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| 2. |
- Ahlgren, Björn, et al.
(författare)
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Investigating Subphotospheric Dissipation in Gamma-Ray Bursts Using Joint Fermi-Swift Observations
- 2019
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 880:2
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Tidskriftsartikel (refereegranskat)abstract
- The jet photosphere has been proposed as the origin for the gamma-ray burst (GRB) prompt emission. In many such models, characteristic features in the spectra appear below the energy range of the Fermi Gamma-ray Burst Monitor (GBM) detectors, so joint fits with X-ray data are important in order to assess the photospheric scenario. Here we consider a particular photospheric model which assumes localized subphotospheric dissipation by internal shocks in a non-magnetized outflow. We investigate it using Bayesian inference and a sample of eight GRBs with known redshifts which are observed simultaneously with Fermi GBM and the Swift X-ray Telescope (XRT). This provides us with an energy range of 0.3. keV-40. MeV and much tighter parameter constraints. We analyze 32 spectra and find that 16 are well described by the model. We also find that the estimates of the bulk Lorentz factor, Gamma, and the fireball luminosity, L-0,L-52, decrease while the fraction of dissipated energy, epsilon(d), increases in the joint fits compared to GBM-only fits. These changes are caused by a small excess of counts in the XRT data, relative to the model predictions from fits to GBM-only data. The fact that our limited implementation of the physical scenario yields 50% accepted spectra is promising, and we discuss possible model revisions in the light of the new data. Specifically, we argue that the inclusion of significant magnetization, as well as removing the assumption of internal shocks, will provide better fits at low energies.
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| 3. |
- Valan, Vlasta, 1990-, et al.
(författare)
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A comprehensive view of blackbody components in the X-ray spectra of GRBs
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 501:4, s. 4974-4997
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Tidskriftsartikel (refereegranskat)abstract
- A small fraction of gamma-ray bursts (GRBs) exhibit blackbody emission in the X-ray spectra, the origin of which is debated. In order to gain a more complete understanding of this phenomenon, we present a search for blackbody components in 116 GRBs with known redshifts observed by Swift X-ray telescope (XRT). A time-resolved spectral analysis is carried out and the significance of the blackbody is assessed with respect to an absorbed power-law model. We report nine new detections and confirm the previously reported blackbody in GRB 171205A. Together with our previous results, there are a total of 19 GRBs with significant blackbody emission in a sample of 199 GRBs observed by Swift over 13 yr. The detections include one short GRB and two low-luminosity GRBs. We estimate fireball parameters from the blackbody components and note that the blackbody luminosity is correlated with the temperature and inferred Lorentz factor. There is a large spread in the properties of the blackbody components and the light curves, which points to different origins for the emission. In about a third of the GRBs, the blackbody is clearly associated with late prompt emission from the jet. The rest of the sample includes cases that are fully consistent with the expectations from a cocoon, as well cases that may be explained by high-latitude emission or more energetic cocoons. These results indicate that thermal emission is associated with all parts of the jet.
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| 5. |
- Valan, Vlasta, 1990-, et al.
(författare)
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Investigating Time Variability of X-Ray Absorption in Swift GRBs
- 2023
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 944:1
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Tidskriftsartikel (refereegranskat)abstract
- The existence of excess absorption in the X-ray spectra of GRBs is well known, but the primary location of the absorbing material is still uncertain. To gain more knowledge about this, we have performed a time-resolved analysis of the X-ray spectra of 199 GRBs observed by the Swift X-ray telescope, searching for evidence of a decreasing column density (N-H,N-intr) that would indicate that the GRBs are ionizing matter in their surroundings. We structured the analysis as Bayesian inference and used an absorbed power law as our baseline model. We also explored alternative spectral models in cases where decreasing absorption was inferred. The analysis reveals seven GRBs that show signs of a decrease in N-H,N-intr, but we note that alternative models for the spectral evolution cannot be ruled out. We conclude that the excess absorption in the vast majority of GRBs must originate on large scales of the host galaxies and/or in the intergalactic medium. Our results also imply that an evolving column density is unlikely to affect the spectral analysis of the early X-ray spectra of GRBs. In line with this, we show that estimating the total N-H,N-intr from early Swift data in Window Timing mode reveals the same increasing trend with redshift as previous results based on data taken at later times, but with tighter constraints.
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| 6. |
- Valan, Vlasta, 1990-
(författare)
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Lessons learned from time-resolved X-ray spectra of gamma-ray bursts
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gamma-ray bursts (GRBs) are among the brightest and most energetic phenomena in the Universe, observed up to great distances. The early X-ray emission ofgamma-ray bursts are usually well described by absorbed power laws. However,in some cases, additional thermal components have been identified. The origin ofthis emission is debated, with proposed explanations including supernova shockbreakout, emission from a cocoon surrounding the jet, and emission from the jetitself. A larger sample of detections is needed to constrain these different models.In this thesis, a time-resolved analysis of 199 GRBs observed by Swift XRT wasperformed to search for thermal components, which revealed 19 cases where thisthermal component is needed to describe GRB X-ray spectra fully. Due to the largespan of blackbody properties, the origin of the blackbody component is thought tobe connected with both the jet itself and the cocoon that surrounds it with nounique origin in all GRBs.This thesis also tackles the question of where the excess X-ray absorption inGRBs originates from. There is an excess absorption above the Galactic valuein the X-ray spectra of all GRBs, and it increases with redshift. However, theexact location where it originates from is still a puzzle, with two possible, but notmutually exclusive, explanations: the host galaxy of the GRB and the inter-galacticmedium and intervening objects. I have analyzed the same sample of 199 GRBsto gain more insights into the origin of this absorption. One possible way to probewhere this excess absorption arises is to search for time variability of the excessabsorption in the X-ray spectra, which would point to the absorber being close tothe GRB. I found that this time decrease of excess absorption is a rare effect. In theGRBs that showed this time decrease, it cannot be excluded that an alternativemodel can explain the observed decrease. This implies that, in the majority ofGRBs, the absorption is happening at a larger scale in the host galaxy and/or inthe intergalactic medium.
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| 7. |
- Valan, Vlasta, 1990-
(författare)
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Thermal components in the early X-ray afterglow of gamma-ray bursts
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gamma-ray bursts (GRBs) are still puzzling scientists even 40 years after their discovery. Questions concerning the nature of the progenitors, the connection with supernovae and the origin of the high-energy emission are still lacking clear answers. Today, it is known that there are two populations of GRBs: short and long. It is also known that long GRBs are connected to supernovae (SNe). The emission observed from GRBs can be divided into two phases: the prompt emission and the afterglow. This thesis presents spectral analysis of the early X-ray afterglow of GRBs observed by the {\it Swift} satellite. For the majority of GRBs the early X-ray afterglows are well described by an absorbed power-law model. However, there exists a number of cases where this power-law component fails in fully describing the observed spectra and an additional blackbody component is needed. In the paper at the end of this thesis, a time-resolved spectral analysis of 74 GRBs observed by the X-ray telescope on board {\it Swift} is presented. Each spectrum is fitted with a power-law and a power-law plus blackbody model. The significance of the added thermal component is then assessed using Monte Carlo simulations. Six new cases of GRBs with thermal components in their spectra are presented, alongside three previously reported cases. The results show that a cocoon surrounding the jet is the most likely explanation for the thermal emission observed in the majority of GRBs. In addition, the observed narrow span in radii points to these GRBs being produced in similar environments.
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| 8. |
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| 9. |
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| 10. |
- Valan, Vlasta, 1990-, et al.
(författare)
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Thermal components in the early X-ray afterglows of GRBs : likely cocoon emission and constraints on the progenitors
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 474:2, s. 2401-2418
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Tidskriftsartikel (refereegranskat)abstract
- The early X-ray afterglows of gamma-ray bursts (GRBs) are usually well described by absorbed power laws. However, in some cases, additional thermal components have been identified. The origin of this emission is debated, with proposed explanations including supernova shock breakout, emission from a cocoon surrounding the jet, as well as emission from the jet itself. A larger sample of detections is needed in order to place constraints on these different models. Here, we present a time-resolved spectral analysis of 74 GRBs observed by Swift X-ray Telescope in a search for thermal components. We report six detections in our sample, and also confirm an additional three cases that were previously reported in the literature. The majority of these bursts have a narrow range of blackbody radii around similar to 2 x 10(12) cm, despite having a large range of luminosities (L-peak similar to 10(47)-10(51) erg s(-1)). This points to an origin connected to the progenitor stars, and we suggest that emission from a cocoon breaking out from a thick wind may explain the observations. For two of the bursts in the sample, an explanation in terms of late prompt emission from the jet is instead more likely. We also find that these thermal components are preferentially detected when the X-ray luminosity is low, which suggests that they may be hidden by bright afterglows in the majority of GRBs.
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