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Register a new userDust properties at z=6.3 in the host galaxy of GRB 050904
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G. Stratta
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We investigate the dust extinction properties in the host galaxy of the Gamma-Ray Burst (GRB) GRB 050904 at z=6.29 by analyzing simultaneous broad band observations of the optical and UV afterglow at three different epochs. We show that the peculiar afterglow spectral energy distribution (SED) observed at 0.5 days and at 1 day after the burst (1.6 and 3 hours rest frame) cannot be explained with dust reddening with any of the extinction curves observed at low redshift. Yet, the extinction curve recently inferred for the most distant BAL QSO at z=6.2 nicely reproduces the SED of GRB 050904 at both epochs. Our result provides an additional, independent indication that the properties of dust evolve beyond z~6. We discuss the implications of this finding within the context of the dust production mechanisms through the cosmic ages.
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We present the results of deep imaging of the field of GRB 050904 with Suprime-Cam on the Subaru 8.2m telescope. We have obtained a narrow-band (130 A) image centered at 9200 A (NB921) and an i-band image with total integration times of 56700 and 24060 s, respectively. The host galaxy was not detected within 1 of the afterglow position. An object was found at 1.5 NE from the position of the afterglow, but clear detection of this object in the i-band image rules out its association with the burst. We obtained a limit of > 26.4 AB magnitude (2 diameter, 3 sigma) in the NB921 image for the host galaxy, corresponding to a flux of 6.0 x 10^{28} erg/s/Hz at rest 1500 A assuming a flat spectrum of the host galaxy. The star formation rate should be less than 7.5 (M_{solar}/yr) based on the conversion rate by Madau et al (1998). This upper limit for the host of GRB 050904 is consistent with the star formation rate of other gamma-ray burst host galaxies around redshift of 2 or less.
Claim of dust extinction for this GRB has been debated in the past. We suggest that the discrepant results occur primarily because most of previous studies have not simultaneously investigated the X-ray to near-IR spectral energy distribution of this GRB. The difficulty with this burst is that the X-ray afterglow is dominated by strong flares at early times and is poorly monitored at late times. In addition, the Z band photometry, which is the most sensitive to dust extinction, has been found to be affected by strong systematics. In this paper we carefully re-analyze the Swift/XRT afterglow observations of this GRB, using extensive past studies of X-ray flare properties when computing the X-ray afterglow flux level and exploiting the recent reanalysis of the optical (UV rest frame) data of the same GRB. We extract the X-ray to optical/near-IR afterglow SED for the three epochs where the best spectral coverage is available: 0.47, 1.25, and 3.4 days after the trigger. A spectral power-law model has been fitted to the extracted SEDs. We discuss that no spectral breaks or chromatic temporal breaks are expected in the epochs of interest. To fit any UV rest-frame dust absorption, we tested the Small Magellanic Cloud (SMC) extinction curve, the mean extinction curve (MEC) found for a sample of QSO at $z>4$ and its corresponding attenuation curve, as well as a starburst attenuation curve, and the extinction curve consistent with a supernova dust origin (SN-type). The SMC extinction curve and the SN-type one provide good fit to the data at all epochs, with an average amount of dust absorption at $lambda_{rest} = 3000 AA$ of $A_{3000} = 0.25pm 0.07$ mag. These results indicate that the primeval galaxy at $z = 6.3$ hosting this GRB has already enriched its ISM with dust.
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]
GRB050904 is the gamma-ray burst with the highest measured redshift. We performed time resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB050904 was a massive star embedded in a dense metal enriched molecular cloud with Z~0.03 Z_solar. This is the first local measurement of metallicity in the close environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high redshift star formation.
GRB050904 is very interesting since it is by far the most distant GRB event known to date($z=6.29$). It was reported that during the prompt high energy emission phase, a very bright optical flare was detected, and it was temporal coincident with an X-ray flare. Here we use two models to explain the optical flare, One is the late internal shock model, in which the optical flare is produced by the synchrotron radiation of the electrons accelerated by the late internal shock, and the X-ray flare is produced by the synchrotron-self-Compton mechanism. The other is the external forward-reverse shock model, in which the optical flare is from the reverse shock emission and the X-ray flare is attributed to the central engine activity. We show that with proper parameters, a bright optical flare can appear in both models. We think the late internal shock model is more favored since in this model the optical flash and the X-ray flare have the same origin, which provides a natural explanation of the temporal coincidence of them. In the forward-reverse shock scenario, fits to the optical flare and the late afterglow suggests that the physical parameters of the reverse shock are much different from that of forward shock, as found in modeling the optical flash of GRB 990123 previously.
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