz Range". International Astronomical Union Colloquium 142 (1994): 693–95. http://dx.doi.org/10.1017/s0252921100077976.

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AbstractFor the first time, solar bursts in the frequency range of (18-23) GHz have been observed with high-time (0.6-1.2 s) and high-frequency resolution (1 GHz), by using the Itapetinga 13.7m diameter antenna. Here, we investigate the microwave type “simple low level (< 10 SFU) bursts” associated with the impulsive phase of solar flares. Observed properties of these simple bursts are: rise time tr ~3 s, decay time td ~ 5 s and spectral index ranging between −1 and −4. These bursts were found to be associated with SF or SN flares as seen in Hα. The above properties suggest that they are likely to be a microwave counterpart of elementary flare bursts. In the majority of the cases the spectral evolution is soft-hard-soft. This suggests a nonthermal gyrosynchrotron mechanism for generating these elementary flare bursts. Estimated parameters of these simple burst sources are height (h ~ 2400 km), electron density (Nє < 8.8 × 109 cm−3 ), and magnetic field (B ~ 300 G).Subject headings: radiation mechanisms: nonthermal — Sun: radio radiation

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6

Houben, L. J. M., L. G. Spitler, S. ter Veen, J. P. Rachen, H. Falcke e M. Kramer. "Constraints on the low frequency spectrum of FRB 121102". Astronomy & Astrophysics 623 (marzo 2019): A42. http://dx.doi.org/10.1051/0004-6361/201833875.

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While repeating fast radio bursts (FRBs) remain scarce in number, they provide a unique opportunity for follow-up observations that enhance our knowledge of their sources and potentially of the FRB population as a whole. Attaining more burst spectra could lead to a better understanding of the origin of these bright, millisecond-duration radio pulses. We therefore performed ∼20 h of simultaneous observations on FRB 121102 with the Effelsberg 100 m radio telescope and the low frequency array (LOFAR) to constrain the spectral behaviour of bursts from FRB 121102 at 1.4 GHz and 150 MHz. This campaign resulted in the detection of nine new bursts at 1.4 GHz but no simultaneous detections with LOFAR. Assuming that the ratio of the fluence at two frequencies scales as a power law, we placed a lower limit of α > −1.2 ± 0.4 on the spectral index for the fluence of the instantaneous broad band emission of FRB 121102. For the derivation of this limit, a realistic fluence detection threshold for LOFAR was determined empirically assuming a burst would be scattered as predicted by the NE2001 model. A significant variation was observed in the burst repeat rate R at L-band. During observations in September 2016, nine bursts were detected, giving R = 1.1 ± 0.4 h−1, while in November no bursts were detected, yielding R < 0.3 h−1 (95% confidence limit). This variation is consistent with earlier seen episodic emission of FRB 121102. In a blind and targeted search, no bursts were found with LOFAR at 150 MHz, resulting in a repeat rate limit of R < 0.16 h−1 (95% confidence limit). Burst repeat rate ratios of FRB 121102 at 3, 2, 1.4, and 0.15 GHz are consistent within the uncertainties with a flattening of its spectrum below 1 GHz.

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Kawabata, K., e H. Ogawa. "Solar Millimeter Wave Bursts". International Astronomical Union Colloquium 104, n. 2 (1989): 191–94. http://dx.doi.org/10.1017/s0252921100154132.

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8

Matsumoto, Hisashi, Zhibin Lin, Joel N. Schrauben e Jan Kleinert. "Ultrafast laser ablation of silicon with ∼GHz bursts". Journal of Laser Applications 33, n. 3 (agosto 2021): 032010. http://dx.doi.org/10.2351/7.0000372.

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Kaufmann, P., F. M. Strauss, J. E. R. Costa e E. Correia. "Circular polarization of solar bursts at 22 GHz". Solar Physics 148, n. 2 (dicembre 1993): 341–58. http://dx.doi.org/10.1007/bf00645094.

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10

Lyu, Fen, En-Wei Liang e D. Li. "Narrowly Banded Spectra with Peak Frequency around 1 GHz of FRB 20201124A: Implications for Energy Function and Radiation Physics". Astrophysical Journal 966, n. 1 (29 aprile 2024): 115. http://dx.doi.org/10.3847/1538-4357/ad3354.

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Abstract The radiation physics of fast radio bursts (FRBs) remains an open question. Current observations have discovered that narrowly banded bursts of FRB 20201124A are active in 0.4–2 GHz, and their spectral peak frequency ( ν p obs ) is mostly toward ∼1 GHz. Utilizing a sample of 1268 bursts of FRB 20201124A detected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we show that the 1σ spectral regime of 71.4% of the events (in-band bursts) is within the FAST bandpass. The intrinsic burst energies ( E BWe obs ) and spectral widths ( σ s obs ) are well measured by fitting the spectral profile with a Gaussian function. The derived E BWe obs and σ s obs distributions are lognormal and centered at log E BWe obs / erg = 37.2 ( σ = 0.76 ) and log σ s obs / GHz = − 1.16 ( σ = 0.17 ) . Our Monte Carlo simulation analysis infers its intrinsic ν p distribution as a normal function centered at ν p,c = 1.16 GHz (σ = 0.22) and its intrinsic energy function as Φ ( E ) ∝ E − 0.60 e − E / E c with E c = 9.49 × 1037 erg. We compare these results with that of typical repeating FRBs 20121102A and 20190520B, which are active over a broad frequency range at several specific frequencies, and discuss possible observational biases on the estimation of the event rate and energy function. Based on these results, we argue that FRB 20201124A likely occurs in a fine-tuned plasma for maser radiation at a narrow frequency range, while FRB 20121102A and FRB 20190520B could involve clumpy plasma conditions that make maser emission around several specific frequencies in a broad range.

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Caballero-Lucas, Francesc, Kotaro Obata e Koji Sugioka. "Enhanced ablation efficiency for silicon by femtosecond laser microprocessing with GHz bursts in MHz bursts(BiBurst)". International Journal of Extreme Manufacturing 4, n. 1 (20 gennaio 2022): 015103. http://dx.doi.org/10.1088/2631-7990/ac466e.

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Abstract Ultrashort laser pulses confine material processing to the laser-irradiated area by suppressing heat diffusion, resulting in precise ablation in diverse materials. However, challenges occur when high speed material removal and higher ablation efficiencies are required. Ultrafast burst mode laser ablation has been proposed as a successful method to overcome these limitations. Following this approach, we studied the influence of combining GHz bursts in MHz bursts, known as BiBurst mode, on ablation efficiency of silicon. BiBurst mode used in this study consists of multiple bursts happening at a repetition rate of 64 MHz, each of which contains multiple pulses with a repetition rate of 5 GHz. The obtained results show differences between BiBurst mode and conventional single pulse mode laser ablation, with a remarkable increase in ablation efficiency for the BiBurst mode, which under optimal conditions can ablate a volume 4.5 times larger than the single pulse mode ablation while delivering the same total energy in the process.

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12

Wan, Junlin, Jianfei Tang, Baolin Tan, Jinhua Shen e Chengming Tan. "Statistical analysis of solar radio fiber bursts and relations with flares". Astronomy & Astrophysics 653 (settembre 2021): A38. http://dx.doi.org/10.1051/0004-6361/202140498.

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Fiber bursts are a type of fine structure that frequently occurs in solar flares. Although observations and theory of fiber bursts have been studied for decades, their microphysical process, emission mechanism, and especially the physical links with the flaring process still remain unclear. We performed a detailed statistical study of fiber bursts observed by the Chinese Solar Broadband Radio Spectrometers in Huairou with high spectral-temporal resolutions in the frequency ranges of 1.10−2.06 GHz and 2.60−3.80 GHz during 2000−2006. We identify more than 900 individual fiber bursts in 82 fiber events associated with 48 solar flares. From the soft X-ray observations of the Geostationary Operational Environmental Satellite, we found that more than 40% of fiber events occurred in the preflare and rising phases of the associated solar flares. Most fiber events are temporally associated with hard X-ray bursts observed by RHESSI or microwave bursts observed by the Nobeyama Radio Polarimaters, which implies that they are closely related to the nonthermal energetic electrons. The results indicate that most fiber bursts have a close temporal relation with energetic electrons. Most fiber bursts are strongly polarized, and their average duration, relative bandwidth, and relative frequency-drift rate are about 1.22 s, 6.31%, and −0.069 s−1. The average duration and relative bandwidth of fiber bursts increase with solar flare class. The fiber bursts associated with X-class flares have a significantly lower mean relative frequency-drift rate. The average durations in the postflare phase are clearly longer than the duration in the preflare and rising phases. The relative drift rate in the rising phase is clearly higher than that in preflare and postflare phases. The hyperbola correlation of the average duration and the relative drift rate of the fiber bursts is very interesting. These characteristics are very important for understanding the formation of solar radio fiber bursts and for revealing the nonthermal processes of the related solar flares.

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Rosa, Reinaldo R., Mauricio J. A. Bolzan, Francisco C. R. Fernandes, H. S. Sawant e Marian Karlický. "Nonlinear analysis of decimetric solar bursts". Proceedings of the International Astronomical Union 5, S264 (agosto 2009): 279–81. http://dx.doi.org/10.1017/s174392130999278x.

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AbstractThe solar radio emissions in the decimetric frequency range (above 1 GHz) are very rich in temporal and spectral fine structures due to nonlinear processes occurring in the magnetic structures on the corresponding active regions. In this paper we characterize the singularity spectrum, f(α), for solar bursts observed at 1.6, 2.0 and 3 GHz. We interpret our findings as evidence of inhomogeneous plasma turbulence driving the underlying plasma emission process and discuss the nonlinear multifractal approach into the context of geoeffective solar active regions.

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Kovalev, Y. Y., e G. M. Larionov. "A Jet Model Interpretation of Multi Frequency Flux Observations of Radio Outbursts in the AGN 0235+16". Symposium - International Astronomical Union 159 (1994): 397. http://dx.doi.org/10.1017/s0074180900175862.

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There are analyzed 1.5 - years observations of a series of bursts in the quasar 0235+16 at 8 frequencies between 0.3 GHz and 15 GHz from July 1981 to December 1982, obtained by Altschuler et al. (1984) and Aller et al. (1985). These observations are compared with a Hedgehog model (see Kovalev and Mikhailutsa, 1980, with full references).

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Zhang, Yu, e Hui-Chun Wu. "Upper Field-strength Limit of Fast Radio Bursts". Astrophysical Journal 929, n. 2 (1 aprile 2022): 164. http://dx.doi.org/10.3847/1538-4357/ac5e2f.

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Abstract Fast radio bursts (FRBs) are cosmological radio transients with an unclear generation mechanism. Known characteristics such as their luminosity, duration, spectrum, and repetition rate, etc., suggest that FRBs are powerful coherent radio signals at GHz frequencies, but the status of FRBs near the source remains unknown. As an extreme astronomical event, FRBs should be accompanied by energy-comparable or even more powerful X/γ-ray counterparts. Here, particle-in-cell simulations of ultrastrong GHz radio pulse interaction with GeV photons show that at ≳3 × 1012 V cm−1 field strengths, quantum cascade can generate dense pair plasmas, which greatly dampen the radio pulse. Thus, in the presence of GeV photons in the source region, GHz radio pulses stronger than 3 × 1012 V cm−1 cannot escape. This result indicates an upper field-strength limit of FRBs at the source.

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Zhang, Yu, e Hui-Chun Wu. "Upper Field-strength Limit of Fast Radio Bursts". Astrophysical Journal 929, n. 2 (1 aprile 2022): 164. http://dx.doi.org/10.3847/1538-4357/ac5e2f.

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Abstract Fast radio bursts (FRBs) are cosmological radio transients with an unclear generation mechanism. Known characteristics such as their luminosity, duration, spectrum, and repetition rate, etc., suggest that FRBs are powerful coherent radio signals at GHz frequencies, but the status of FRBs near the source remains unknown. As an extreme astronomical event, FRBs should be accompanied by energy-comparable or even more powerful X/γ-ray counterparts. Here, particle-in-cell simulations of ultrastrong GHz radio pulse interaction with GeV photons show that at ≳3 × 1012 V cm−1 field strengths, quantum cascade can generate dense pair plasmas, which greatly dampen the radio pulse. Thus, in the presence of GeV photons in the source region, GHz radio pulses stronger than 3 × 1012 V cm−1 cannot escape. This result indicates an upper field-strength limit of FRBs at the source.

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Faber, Jakob T., Vishal Gajjar, Andrew P. V. Siemion, Steve Croft, Daniel Czech, David DeBoer, Julia DeMarines et al. "Re-analysis of Breakthrough Listen Observations of FRB 121102: Polarization Properties of Eight New Spectrally Narrow Bursts". Research Notes of the AAS 5, n. 1 (1 gennaio 2021): 17. http://dx.doi.org/10.3847/2515-5172/abde48.

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Abstract We report polarization properties for eight narrowband bursts from FRB 121102 that have been re-detected in a high-frequency (4–8 GHz) Breakthrough Listen observation with the Green Bank Telescope, originally taken on 2017 August 26. The bursts were found to exhibit nearly 100% linear polarization, Faraday rotation measures bordering 9.3 × 104 rad m−2, and stable polarization position angles, all of which agree with burst properties previously reported for FRB 121102 at the same epoch. We confirm that these detections are indeed physical bursts with limited spectral occupancies and further support the use of sub-banded search techniques in FRB detection.

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Schwarz, Simon, Stefan Rung, Cemal Esen e Ralf Hellmann. "Enhanced ablation efficiency using GHz bursts in micromachining fused silica". Optics Letters 46, n. 2 (8 gennaio 2021): 282. http://dx.doi.org/10.1364/ol.415959.

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Qin, Zhihai, e Guangli Huang. "Some characteristics of pulsations in radio bursts at 9.375 GHz". Astrophysics and Space Science 218, n. 2 (1994): 213–22. http://dx.doi.org/10.1007/bf00627760.

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20

Kumar, P., R. M. Shannon, C. Flynn, S. Osłowski, S. Bhandari, C. K. Day, A. T. Deller et al. "Extremely band-limited repetition from a fast radio burst source". Monthly Notices of the Royal Astronomical Society 500, n. 2 (5 novembre 2020): 2525–31. http://dx.doi.org/10.1093/mnras/staa3436.

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ABSTRACT The fast radio burst (FRB) population is observationally divided into sources that have been observed to repeat and those that have not. There is tentative evidence that the bursts from repeating sources have different properties than the non-repeating ones. In order to determine the occurrence rate of repeating sources and characterize the nature of repeat emission, we have been conducting sensitive searches for repetitions from bursts detected with the Australian Square Kilometre Array Pathfinder (ASKAP) with the 64-m Parkes radio telescope, using the recently commissioned Ultra-wideband Low (UWL) receiver system, over a band spanning 0.7–4.0 GHz. We report the detection of a repeat burst from the source of FRB 20190711A. The detected burst is 1 ms wide and has a bandwidth of just 65 MHz. We find no evidence of any emission in the remaining part of the 3.3 GHz UWL band. While the emission bandwidths of the ASKAP and UWL bursts show ν−4 scaling consistent with a propagation effect, the spectral occupancy is inconsistent with diffractive scintillation. This detection rules out models predicting broad-band emission from the FRB 20190711A source and puts stringent constraints on the emission mechanism. The low spectral occupancy highlights the importance of sub-banded search methods in detecting FRBs.

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Katz, J. I. "Fast radio bursts — A brief review: Some questions, fewer answers". Modern Physics Letters A 31, n. 14 (5 maggio 2016): 1630013. http://dx.doi.org/10.1142/s0217732316300135.

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Fast radio bursts (FRBs) are millisecond bursts of radio radiation at frequencies of about 1 GHz, recently discovered in pulsar surveys. They have not yet been definitively identified with any other astronomical object or phenomenon. The bursts are strongly dispersed, indicating passage through a high column density of low density plasma. The most economical interpretation is that this is the intergalactic medium, indicating that FRB are at “cosmological” distances with redshifts in the range 0.3–1.3. Their inferred brightness temperatures are as high as 10[Formula: see text] K, implying coherent emission by “bunched” charges, as in radio pulsars. I review the astronomical sites, objects and emission processes that have been proposed as the origin of FRB, with particular attention to soft gamma repeaters (SGRs) and giant pulsar pulses.

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Krüger, A., B. Kliem, J. Hildebrandt, V. P. Nefedev, B. V. Agalakov e G. Ya Smolkov. "An Attempt to Classify Solar Microwave-Bursts by Source Localization Characteristics and Dynamics of Flare-Energy Release". Symposium - International Astronomical Union 188 (1998): 205–6. http://dx.doi.org/10.1017/s0074180900114767.

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An overview of spatially resolved observations of solar radio bursts obtained by the Siberian Solar Radio Telescope at 5.8 GHz during the last ten years reveals the occurrence of different classes of burst emission defined by their source localization characteristics. Four major classes of bursts according to the source position relative to sunspots, the source size and structure, and the source height, could be tentatively distinguished and compared with burst spectral characteristics as well as with soft X-ray emission oberved by YOHKOH. These findings are in favour of a magnetic origin of the underlying flare process.

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Farah, W., C. Flynn, M. Bailes, A. Jameson, T. Bateman, D. Campbell-Wilson, C. K. Day et al. "Five new real-time detections of fast radio bursts with UTMOST". Monthly Notices of the Royal Astronomical Society 488, n. 3 (4 luglio 2019): 2989–3002. http://dx.doi.org/10.1093/mnras/stz1748.

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Abstract We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between 2017 June and 2018 December. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $\mu$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering time-scales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with three peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky−1 d−1 to a fluence limit of 8 Jy ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index α = −1.6 (Fν ∝ να). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio, and X-ray follow-up has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.

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Bartulevicius, Tadas, Karolis Madeikis, Laurynas Veselis, Virginija Petrauskiene e Andrejus Michailovas. "Active fiber loop for synthesizing GHz bursts of equidistant ultrashort pulses". Optics Express 28, n. 9 (16 aprile 2020): 13059. http://dx.doi.org/10.1364/oe.389056.

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Bonamis, Guillaume, Eric Audouard, Clemens Hönninger, John Lopez, Konstantin Mishchik, Eric Mottay e Inka Manek-Hönninger. "Systematic study of laser ablation with GHz bursts of femtosecond pulses". Optics Express 28, n. 19 (3 settembre 2020): 27702. http://dx.doi.org/10.1364/oe.400624.

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Jin, S. Z., R. Y. Zhao e Q. J. Fu. "Solar microwave bursts recorded at 2.84 GHz with millisecond time resolution". Solar Physics 104, n. 2 (aprile 1986): 391–413. http://dx.doi.org/10.1007/bf00159090.

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Kumar, P., R. M. Shannon, M. E. Lower, S. Bhandari, A. T. Deller, C. Flynn e E. F. Keane. "Circularly polarized radio emission from the repeating fast radio burst source FRB 20201124A". Monthly Notices of the Royal Astronomical Society 512, n. 3 (14 marzo 2022): 3400–3413. http://dx.doi.org/10.1093/mnras/stac683.

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ABSTRACT The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterize the emission properties in a manner impossible with one-off bursts. Here, we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the ultra-wideband low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early 2021 April. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the Parkes UWL, none display any emission in the range 1.1–4 GHz. All UWL bursts are highly polarized, with their Faraday rotation measures (RMs) showing apparent variations. We obtain an average RM of −614 rad m−2 for this FRB source with a standard deviation of 16 rad m−2 in the UWL bursts. In one of the UWL bursts, we see evidence of significant circularly polarized emission with a fractional extent of 47 ± 1 per cent. Such a high degree of circular polarization has never been seen before in bursts from repeating FRB sources. We also see evidence for significant variation in the linear polarization position angle in the pulse profile of this UWL repeat burst. Models for repeat burst emission will need to account for the increasing diversity in the burst polarization properties.

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Bouwhuis, Mieke, Keith W. Bannister, Jean-Pierre Macquart, R. M. Shannon, David L. Kaplan, John D. Bunton, Bärbel S. Koribalski e M. T. Whiting. "A search for fast-radio-burst-like emission from Fermi gamma-ray bursts". Monthly Notices of the Royal Astronomical Society 497, n. 1 (6 luglio 2020): 125–29. http://dx.doi.org/10.1093/mnras/staa1889.

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ABSTRACT We report the results of the rapid follow-up observations of gamma-ray bursts (GRBs) detected by the Fermi satellite to search for associated fast radio bursts. The observations were conducted with the Australian Square Kilometre Array Pathfinder at frequencies from 1.2 to 1.4 GHz. A set of 20 bursts, of which four were short GRBs, were followed up with a typical latency of about 1 min, for a duration of up to 11 h after the burst. The data were searched using 4096 dispersion measure trials up to a maximum dispersion measure of 3763 pc cm−3, and for pulse widths w over a range of duration from 1.256 to 40.48 ms. No associated pulsed radio emission was observed above $26 \, {\rm Jy\, ms}\, (w/1\, {\rm ms})^{-1/2}$ for any of the 20 GRBs.

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29

McIntyre, V. J., e W. J. Zealey. "Starbursts in Ring and Irregular Galaxies". Publications of the Astronomical Society of Australia 9, n. 2 (1991): 251–52. http://dx.doi.org/10.1017/s1323358000024012.

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AbstractThis paper presents the current status of a Ph.D. project undertaken to search for extended-scale bursts of star formation (> 1 kpc) in irregular and collisionally produced ring galaxies, principally in the southern half of the sky. Results of recent 8.4 GHz radio continuum observations and UBVRI CCD imaging of some of the program galaxies are presented.

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30

Zhang, Yong-Kun, Di Li, Bing Zhang, Shuo Cao, Yi Feng, Wei-Yang Wang, Yuanhong Qu et al. "FAST Observations of FRB 20220912A: Burst Properties and Polarization Characteristics". Astrophysical Journal 955, n. 2 (27 settembre 2023): 142. http://dx.doi.org/10.3847/1538-4357/aced0b.

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Abstract We report the observations of FRB 20220912A using the Five-hundred-meter Aperture Spherical radio Telescope. We conducted 17 observations totaling 8.67 hr and detected a total of 1076 bursts with an event rate up to 390 hr−1. The cumulative energy distribution can be well described using a broken power-law function with the lower- and higher-energy slopes of −0.38 ± 0.02 and −2.07 ± 0.07, respectively. We also report the L-band (1–1.5 GHz) spectral index of the synthetic spectrum of FRB 20220912A bursts, which is −2.6 ± 0.21. The average rotation measure value of the bursts from FRB 20220912A is −0.08 ± 5.39 rad m−2, close to 0 rad m−2 and was relatively stable over 2 months. Most bursts have nearly 100% linear polarization. About 45% of the bursts have circular polarization with Signal-to-Noise ratio > 3, and the highest circular polarization degree can reach 70%. Our observations suggest that FRB 20220912A is located in a relatively clean local environment with complex circular polarization characteristics. These various behaviors imply that the mechanism of circular polarization of FRBs likely originates from an intrinsic radiation mechanism, such as coherent curvature radiation or inverse Compton scattering inside the magnetosphere of the FRB engine source (e.g., a magnetar).

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31

Metzner, Daniel, Markus Olbrich, Peter Lickschat, Alexander Horn e Steffen Weißmantel. "X-ray generation by laser ablation using MHz to GHz pulse bursts". Journal of Laser Applications 33, n. 3 (agosto 2021): 032014. http://dx.doi.org/10.2351/7.0000403.

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32

Lopez, John, Samba Niane, Guillaume Bonamis, Pierre Balage, Eric Audouard, Clemens Hönninger, Eric Mottay e Inka Manek-Hönninger. "Percussion drilling in glasses and process dynamics with femtosecond laser GHz-bursts". Optics Express 30, n. 8 (29 marzo 2022): 12533. http://dx.doi.org/10.1364/oe.455553.

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33

Pyatunina, T. B. "Structure and Variability of Sources from Geodetic VLBI-Data". International Astronomical Union Colloquium 164 (1998): 157–58. http://dx.doi.org/10.1017/s0252921100044985.

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Abstract8 GHz maps of 39 radiosources have been obtained with ≈ 0.5 mas resolution from geodetic VLBI-data. Structure variability has been investigated on ≈ monthly interval during the bursts of radio emission of two sources 0059+581 and 0202+149 in period from June 1994 to February 1996. Bright jet components in the sources show no steady outward motion, but may be some oscillations near stationary positions.

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34

Kojima, Y., K. Fujisawa e K. Motogi. "The bursting variability of 6.7 GHz methanol maser of G33.641-0.228". Proceedings of the International Astronomical Union 13, S336 (settembre 2017): 336–37. http://dx.doi.org/10.1017/s1743921317011395.

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AbstractFrom 2014 to 2015, we conducted a total of 469 days observation of the 6.7 GHz methanol maser in a star forming region G33.641-0.228, known to be a bursting maser source. As a result, eleven bursts were detected. On MJD 57364, the flux density grew by more than six times w.r.t the day before. Moreover, during the largest burst, the flux density repeatedly increased and decreased rapidly with time-scale as short as 0.24 day. Since these characteristics of the burst are similar to the solar burst, we speculate that the burst of the 6.7 GHz methanol maser in G33.641-0.228 might occur with a similar mechanism of the solar burst.

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35

Hilmarsson, G. H., L. G. Spitler, R. A. Main e D. Z. Li. "Polarization properties of FRB 20201124A from detections with the Effelsberg 100-m radio telescope". Monthly Notices of the Royal Astronomical Society 508, n. 4 (13 ottobre 2021): 5354–61. http://dx.doi.org/10.1093/mnras/stab2936.

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ABSTRACT The repeating fast radio burst (FRB) source, FRB 20201124A, was found to be highly active in 2021 March and April. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 2021 April 9 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multicomponent bursts in our sample yields a DM of 411.6 ± 0.6 pc cm−3. We find that the rotation measure (RM) of the bursts varies around their weighted mean value of −601 rad m−2 with a standard deviation of 11.1 rad m−2. This RM magnitude is 10 times larger than the expected Galactic contribution along this line of sight (LoS). We estimate an LoS magnetic field strength of 4–6 µG, assuming that the entire host galaxy DM contributes to the RM. Further polarization measurements will help determine FRB 20201124A’s RM stability. The bursts are highly linearly polarized, with some showing signs of circular polarization, the first for a repeating FRB. Their polarization position angles (PAs) are flat across the burst envelopes and vary between bursts. We argue that the varying polarization fractions and PAs of FRB 20201124A are similar to known magnetospheric emission from pulsars, while the observed circular polarization, combined with the RM variability, is hard to explain with Faraday conversion. The high linear polarization fractions, flat PAs, and downward drift from FRB 20201124A bursts are similar to previous repeating sources, while the observed circular polarization is a newly seen behaviour among repeaters.

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36

Mészárosová, Hana, Ján Rybák, Marian Karlický e Karel Jiřička. "Separation of solar radio bursts in a complex spectrum". Proceedings of the International Astronomical Union 6, S274 (settembre 2010): 150–52. http://dx.doi.org/10.1017/s1743921311006788.

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AbstractRadio spectra, observed during solar flares, are usually very complex (many bursts and fine structures). We have developed a new method to separate them into individual bursts and analyze them separately. The method is used in the analysis of the 0.8–2.0 GHz radio spectrum of the April 11, 2001 event, which was rich in drifting pulsating structures (DPSs). Using this method we showed that the complex radio spectrum consists of at least four DPSs separated with respect to their different frequency drifts (−115, −36, −23, and −11 MHz s−1). These DPSs indicate a presence of at least four plasmoids expected to be formed in a flaring current sheet. These plasmoids produce the radio emission on close frequencies giving thus a mixture of superimposed DPSs observed in the radio spectrum.

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37

Ghisellini, Gabriele, e Nicola Locatelli. "Coherent curvature radiation and fast radio bursts". Astronomy & Astrophysics 613 (maggio 2018): A61. http://dx.doi.org/10.1051/0004-6361/201731820.

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Fast radio bursts are extragalactic radio transient events lasting a few milliseconds with a ~Jy flux at ~1 GHz. We propose that these properties suggest a neutron star progenitor, and focus on coherent curvature radiation as the radiation mechanism. We study for which sets of parameters the emission can fulfil the observational constraints. Even if the emission is coherent, we find that self-absorption can limit the produced luminosities at low radio frequencies and that an efficient re-acceleration process is needed to balance the dramatic energy losses of the emitting particles. Self-absorption limits the luminosities at low radio frequency, while coherence favours steep optically thin spectra. Furthermore, the magnetic geometry must have a high degree of order to obtain coherent curvature emission. Particles emit photons along their velocity vectors, thereby greatly reducing the inverse Compton mechanism. In this case we predict that fast radio bursts emit most of their luminosities in the radio band and have no strong counterpart in any other frequency bands.

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38

Aggarwal, Kshitij, Devansh Agarwal, Evan F. Lewis, Reshma Anna-Thomas, Jacob Cardinal Tremblay, Sarah Burke-Spolaor, Maura A. McLaughlin e Duncan R. Lorimer. "Comprehensive Analysis of a Dense Sample of FRB 121102 Bursts". Astrophysical Journal 922, n. 2 (25 novembre 2021): 115. http://dx.doi.org/10.3847/1538-4357/ac2577.

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Abstract We present an analysis of a densely repeating sample of bursts from the first repeating fast radio burst, FRB 121102. We reanalyzed the data used by Gourdji et al. and detected 93 additional bursts using our single-pulse search pipeline. In total, we detected 133 bursts in three hours of data at a center frequency of 1.4 GHz using the Arecibo telescope, and develop robust modeling strategies to constrain the spectro-temporal properties of all of the bursts in the sample. Most of the burst profiles show a scattering tail, and burst spectra are well modeled by a Gaussian with a median width of 230 MHz. We find a lack of emission below 1300 MHz, consistent with previous studies of FRB 121102. We also find that the peak of the log-normal distribution of wait times decreases from 207 to 75 s using our larger sample of bursts, as compared to that of Gourdji et al. Our observations do not favor either Poissonian or Weibull distributions for the burst rate distribution. We searched for periodicity in the bursts using multiple techniques, but did not detect any significant period. The cumulative burst energy distribution exhibits a broken power-law shape, with the lower- and higher-energy slopes of −0.4 ± 0.1 and −1.8 ± 0.2, with the break at (2.3 ± 0.2) × 1037 erg. We provide our burst fitting routines as a Python package burstfit 4 4 https://github.com/thepetabyteproject/burstfit that can be used to model the spectrogram of any complex fast radio burst or pulsar pulse using robust fitting techniques. All of the other analysis scripts and results are publicly available. 5 5 https://github.com/thepetabyteproject/FRB121102

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39

Sawant, H. S., R. R. Rosa, J. R. Cecatto e N. Gopalswamy. "Solar simple bursts observed with spectral resolution in the 18-23 GHz range". Astrophysical Journal Supplement Series 90 (febbraio 1994): 693. http://dx.doi.org/10.1086/191891.

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40

Sobacchi, Emanuele, Yuri Lyubarsky, Andrei M. Beloborodov e Lorenzo Sironi. "Self-modulation of fast radio bursts". Monthly Notices of the Royal Astronomical Society 500, n. 1 (20 ottobre 2020): 272–81. http://dx.doi.org/10.1093/mnras/staa3248.

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ABSTRACT Fast radio bursts (FRBs) are extreme astrophysical phenomena entering the realm of non-linear optics, a field developed in laser physics. A classical non-linear effect is self-modulation. We examine the propagation of FRBs through the circumburst environment using the idealized setup of a monochromatic linearly polarized GHz wave propagating through a uniform plasma slab of density N at distance R from the source. We find that self-modulation occurs if the slab is located within a critical radius Rcrit ∼ 1017(N/102 cm−3)(L/1042 erg s−1) cm, where L is the isotropic equivalent of the FRB luminosity. Self-modulation breaks the burst into pancakes transverse to the radial direction. When R ≲ Rcrit, the transverse size of the pancakes is smaller than the Fresnel scale. The pancakes are strongly diffracted as the burst exits the slab, and interference between the pancakes produces a frequency modulation of the observed intensity with a sub-GHz bandwidth. When R ∼ Rcrit, the transverse size of the pancakes becomes comparable with the Fresnel scale, and the effect of diffraction is weaker. The observed intensity is modulated on a time-scale of 10 µm, which corresponds to the radial width of the pancakes. Our results suggest that self-modulation may cause the temporal and frequency structure observed in FRBs.

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41

Marthi, V. R., T. Gautam, D. Z. Li, H.-H. Lin, R. A. Main, A. Naidu, U.-L. Pen e R. S. Wharton. "Detection of 15 bursts from the fast radio burst 180916.J0158+65 with the upgraded Giant Metrewave Radio Telescope". Monthly Notices of the Royal Astronomical Society: Letters 499, n. 1 (26 settembre 2020): L16—L20. http://dx.doi.org/10.1093/mnrasl/slaa148.

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ABSTRACT We report the findings of an upgraded Giant Metrewave Radio Telescope (uGMRT) observing campaign for FRB 180916.J0158+65, which was recently found to show a 16.35-d periodicity of its active cycle. We observed the source at 550–750 MHz for ∼2 h during each of three successive cycles at the peak of its expected active period. We find 0, 12 and 3 bursts, respectively, implying a highly variable bursting rate even within the active phase. We consistently detect faint bursts with spectral energies only an order of magnitude higher than the Galactic burst source SGR 1935+2154. The times of arrival of the detected bursts rule out many possible aliased solutions, strengthening the findings of the 16.35-d periodicity. A periodicity search over a short time-scale returned no highly significant candidates. Two of the beamformer-detected bursts were bright enough to be clearly detected in the imaging data, achieving subarcsec localization, and proving to be a proof-of-concept for FRB imaging with the GMRT. We provide a 3σ upper limit of the persistent radio flux density at 650 MHz of 66 μJy, which, combined with the European VLBI Network and Very Large Array limits at 1.6 GHz, further constrains any potential radio counterpart. These results demonstrate the power of the uGMRT for providing targeted observations to detect and localize known repeating FRBs.

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42

Kuzmenko, Irina. "CORONAL JETS AS A CAUSE OF MICROWAVE NEGATIVE BURSTS". Solar-Terrestrial Physics 6, n. 3 (22 settembre 2020): 23–28. http://dx.doi.org/10.12737/stp-63202003.

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We have investigated the cause of three “isolated” negative radio bursts recorded one after another at several frequencies in the 1–17 GHz range at the Nobeyama Radio Observatory, Ussuriysk Astrophysical Observatory, and Learmonth Solar Observatory on April 10–11, 2014. The cause of the rarely observed “isolated” negative bursts is the absorption of radio emission from the quiet Sun’s regions or a radio source in the material of a large eruptive filament. Analysis of observations in different spectral ranges using images from the Nobeyama radioheliograph and the Solar Dynamics Observatory/Atmospheric Imaging Assembly has shown that the cause of all the three radio emission depressions was the screening of the limb radio source by the material of recurrent coronal jets. Parameters of the absorbing material were estimated using a previously developed model. These estimates confirmed the absorption of solar radio emission in cold material with a temperature of ~104 K at the bottom of the jets.

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43

Kuzmenko, Irina. "CORONAL JETS AS A CAUSE OF MICROWAVE NEGATIVE BURSTS". Solnechno-Zemnaya Fizika 6, n. 3 (22 settembre 2020): 26–32. http://dx.doi.org/10.12737/szf-63202003.

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Abstract (sommario):

We have investigated the cause of three “isolated” negative radio bursts recorded one after another at several frequencies in the 1–17 GHz range at the Nobeyama Radio Observatory, Ussuriysk Astrophysical Observatory, and Learmonth Solar Observatory on April 10–11, 2014. The cause of the rarely observed “isolated” negative bursts is the absorption of radio emission from the quiet Sun’s regions or a radio source in the material of a large eruptive filament. Analysis of observations in different spectral ranges using images from the Nobeyama radioheliograph and the Solar Dynamics Observatory/Atmospheric Imaging Assembly has shown that the cause of all the three radio emission depressions was the screening of the limb radio source by the material of recurrent coronal jets. Parameters of the absorbing material were estimated using a previously developed model. These estimates confirmed the absorption of solar radio emission in cold material with a temperature of ~104 K at the bottom of the jets.

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44

Rajwade, K. M., D. Agarwal, D. R. Lorimer, N. M. Pingel, D. J. Pisano, M. Ruzindana, B. Jeffs, K. F. Warnick, D. A. Roshi e M. A. McLaughlin. "A 21 cm pilot survey for pulsars and transients using the Focal L-Band Array for the Green Bank Telescope". Monthly Notices of the Royal Astronomical Society 489, n. 2 (13 agosto 2019): 1709–18. http://dx.doi.org/10.1093/mnras/stz2207.

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Abstract Phased array feed (PAF) receivers are at the forefront of modern day radio astronomy. PAFs are currently being developed for spectral line and radio continuum surveys and to search for pulsars and fast radio bursts. Here, we present results of the pilot survey for pulsars and fast radio bursts using the Focal plane L-band Array for the Green Bank Telescope (FLAG) receiver operating in the frequency range of 1.3–1.5 GHz. With a system temperature of ∼18 K, the receiver provided unprecedented sensitivity to the survey over an instantaneous field of view (FoV) of 0.1 deg2. For the survey, we implemented both time and frequency domain search pipelines designed to find pulsars and fast radio bursts that were validated by test pulsar observations. Although no new sources were found, we were able to demonstrate the capability of this instrument from observations of known pulsars. We report an upper limit on the rate of fast radio bursts above a fluence of 0.36 Jy ms to be 1.3 × 106 events per day per sky. Using population simulations, we show that the FLAG will find a factor of 2–3 more pulsars in same survey duration compared to its single pixel counterpart at the Green Bank Telescope. We also demonstrate that the new phased array receiver, ALPACA for the Arecibo telescope, will be a superior survey instrument and will find pulsars at a higher rate than most contemporary receivers by a factor of 2–10.

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45

Vukotic, B., e M. M. Cirkovic. "On the timescale forcing in astrobiology". Serbian Astronomical Journal, n. 175 (2007): 45–50. http://dx.doi.org/10.2298/saj0775045v.

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We investigate the effects of correlated global regulation mechanisms, especially Galactic gamma-ray bursts (GRBs), on the temporal distribution of hypothetical inhabited planets, using simple Monte Carlo numerical experiments. Starting with recently obtained models of planetary ages in the Galactic Habitable Zone (GHZ), we obtain that the times required for biological evolution on habitable planets of the Milky Way are highly correlated. These results run contrary to the famous anti-SETI anthropic argument of Carter, and give tentative support to the ongoing and future SETI observation projects.

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46

Leaird, D. E., A. M. Weiner, S. Kamei, M. Ishii, A. Sugita e K. Okamoto. "Generation of flat-topped 500-GHz pulse bursts using loss engineered arrayed waveguide gratings". IEEE Photonics Technology Letters 14, n. 6 (giugno 2002): 816–18. http://dx.doi.org/10.1109/lpt.2002.1003103.

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47

Zhdanov, Dmitriy, Alexander Altyntsev, Nataliya Meshalkina e Sergey Anfinogentov. "Statistical analysis of microflares as observed by the 4–8 GHz spectropolarimeter". Solar-Terrestrial Physics 9, n. 3 (30 settembre 2023): 102–11. http://dx.doi.org/10.12737/stp-93202312.

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Radio observations of weak events are one of the promising methods for studying energy release and non-thermal processes in the solar corona. The development of instrumental capabilities allows for radio observations of weak transient coronal events, such as quasi-stationary brightenings and weak flares of X-ray class B and below, which were previously inaccessible for analysis. We have measured the spectral parameters of microwave radiation for thirty weak solar flares with X-ray classes ranging from A to C1.5, using observations from the Badary Broadband Microwave Spectropolarimeter (BBMS). The spectra indicate that plasma heating is caused by the appearance of non-thermal electron fluxes, which can be detected by bursts of microwave radiation, predominantly with an amplitude ~5–6 solar flux units (SFU) at 4–5 GHz frequencies. One solar flux unit (SFU) of radio emission is equal to 10–22 W/(m•Hz). The range of low-frequency spectrum growth indices fα varies widely from α=0.3 to 15. The distribution of high-frequency decay indices is similar to the distributions of regular flares. One of the explanations for the appearance of large fα values is the Razin effect, which can influence the shape of the gyrosynchrotron spectrum during the generation of bursts in dense plasma under relatively weak magnetic fields. We have detected two events in which the appearance of non-thermal electrons led to the generation of narrowband bursts at frequencies near the double plasma frequency. SRH test trials have shown the potential for measuring the structure of flare sources with fluxes of the order of 1 SFU, indicating the high diagnostic potential of the radioheliograph for detecting acceleration processes in weak flare events and their localization in active regions.

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48

Zhdanov, Dmitriy, Alexander Altyntsev, Nataliya Meshalkina e Sergey Anfinogentov. "Statistical analysis of microflares as observed by the 4–8 GHz spectropolarimeter". Solnechno-Zemnaya Fizika 9, n. 3 (29 settembre 2023): 111–21. http://dx.doi.org/10.12737/szf-93202312.

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Abstract (sommario):

Radio observations of weak events are one of the promising methods for studying energy release and non-thermal processes in the solar corona. The development of instrumental capabilities allows for radio observations of weak transient coronal events, such as quasi-stationary brightenings and weak flares of X-ray class B and below, which were previously inaccessible for analysis. We have measured the spectral parameters of microwave radiation for thirty weak solar flares with X-ray classes ranging from A to C1.5, using observations from the Badary Broadband Microwave Spectropolarimeter (BBMS). The spectra indicate that plasma heating is caused by the appearance of non-thermal electron fluxes, which can be detected by bursts of microwave radiation, predominantly with an amplitude ~5–6 solar flux units (SFU) at 4–5 GHz frequencies. One solar flux unit (SFU) of radio emission is equal to 10–22 W/(m•Hz). The range of low-frequency spectrum growth indices fα varies widely from α=0.3 to 15. The distribution of high-frequency decay indices is similar to the distributions of regular flares. One of the explanations for the appearance of large fα values is the Razin effect, which can influence the shape of the gyrosynchrotron spectrum during the generation of bursts in dense plasma under relatively weak magnetic fields. We have detected two events in which the appearance of non-thermal electrons led to the generation of narrowband bursts at frequencies near the double plasma frequency. SRH test trials have shown the potential for measuring the structure of flare sources with fluxes of the order of 1 SFU, indicating the high diagnostic potential of the radioheliograph for detecting acceleration processes in weak flare events and their localization in active regions.

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49

Mir, Hanan, Fabian Meyer, Andreas A. Brand, Katrin Erath-Dulitz e Jan Frederik Nekarda. "Study of GHz-Burst Femtosecond Laser Micro-Punching of 4H-SiC Wafers". Solid State Phenomena 344 (6 giugno 2023): 29–33. http://dx.doi.org/10.4028/p-q6725d.

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The micromaching of silicon carbide using femtosecond laser pulses is becoming an important field of research. High-repetition-rate sub-pulse trains, so-called pulse bursts, are a particularly promising route towards completely new process regimes. We report on the results of micro-punching n-type 4H-silicon carbide wafers using GHz pulse burst in order to systematically investigate the influence of the temporal energy distribution on laser processing. Pulse-burst experiments are performed at a laser wavelength of λ= 1030 nm using a single GHz burst containing a varying number of pulses and then compared with standard single femtosecond pulse exposures. The pulse energy is swept across the ablation threshold. For each set of parameters, the micromachining efficiency is evaluated in terms of ablation efficiency and burr characteristics. Scanning electron micrographs provide qualitative information about the machining quality. The characteristics of the laser modification are discussed in relation to an increase in the number of pulses in a burst envelope and to an increase in pulse energy. We observe that, compared to a single pulse, a GHz burst comprised of 10 lower-energy pulses leads to an increase in the ablation rate by a factor of ≤ 10.

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50

Tan, Cheng-Ming, Karl Ludwig Klein, Yi-Hua Yan, Satoshi Masuda, Bao-Lin Tan, Jing Huang e Guo-Wu Yuan. "Energy and spectral analysis of confined solar flares from radio and X-ray observations". Research in Astronomy and Astrophysics 21, n. 11 (1 dicembre 2021): 274. http://dx.doi.org/10.1088/1674-4527/21/11/274.

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Abstract The energy and spectral shape of radio bursts may help us understand the generation mechanism of solar eruptions, including solar flares, coronal mass ejections, eruptive filaments, and various scales of jets. The different kinds of flares may have different characteristics of energy and spectral distribution. In this work, we selected 10 mostly confined flare events during October 2014 to investigate their overall spectral behaviour and the energy emitted in microwaves by using radio observations from microwaves to interplanetary radio waves, and X-ray observations of GOES, RHESSI, and Fermi/GBM. We found that: all the confined flare events were associated with a microwave continuum burst extending to frequencies of 9.4 ∼ 15.4 GHz, and the peak frequencies of all confined flare events are higher than 4.995 GHz and lower than or equal to 17 GHz. The median value is around 9 GHz. The microwave burst energy (or fluence) and the peak frequency are found to provide useful criteria to estimate the power of solar flares. The observations imply that the magnetic field in confined flares tends to be stronger than that in 412 flares studied by Nita et al. (2004). All 10 events studied did not produce detectable hard X-rays with energies above ∼300 keV indicating the lack of efficient acceleration of electrons to high energies in the confined flares.

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