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Register a new userDust extinction for an unbiased sample of GRB afterglows
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In this paper we compute rest-frame extinctions for the afterglows of a sample of gamma-ray bursts complete in redshift. The selection criteria of the sample are based on observational high-energy parameters of the prompt emission and therefore our sample should not be biased against dusty sight-lines. It is therefore expected that our inferences hold for the general population of gamma-ray bursts. Our main result is that the optical/near-infrared extinction of gamma-ray burst afterglows in our sample does not follow a single distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are highly absorbed. The true percentage of gamma-ray burst afterglows showing high absorption could be even higher since a fair fraction of the events without reliable redshift measurement are probably part of this class. These events may be due to highly dusty molecular clouds/star forming regions associated with the gamma-ray burst progenitor or along the afterglow line of sight, and/or to massive dusty host galaxies. No clear evolution in the dust extinction properties is evident within the redshift range of our sample, although the largest extinctions are at z~1.5-2, close to the expected peak of the star formation rate. Those events classified as dark are characterized, on average, by a higher extinction than typical events in the sample. A correlation between optical/near-infrared extinction and hydrogen-equivalent column density based on X-ray studies is shown although the observed NH appears to be well in excess compared to those observed in the Local Group. Dust extinction does not seem to correlate with GRB energetics or luminosity.
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In order to study the effect of dust extinction on the afterglow of gamma-ray bursts (GRBs), we carry out numerical calculations with high precision based on rigorous Mie theory and latest optical properties of interstellar dust grains, and analyze the different extinction curves produced by dust grains with different physical parameters. Our results indicate that the absolute extinction quantity is substantially determined by the medium density and metallicity. However, the shape of the extinction curve is mainly determined by the size distribution of the dust grains. If the dust grains aggregate to form larger ones, they will cause a flatter or grayer extinction curve with lower extinction quantity. On the contrary, if the dust grains are disassociated to smaller ones due to some uncertain processes, they will cause a steeper extinction curve with larger amount of extinction. These results might provide an important insight into understanding the origin of the optically dark GRBs.
The unequivocal, spectroscopic detection of the 2175 bump in extinction curves outside the Local Group is rare. To date, the properties of the bump have been examined in only two GRB afterglows (GRB 070802 and GRB 080607). In this work we analyse in detail the detections of the 2175 extinction bump in the optical spectra of the two further GRB afterglows: GRB 080605 and 080805. We gather all available optical/NIR photometric, spectroscopic and X-ray data to construct multi-epoch SEDs for both GRB afterglows. We fit the SEDs with the Fitzpatrick & Massa (1990) model with a single or broken PL. We also fit a sample of 38 GRB afterglows, known to prefer a SMC-type extinction curve, with the same model. We find that the SEDs of GRB 080605 and GRB 080805 at two epochs are fit well with a single PL with a derived extinction of A_V = 0.52(+0.13 -0.16) and 0.50 (+0.13 -0.10), and 2.1(+0.7-0.6) and 1.5+/-0.2 respectively. While the slope of the extinction curve of GRB 080805 is not well-constrained, the extinction curve of GRB 080605 has an unusual very steep far-UV rise together with the 2175 bump. Such an extinction curve has previously been found in only a small handful of sightlines in the MW. One possible explanation of such an extinction curve may be dust arising from two different regions with two separate grain populations, however we cannot distinguish the origin of the curve. We finally compare the four 2175 bump sightlines to the larger GRB afterglow sample and to Local Group sightlines. We find that while the width and central positions of the bumps are consistent with what is observed in the Local Group, the relative strength of the detected bump (A_bump) for GRB afterglows is weaker for a given A_V than for almost any Local Group sightline. Such dilution of the bump strength may offer tentative support to a dual dust-population scenario.
The VHE component from at least two GRBs, i.e., GRB180720B and GRB190114C, has been detected in the afterglow phase. We systematically analyzed 199 GRBs detected by Fermi-LAT during 2008-2019. If an additional high-energy component exists in the afterglows of Fermi-LAT GRBs, the best-fit spectral model could be a broken power-law (BPL) model with an upturn above a break energy. We compare the afterglow spectra using PL and BPL representations. Out of the 30 GRBs with >10GeV photons that arrived after T90, 25 GRBs are tentatively or significantly detected at 0.1-200 GeV after 2*T90. The spectrum of GRB131231A shows an upturn above a break of 1.6+-0.8~GeV, supporting the BPL model. For GRB131231A, we performed a modeling of its X-ray and gamma-ray spectra, and found that the SSC model can explain the upturn with acceptable parameter values. In the cases of GRBs 190114C, 171210A, 150902A, 130907A, 130427A, and 090902B, the improvement of the BPL fit compared to the PL fit is tentative or marginal. There is no conclusive evidence that an additional higher energy component commonly exists in Fermi-LAT GRB afterglows, except for a group of Fermi-LAT GRBs mentioned above. Such an additional high-energy component may be explained by the synchrotron self-Compton mechanism. Current and future VHE observations will provide important constraints on the issue.
To date, nearly all multi-wavelength modeling of long-duration gamma-ray bursts has ignored synchrotron radiation from the significant population of electrons expected to pass the shock without acceleration into a power-law distribution. We investigate the effect of including the contribution of thermal, non-accelerated electrons to synchrotron absorption and emission in the standard afterglow model, and show that these thermal electrons provide an additional source of opacity to synchrotron self-absorption, and yield an additional emission component at higher energies. The extra opacity results in an increase in the synchrotron self-absorption frequency by factors of 10--100 for fiducial parameters. The nature of the additional emission depends on the details of the thermal population, but is generally observed to yield a spectral peak in the optical brighter than radiation from the nonthermal population by similar factors a few seconds after the burst, remaining detectable at millimeter and radio frequencies several days later.
Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction -- the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]
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