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Register a new userThe 2175 AA extinction feature in the optical afterglow spectrum of GRB 180325A at z=2.25
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The UV extinction feature at 2175 AA is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 AA bump on the sightline to the gamma-ray burst (GRB) afterglow GRB 180325A at z=2.2486, the only unambiguous detection over the past ten years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well-described by a single power-law and an extinction law with R_V~4.4, A_V~1.5, and the 2175 AA extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of [Zn/H]>-0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with an equivalent width of Wr(lambda 1656) = 0.85+/-0.05. We also detect optical emission lines from the host galaxy. Based on the Halpha emission line flux, the derived dust-corrected star-formation rate is ~46+/-4 M_sun/yr and the predicted stellar mass is log M*/M_sun~9.3+/-0.4, suggesting the host galaxy is amongst the main-sequence star-forming galaxies.
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We report the clear detection of the 2175A dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshift of z=2.45. This is the highest redshift for a detected 2175A dust bump to date, and it is the first clear detection of the 2175A bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives A_V=0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, an LMC type extinction gives the best fit to the extinction curve of the host of GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H]=-0.46+/-0.38, [Zn/H]=-0.50+/-0.68). In particular, the spectrum of GRB070802 is unique for a GRB spectrum in that it shows clear CI absorption features, leading us to propose a correlation between the presence of the bump and CI. The gas to dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies between typical MW and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. 2003 that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.
We present optical photometry of the GRB 060912A afterglow obtained with ground-based telescopes, from about 100 sec after the GRB trigger till about 0.3 day later, supplemented with the Swift optical afterglow data released in its official website. The optical light curve (LC) displays a smooth single power-law decay throughout the observed epochs, with a power-law index of about -1 and no significant color evolution. This is in contrast to the X-ray LC which has a plateau phase between two normal power-law decays of a respective index of about -1 and -1.2. It is shown by our combined X-ray and optical data analysis that this asynchronous behavior is difficult to be reconciled with the standard afterglow theory and energy injection hypothesis. We also construct an optical-to-X-ray spectral energy distribution at about 700 sec after the GRB trigger. It displays a significant flux depression in the B-band, reminding us of the possibility of a host-galaxy (at z=0.937) 2175-A dust absorption similar to the one that characterizes the Milky Way extinction law. Such an identification, although being tentative, may be confirmed by our detailed analysis using both template extinction laws and the afterglow theory. So far the feature is reported in very few GRB afterglows. Most seem to have a host galaxy either unusually bright for a GRB, just like this one, or of an early type, supporting the general suggestion of an anti-correlation between the feature and star-forming activities.
Context. Gamma-ray burst (GRBs) afterglows probe sightlines to star-forming regions in distant star-forming galaxies. Here we present a study of the peculiar afterglow spectrum of the z = 0.889 Swift GRB 140506A. Aims. Our aim is to understand the origin of the very unusual properties of the absorption along the line-of-sight. Methods. We analyse spectroscopic observations obtained with the X-shooter spectrograph mounted on the ESO/VLT at two epochs 8.8 h and 33 h after the burst as well as imaging from the GROND instrument. We also present imaging and spectroscopy of the host galaxy obtained with the Magellan telescope. Results. The underlying afterglow appears to be a typical afterglow of a long-duration GRB. However, the material along the line-of- sight has imprinted very unusual features on the spectrum. Firstly, there is a very broad and strong flux drop below 8000 AA (4000 AA in the rest frame), which seems to be variable between the two spectroscopic epochs. We can reproduce the flux-drops both as a giant 2175 AA extinction bump and as an effect of multiple scattering on dust grains in a dense environment. Secondly, we detect absorption lines from excited H i and He i. We also detect molecular absorption from CH+ . Conclusions. We interpret the unusual properties of these spectra as reflecting the presence of three distinct regions along the line-of-sight: the excited He i absorption originates from an H ii-region, whereas the Balmer absorption must originate from an associated photodissociation region. The strong metal line and molecular absorption and the dust extinction must originate from a third, cooler region along the line-of-sight. The presence of (at least) three separate regions is reflected in the fact that the different absorption components have different velocities relative to the systemic redshift of the host galaxy.
The 2175 {AA} ultraviolet (UV) extinction bump in interstellar medium (ISM) of the Milky Way was discovered in 1965. After intensive exploration of more than a half century, however, its exact origin still remains a big conundrum that is being debated. Here we propose a mixture model by which the extinction bump in ISM is argued possibly relevant to the clusters of hydrogenated T-carbon (HTC) molecules (C40H16) that have intrinsically a sharp absorption peak at the wavelength 2175 {AA}. By linearly combining the calculated absorption spectra of HTC mixtures, graphite, MgSiO3 and Fe2SiO4, we show that the UV extinction curves of optional six stars can be nicely fitted. This present work poses an alternative explanation toward understanding the physical origin of the 2175 {AA} extinction bump in ISM of the Milky Way.
We present a study of the extinction and depletion-derived dust properties of gamma-ray burst (GRB) absorbers at $1<z<3$ showing the presence of neutral carbon (ion{C}{I}). By modelling their parametric extinction laws, we discover a broad range of dust models characterizing the GRB ion{C}{I} absorption systems. In addition to the already well-established correlation between the amount of ion{C}{I} and visual extinction, $A_V$, we also observe a correlation with the total-to-selective reddening, $R_V$. All three quantities are also found to be connected to the presence and strength of the 2175,{AA} dust extinction feature. While the amount of ion{C}{I} is found to be correlated with the SED-derived dust properties, we do not find any evidence for a connection with the depletion-derived dust content as measured from [Zn/Fe] and $N$(Fe)$_{rm dust}$. To reconcile this, we discuss a scenario where the observed extinction is dominated by the composition of dust particles confined in the molecular gas-phase of the ISM. We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinction in GRB ion{C}{I} absorbers is primarily produced by carbon-rich dust in the molecular cloud and is therefore only observable in the extinction curves and not in the depletion patterns. This also indicates that the 2175,{AA} dust extinction feature is caused by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high-$z$ absorbers.
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