| 1. |
- Preece, R., et al.
(författare)
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The First Pulse of the Extremely Bright GRB 130427A : A Test Lab for Synchrotron Shocks
- 2014
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 343:6166, s. 51-54
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray burst (GRB) 130427A is one of the most energetic GRBs ever observed. The initial pulse up to 2.5 seconds is possibly the brightest well-isolated pulse observed to date. A fine time resolution spectral analysis shows power-law decays of the peak energy from the onset of the pulse, consistent with models of internal synchrotron shock pulses. However, a strongly correlated power-law behavior is observed between the luminosity and the spectral peak energy that is inconsistent with curvature effects arising in the relativistic outflow. It is difficult for any of the existing models to account for all of the observed spectral and temporal behaviors simultaneously.
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| 2. |
- Axelsson, Magnus, et al.
(författare)
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GRB110721A : AN EXTREME PEAK ENERGY AND SIGNATURES OF THE PHOTOSPHERE
- 2012
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Ingår i: Astrophysical Journal Letters. - 2041-8205. ; 757:2
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Tidskriftsartikel (refereegranskat)abstract
- GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for similar to 24.5 s (in the GBM) and had a peak flux of (5.7 +/- 0.2) x 10(-5) erg s(-1) cm(-2). The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15 +/- 2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy events to achieve continuous 10-100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89 +/- 0.10. The temperature of the blackbody component also decreased, starting from similar to 80 keV, and the decay showed a significant break after similar to 2 s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.
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| 3. |
- Ahumada, T., et al.
(författare)
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Discovery and confirmation of the shortest gamma-ray burst from a collapsar
- 2021
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Ingår i: Nature Astronomy. - : Springer Nature. - 2397-3366. ; 5:9, s. 917-927
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Tidskriftsartikel (refereegranskat)abstract
- Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. The duration and hardness distribution of GRBs has two clusters1, now understood to reflect (at least) two different progenitors2. Short-hard GRBs (SGRBs; T90 < 2 s) arise from compact binary mergers, and long-soft GRBs (LGRBs; T90 > 2 s) have been attributed to the collapse of peculiar massive stars (collapsars)3. The discovery of SN 1998bw/GRB 980425 (ref. 4) marked the first association of an LGRB with a collapsar, and AT 2017gfo (ref. 5)/GRB 170817A/GW170817 (ref. 6) marked the first association of an SGRB with a binary neutron star merger, which also produced a gravitational wave. Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi satellite and the Interplanetary Network localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova, but which is consistent with being the supernova. Although the GRB duration is short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirm a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.
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| 4. |
- Burgess, J. M., et al.
(författare)
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Time-resolved analysis of fermi gamma-ray bursts with fast- and slow-cooled synchrotron photon models
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 784:1, s. 17-
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved spectroscopy is performed on eight bright, long gamma-ray bursts (GRBs) dominated by single emission pulses that were observed with the Fermi Gamma-Ray Space Telescope. Fitting the prompt radiation of GRBs by empirical spectral forms such as the Band function leads to ambiguous conclusions about the physical model for the prompt radiation. Moreover, the Band function is often inadequate to fit the data. The GRB spectrum is therefore modeled with two emission components consisting of optically thin non-thermal synchrotron radiation from relativistic electrons and, when significant, thermal emission from a jet photosphere, which is represented by a blackbody spectrum. To produce an acceptable fit, the addition of a blackbody component is required in five out of the eight cases. We also find that the low-energy spectral index a is consistent with a synchrotron component with a = -0.81 +/- 0.1. This value lies between the limiting values of a = -2/3 and a = -3/2 for electrons in the slow-and fast-cooling regimes, respectively, suggesting ongoing acceleration at the emission site. The blackbody component can be more significant when using a physical synchrotron model instead of the Band function, illustrating that the Band function does not serve as a good proxy for a non-thermal synchrotron emission component. The temperature and characteristic emission-region size of the blackbody component are found to, respectively, decrease and increase as power laws with time during the prompt phase. In addition, we find that the blackbody and non-thermal components have separate temporal behaviors as far as their respective flux and spectral evolutions.
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| 5. |
- Basak, Rupal, et al.
(författare)
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Surprise in simplicity : an unusual spectral evolution of a single pulse GRB 151006A
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 472:1, s. 891-903
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed analysis of GRB 151006A, the first gamma-ray burst (GRB) detected by AstroSat Cadmium-Zinc-Telluride Imager (CZTI). We study the long-term spectral evolution by exploiting the capabilities of Fermi and Swift satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands shows a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft evolution until similar to 16-20 s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in the case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, blackbody+Band and two blackbodies+power law. Interestingly, the Fermi Large Area Telescope also detects its first photon (> 100 MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself that, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The Swift data at late epochs (> T-90 of the GRB) also show a significant spectral evolution consistent with the early second phase. The CZTI data (100-350 keV), though having low significance (1 sigma), show high values of polarization in the two epochs (77-94 per cent), in agreement with our interpretation.
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| 6. |
- Chattopadhyay, Tanmoy, et al.
(författare)
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Hard X-Ray Polarization Catalog for a Five-year Sample of Gamma-Ray Bursts Using AstroSat CZT Imager
- 2022
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Ingår i: Astrophysical Journal. - : IOP Publishing Ltd. - 0004-637X .- 1538-4357. ; 936:1
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Tidskriftsartikel (refereegranskat)abstract
- The Cadmium Zinc Telluride Imager (CZTI) on board AstroSat has been regularly detecting gamma-ray bursts (GRBs) since its launch in 2015. Its sensitivity to polarization measurements at energies above 100 keV allows CZTI to attempt spectropolarimetric studies of GRBs. Here, we present the first catalog of GRB polarization measurements made by CZTI during its first five years of operation. This includes the time-integrated polarization measurements of the prompt emission of 20 GRBs in the energy range 100-600 keV. The sample includes the bright GRBs that were detected within an angle range of 0 degrees-60 degrees and 120 degrees-180 degrees where the instrument has useful polarization sensitivity and is less prone to systematics. We implement a few new modifications in the analysis to enhance the polarimetric sensitivity of the instrument. The majority of the GRBs in the sample are found to possess less/null polarization across the total bursts' duration in contrast to a small fraction of five GRBs that exhibit high polarization. The low polarization across the bursts might be due either to the burst being intrinsically weakly polarized or to a varying polarization angle within the burst even when it is highly polarized. In comparison to POLAR measurements, CZTI has detected a larger number of cases with high polarization. This may be a consequence of the higher energy window of CZTI observations, which results in the sampling of a shorter duration of burst emissions than POLAR, thereby probing emissions with less temporal variation in polarization properties.
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| 7. |
- Iyyani, Shabnam, et al.
(författare)
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Variable jet properties in GRB 110721A : time resolved observations of the jet photosphere
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 433:4, s. 2739-2748
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Tidskriftsartikel (refereegranskat)abstract
- Fermi Gamma-ray Space Telescope observations of GRB 110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at similar to 100 keV, and a non-thermal component, which peaks at similar to 1000 keV. We use the photospheric component to calculate the properties of the relativistic outflow. We find a strong evolution in the flow properties: the Lorentz factor decreases with time during the bursts from G similar to 1000 to similar to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown efficiency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the flow nozzle measured from the central engine, r(0), increases by more than two orders of magnitude. Assuming a moderately magnetized outflow we estimate that r(0) varies from 10(6) to similar to 10(9) cm during the burst. We suggest that the maximal value reflects the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for gamma-ray burst pulses.
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| 8. |
- Burgess, J. Michael, et al.
(författare)
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AN OBSERVED CORRELATION BETWEEN THERMAL AND NON-THERMAL EMISSION IN GAMMA-RAY BURSTS
- 2014
-
Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 784:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent observations by the Fermi Gamma-ray Space Telescope have confirmed the existence of thermal and non-thermal components in the prompt photon spectra of some gamma-ray bursts (GRBs). Through an analysis of six bright Fermi GRBs, we have discovered a correlation between the observed photospheric and non-thermal gamma-ray emission components of several GRBs using a physical model that has previously been shown to be a good fit to the Fermi data. From the spectral parameters of these fits we find that the characteristic energies, E-p and kT, of these two components are correlated via the relation E-p proportional to T-alpha which varies from GRB to GRB. We present an interpretation in which the value of the index alpha indicates whether the jet is dominated by kinetic or magnetic energy. To date, this jet composition parameter has been assumed in the modeling of GRB outflows rather than derived from the data.
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| 9. |
- Chand, Vikas, et al.
(författare)
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Violation of Synchrotron Line of Death by the Highly Polarized GRB 160802A
- 2018
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 862:2
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Tidskriftsartikel (refereegranskat)abstract
- GRB 160802A is one of the brightest gamma-ray bursts (GRBs) observed by the Fermi Gamma-ray Burst Monitor (GBM) in the energy range of 10-1000 keV, while at the same time it is surprisingly faint at energies greater than or similar to 2 MeV. An observation with the AstroSat/CZT Imager also provides the polarization that helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the Fermi/GBM data, and find two main bursting episodes that are clearly separated in time, one of which is particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behavior corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90% cases, crosses even the slow cooling limit (alpha = -2/3) of an optically thin synchrotron shock model. Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarization in 100-300 keV is too high, pi = 85 +/- 29%, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to alpha = + 0.5, in principle can produce high polarization, but only beyond the spectral peak, which in our case lies close to 200-300 keV during the time when most of the polarization signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.
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| 10. |
- Chattopadhyay, Tanmoy, et al.
(författare)
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Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts
- 2021
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Ingår i: Journal of astrophysics and astronomy. - : Springer Nature. - 0250-6335 .- 0973-7758. ; 42:2
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Tidskriftsartikel (refereegranskat)abstract
- Cadmium-Zinc-Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100-300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (similar to 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100-900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected similar to 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.
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| 11. |
- Iyyani, Shabnam, 1988-, et al.
(författare)
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Extremely narrow spectrum of GRB110920A : further evidence for localised, subphotospheric dissipation
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 450:2, s. 1651-1663
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Tidskriftsartikel (refereegranskat)abstract
- Much evidence points towards that the photosphere in the relativistic outflow in GRBs plays an important role in shaping the observed MeV spectrum. However, it is unclear whether the spectrum is fully produced by the photosphere or whether a substantial part of the spectrum is added by processes far above the photosphere. Here we make a detailed study of the $\gamma$−ray emission from single pulse GRB110920A which has a spectrum that becomes extremely narrow towards the end of the burst. We show that the emission can be interpreted as Comptonisation of thermal photons by cold electrons in an unmagnetised outflow at an optical depth of $\tau \sim 20$. The electrons receive their energy by a local dissipation occurring close to the saturation radius. The main spectral component of GRB110920A and its evolution is thus, in this interpretation, fully explained by the emission from the photosphere including localised dissipation at high optical depths.
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| 12. |
- Sharma, Vidushi, et al.
(författare)
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Time-varying Polarized Gamma-Rays from GRB 160821A : Evidence for Ordered Magnetic Fields
- 2019
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Ingår i: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 882:1
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Tidskriftsartikel (refereegranskat)abstract
- GRB 160821A is the third most energetic gamma-ray burst observed by the Fermi gamma-ray space telescope. Based on the observations made by the Cadmium Zinc Telluride Imager on board AstroSat, here we report the most conclusive evidence to date of (i) high linear polarization (66(27)(+26)%; 5.3 sigma detection), and (ii) variation of polarization angle with time, occurring twice during the rise and decay phase of the burst at 3.5 sigma and 3.1 sigma detections, respectively. All confidence levels are reported for two parameters of interest. These observations strongly suggest synchrotron radiation produced in magnetic field lines that are highly ordered on angular scales of 1/Gamma, where Gamma is the Lorentz factor of the outflow.
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