Optical Afterglows Of Gamma-ray Bursts Research Articles

Are GRB optical afterglows relatively brighter at high z?

Abstract The redshift distribution of gamma-ray bursts (GRBs) is strongly biased by selection effects. We investigate, via Monte Carlo simulations, one possible selection effect that may be modifying the Swift GRB redshift distribution. We show how telescope response times to acquire a GRB redshift may, via the Malmquist effect and GRB optical afterglow (OA) brightness distribution, introduce a bias into the average of the observed redshift distribution. It is difficult to reconcile a recently reported correlated trend between telescope response time and average redshifts unless we employ a redshift-dependent OA distribution. Simulations of this selection effect suggest that GRB OAs may have been either intrinsically brighter early in the Universe or suffered less local host galaxy extinction.

Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: implications for molecular hydrogen searches

Aims. We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration gamma-ray bursts (GRBs) and to determine their physical conditions. Methods. A detailed analysis of high-quality VLT/UVES spectra of the optical afterglow of GRB 050730 and other Swift-era bursts is presented. Results. We report the detection of a significant number of previously unidentified allowed transition lines of Fe + , involving the fine structure of the ground term ( 6 D7/2, 6 D5/2, 6 D3/2, 6 D1/2) and that of other excited levels ( 4 F9/2, 4 F7/2, 4 F5/2, 4 F3/2, 4 D7/2, 4 D5/2), from the zabs = 3.969, log N(H 0 ) = 22.10, damped Lyman-α (DLA) system located in the host galaxy of GRB 050730. No molecular hydrogen (H2) is detected down to a molecular fraction of log f < −8.0. We derive accurate metal abundances for Fe + ,S + ,N 0 , Ni + , and, for the first time in this system, Si + and Ar 0 . The absorption lines are best-fit as a single narrow velocity component at zabs = 3.96857. The time-dependent evolution of the observed Fe + energy-level populations is modelled by assuming the excitation

LOOKING INTO THE FIREBALL: ROTSE-III ANDSWIFTOBSERVATIONS OF EARLY GAMMA-RAY BURST AFTERGLOWS

We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the Robotic Optical Transient Search Experiment (ROTSE-III) telescope network from 2005 March through 2007 June. This set is comprised of 12 afterglows with early optical and Swift/X-Ray Telescope observations, with a median ROTSE-III response time of 45 s after the start of γ-ray emission (8 s after the GCN notice time). These afterglows span 4 orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100 ≲ Γ_0 ≲ 1000, consistent with expectations.

A UNIFIED REPRESENTATION OF GAS-PHASE ELEMENT DEPLETIONS IN THE INTERSTELLAR MEDIUM

A study of gas-phase element abundances reported in the literature for 17 different elements sampled over 243 sight lines in the local part of our Galaxy reveals that the depletions into solid form (dust grains) are extremely well characterized by trends that employ only three kinds of parameters. One is an index that describes the overall level of depletion applicable to the gas in any particular sight line, and the other two represent linear coefficients that describe how to derive each element's depletion from this sight-line parameter. The information from this study reveals the relative proportions of different elements that are incorporated into dust at different stages of grain growth. An extremely simple scheme is proposed for deriving the dust contents and metallicities of absorption-line systems that are seen in the spectra of distant quasars or the optical afterglows of gamma-ray bursts. Contrary to presently accepted thinking, the elements sulfur and krypton appear to show measurable changes in their depletions as the general levels of depletions of other elements increase, although more data are needed to ascertain whether or not these findings truly compelling. Nitrogen appears to show no such increase. The incorporation of oxygen into solid form in the densest gas regions far exceeds the amounts that can take the form of silicates or metallic oxides; this conclusion is based on differential measurements of depletion and thus is unaffected by uncertainties in the solar abundance reference scale.

PROMPT OBSERVATIONS OF THE EARLY-TIME OPTICAL AFTERGLOW OF GRB 060607A

Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) observed the early-time optical afterglow of gamma-ray burst (GRB) 060607A and obtained a densely sampled multiwavelength light curve that begins only tens of seconds after the GRB. Located at Cerro Tololo Inter-American Observatory in Chile, PROMPT is designed to observe the afterglows of γ-ray bursts using multiple automated 0.4 m telescopes that image simultaneously in many filters when the afterglow is bright and may be highly variable. The data span the interval from 44 s after the GRB trigger to 3.3 hr in the Bgri filters. We observe an initial peak in the light curve at approximately 3 minutes, followed by re-brightenings peaking around 40 minutes and again at 66 minutes. Although our data overlap with the early Swift γ-ray and X-ray light curves, we do not see a correlation between the optical and high-energy flares. We do not find evidence for spectral evolution throughout the observations. We model the variations in the light curves and find that the most likely cause of the re-brightening episodes is a refreshment of the forward shock preceded by a rapidly fading reverse shock component, although other explanations are plausible.

Gamma-ray burst optical afterglow and redshift selection effects: the learning curve effect at work

Abstract We show how the observed gamma-ray burst (GRB) optical afterglow (OA) and redshift distributions are changing in time from selection effects. For a subset of Swift triggered long-duration bursts, we show that the mean time taken to acquire spectroscopic redshifts for a GRB OA has evolved to shorter times. We identify a strong correlation between the mean time taken to acquire a spectroscopic redshift and the measured redshift. This correlation reveals that shorter response times favour smaller redshift bursts. This is compelling evidence for a selection effect that biases longer response times with relatively brighter high-redshift bursts. Conversely, for shorter response times, optically fainter bursts that are relatively closer are bright enough for spectroscopic redshifts to be acquired. This selection effect could explain why the average redshift, 〈z〉≈ 2.8 measured in 2005, has evolved to 〈z〉≈ 2, by mid 2008. Understanding these selection effects provides an important tool for separating the contributions of intrinsically faint bursts, those obscured by host galaxy dust and bursts not seen in the optical because their OAs are observed at late times. The study highlights the importance of rapid response telescopes capable of spectroscopy, and identifies a new redshift selection effect that has not been considered previously, namely the response time to measure the redshift.

Optical afterglows of gamma-ray bursts: a bimodal distribution?

The luminosities of the optical afterglows of gamma-ray bursts, 12 h (rest-frame time) after the trigger, show a surprising clustering, with a minority of events being at a significantly smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre-Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic burst is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflect a real bimodality: either the optical luminosity distribution of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength-independent extinction. This population of intrinsically weak or grey-absorbed events can be associated with dark bursts.

Observations of the Optical Afterglow of GRB 050319: The Wind-to-ISM Transition in View

The collapse of a massive star is believed to be the most probable progenitor of a long gamma-ray burst (GRB). Such a star is expected to have its environment modified by the stellar wind. The effect of such a circumstellar wind medium is expected to be seen in the evolution of a GRB afterglow, but so far this has not been conclusively found. We claim that a signature of the transition from wind to constant density medium of the circumburst medium is visible in the afterglow of GRB 050319. Along with the optical observations of the afterglow of GRB 050319, we present a model for the multiband afterglow of GRB 050319. We show that the break seen in the optical light curve at ~0.02 days could be explained as being due to the transition from wind to constant density medium of the circumburst medium, in which case, to our knowledge, this could be the first ever detection of such a transition at any given frequency band. Detection of such a transition could also serve as confirmation of the massive star collapse scenario for GRB progenitors, independent of supernova signatures.

GRB 060505: A Possible Short‐Duration Gamma‐Ray Burst in a Star‐forming Region at a Redshift of 0.09

On 2006 May 5, a four second duration, low energy, ~10^49 erg, Gamma-Ray Burst (GRB) was observed, spatially associated with a z=0.0894 galaxy. Here, we report the discovery of the GRB optical afterglow and observations of its environment using Gemini-south, Hubble Space Telescope (HST), Chandra, Swift and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star forming region suggests that the progenitor delay time, from birth to explosion, is smaller than about 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in case of ``maximal'' extinction) at about two weeks after the burst, and limits the ejected mass of radioactive Nickel 56 to be less than about 2x10^-4 solar mass (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as for short-duration GRBs.

Very Different X-Ray-to-Optical Column Density Ratios in γ-Ray Burst Afterglows: Ionization in GRB Environments

We compare the H I column densities from Lyalpha absorption to the metal column densities from X-ray absorption in gamma-ray burst (GRB) afterglows. Eight bursts of 17 entering the sample have significant extragalactic X-ray absorptions. Of these we find a range in metallicities from subsolar to a few hundred times the solar value. There is a complete lack of correlation between the X-ray and optical column densities. This, and the large range and extreme values of these ``metallicities,'' suggests that the column densities derived by one or both methods are not a reliable indication of the true total column densities toward GRBs. Ionization of the GRB's gas cloud to large distances along the line of sight seems the most likely cause. From the lower limit on the total column density and the UV luminosity of the GRBs, we derive a maximum distance to the majority of the gas surrounding GRBs of ~3 pc, suggesting that the gas probed by optical afterglow spectra is not the cloud in which the burst occurs. This is an encouraging result for the use of GRB optical afterglows as probes of the interstellar medium (ISM) in their host galaxies, as the ISM observed is less likely to be strongly affected by the GRB or its progenitor.

Three intervening galaxy absorbers towards GRB 060418: faint and dusty?

Abstract We present an analysis of three strong, intervening Mg II absorption systems (zabs = 0.603, 0.656, 1.107) towards the optical afterglow of gamma-ray burst (GRB) 060418. From high-resolution Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we measure metal column densities and find that the highest redshift absorber exhibits a large amount of dust depletion compared with damped Lyman absorbers (DLAs) seen in quasi-stellar object (QSO) spectra. The intervening zabs = 1.107 absorber is also unusual in exhibiting a clear 2175-Å bump, the first time this feature has been definitively detected in a GRB spectrum. The GRB afterglow spectrum is best fitted with a two-component extinction curve: a Small Magellanic Cloud (SMC) extinction law at z = 1.49 (the redshift of the host) with E(B−V) = 0.07 ± 0.01 and a Galactic extinction curve at z∼ 1.1 with E(B−V) = 0.08 ± 0.01. We also present a moderately deep New Technology Telescope (NTT) R-band image of the GRB 060418 field and spectroscopy of four galaxies within 1 arcmin. None of these objects has a redshift that matches any of the intervening absorbers, and we conclude that the galaxies responsible for the two intervening Mg ii absorbers at z∼ 0.6 have luminosities .

An Optical Afterglow Model for Bright Linear Type II Supernovae

We show that a gamma-ray burst (GRB) afterglow model combined with an underlying plateau Type II supernova can fit the data of at least one bright linear Type II supernova, SN 1979C. We suggest that the bright linear subclass of Type IIs could be represented by this model. We describe a scenario in which a hydrogen-rich massive star core collapses, causing a jet explosion that punctures through the star and initiates a shock that ejects the hydrogen-rich envelope. The jet is responsible for a low-density high-energy ejecta causing a GRB optical afterglow. The jet contains enough energy to create an asymmetric shock wave that ejects the He core and the overlying H envelope. We fit the observational data of SN 1979C with an analytic GRB model and a parameterized Type II light curve from the data of SN 1969L. We also show that a numerical simulation of a massive star experiencing mass loss can also fit the data with ejected mass of 12 M☉, initial radius ~300 R☉, and explosion energy of 1 × 1051 ergs coupled to a GRB afterglow. We suggest that the energy of the jet could have been dissipated by the hydrogen envelope and thus not cause the ejecta to expand with the high velocities typically seen in Type Ic supernovae associated with GRBs. The model presented here could be an example of a failed GRB and X-ray flashes.

A Flash in the Dark: UVES Very Large Telescope High‐Resolution Spectroscopy of Gamma‐Ray Burst Afterglows

We present the first high-resolution (R ¼ 20;000 45;000, corresponding to 14 km s � 1 at 4200 8 to 6.6 km s � 1 at 9000 8) observations of the optical afterglow of gamma-ray bursts. GRB 020813 and GRB 021004 were observed by UVES at the Very Large Telescope 22.19 and 13.52 hr after the trigger, respectively. These spectra show that the interstellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spans a total velocity range of up to about 3000 km s � 1 . Several narrow components are resolved down to aw idth of af ew tens of km s � 1 . In the case of GRB 021004 we detected both low- and highionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the varioussystemsareconsistentwitharemarkablynarrowrangewithnocleartrendwithsystemvelocity.Thiscanbe interpreted as due to density fluctuations on top of a regular R � 2 wind density profile. Subject headingg cosmology: observations — galaxies: abundances — galaxies: ISM — gamma rays: bursts Online material: color figures

On the Light Curve and Spectrum of SN 2003dh Separated from the Optical Afterglow of GRB 030329

The net optical light curves and spectra of the supernova (SN) 2003dh are obtained from the published spectra of GRB 030329, covering about 6 days before SN maximum to about 60 days after. The bulk of the U-band flux is subtracted from the observed spectra using early-time afterglow templates, because strong line blanketing greatly depresses the UV and U-band SN flux in a metal-rich, fast-moving SN atmosphere. The blue-end spectra of the gamma-ray burst (GRB) connected hypernova SN 1998bw is used to determine the amount of subtraction. The subtraction of a host galaxy template affects the late-time results. The derived SN 2003dh light curves are narrower than those of SN 1998bw, rising as fast before maximum, reaching a possibly fainter maximum, and then declining ~ 1.2-1.4 times faster. We then build UVOIR bolometric SN light curve. Allowing for uncertainties, it can be reproduced with a spherical ejecta model of Mej ~ 7 ± 3 M☉, EK ~ 3.5 ± 1.5 × 1052 ergs, with EK/Mej ~ 5 following previous spectrum modeling, and M(56Ni) ~ 0.4 M☉. This suggests a progenitor main-sequence mass of ~25-40 M☉, lower than SN 1998bw but significantly higher than normal Type Ic SNe and the GRB-unrelated hypernova SN 2002ap.

Early re-brightenings in GRB afterglows as signatures of low-to-high density boundary

The association of long gamma-ray bursts (GRBs) with star forming regions and the idea of massive stars as progenitors of GRBs are widely accepted. Because of their short lifetimes, it is very likely that massive stars are still embedded in dense molecular clouds when they give birth to GRBs. Stellar winds from GRB progenitors can create low-density bubbles with sizes and densities strongly depending on the initial ambient density. A boundary between the bubble and the dense molecular cloud must exist with the density at the boundary increasing from that of the bubble to that of the outer cloud. We have calculated the lightcurves of the afterglows in such environments with three regions: the stellar wind region, the boundary, and the molecular cloud. We show that the interaction between the cylindrical jet and the density boundary can result in a re-brightening of the afterglow occurring as early as ∼1 day after the GRB. We compare our models with the optical afterglows of GRB 970508, GRB 000301C, and GRB 030226. We find that the values of our model parameters, including the radius of the wind bubble, the densities in the bubble and in the outer molecular cloud are within typical ranges.

On the X‐Ray Lines in the Afterglows of Gamma‐Ray Bursts

The observation of X-ray lines in the afterglow of GRB 011211 has been reported, and challenged. The lines were interpreted as blueshifted X-rays characteristic of a set of photoionized metals, located in a section of a supernova shell illuminated by a gamma-ray burst (GRB) emitted a couple of days after the supernova explosion. We show that the most prominent reported lines coincide with the ones predicted in the cannonball model of GRBs. In this model, the putative signatures are hydrogen lines, boosted by the (highly relativistic) motion of the cannonballs (CBs). The corresponding Doppler boost can be extracted from the fit to the observed I-, R-, and V-band light curves of the optical afterglow of GRB 011211, so that, since the redshift is also known, the line energies are (in the CB model) predicted. We also discuss other GRBs of known redshift which show spectral features generally interpreted as Fe lines, or Fe recombination edges. The ensemble of results is very encouraging from the CB model's point of view, but the data on each individual GRB are not good enough to draw (any) objectively decisive conclusions. We outline a strategy for X-ray observers to search for lines which, in the CB model, move predictably from higher to lower energies.

Are the hosts of gamma-ray bursts sub-luminous and blue galaxies?

We present K-band imaging observations of ten Gamma-Ray Burst (GRB) host galaxies. We compare their observed and absolute K magnitudes as well as their R-K colours with those of other distant sources detected in various optical, near-infrared, mid-infrared and submillimeter deep surveys. We find that the GRB host galaxies, most of them lying at 0.5<z<1.5, exhibit very blue colours, comparable to those of the faint blue star-forming sources at high redshift. They are sub-luminous in the K-band, suggesting a low stellar mass content. We do not find any GRB hosts harbouring R- and K-band properties similar to those characterizing the luminous infrared/submillimeter sources and the extremely red starbursts. Should GRBs be regarded as an unbiased probe of star-forming activity, this lack of luminous and/or reddened objects among the GRB host sample might reveal that the detection of GRB optical afterglows is likely biased toward unobscured galaxies. It would moreover support the idea that a large fraction of the optically-dark GRBs occur within dust-enshrouded regions of star formation. On the other hand, our result might also simply reflect intrinsic properties of GRB host galaxies experiencing a first episode of very massive star formation and characterized by a rather weak underlying stellar population. Finally, we compute the absolute B magnitudes for the whole sample of GRB host galaxies with known redshifts and detected at optical wavelengths. We find that the latter appear statistically even less luminous than the sub-luminous blue sources which mostly contributed to the B-band light emitted at high redshift. This indicates that the formation of GRBs could be favoured in particular systems with very low luminosities and, therefore, low metallicities. (Abridged)

Optical Spectropolarimetry of the GRB 020813 Afterglow

The optical afterglow of gamma-ray burst 020813 was observed for 3 hours with the LRIS spectropolarimeter at the Keck-I telescope, beginning 4.7 hours after the burst was detected by HETE-2. The spectrum reveals numerous metal absorption lines that we identify with two systems at z=1.223 and z=1.255. We also detect an O II 3727 emission line at z=1.255 and we identify this galaxy as the likely host of the GRB. After a correction for Galactic interstellar polarization, the optical afterglow has a linear polarization of 1.8-2.4% during 4.7-7.9 hours after the burst. A measurement of p = 0.80% +/- 0.16% on the following night by Covino et al. demonstrates significant polarization variability over the next 14 hours. The lack of strong variability in the position angle of linear polarization indicates that the magnetic field in the jet is likely to be globally ordered rather than composed of a number of randomly oriented cells. Within the framework of afterglow models with collimated flows, the relatively low observed polarization suggests that the magnetic field components perpendicular and parallel to the shock front are only different by about 20%.

The Optical Afterglow of the Gamma-Ray Burst GRB 011211

The optical afterglows of gamma-ray bursts can be used to probe the physics, geometry, and environments of gamma-ray bursts. In this article I discuss the how spectra and photometry can be used to constrain fireball parameters, describe several types of breaks that might be observed in the optical decay, and briefly review the late-time bumps and rapid variations in optical light curves.

Hydrodynamics of Relativistic Blast Waves in a Density-Jump Medium and Their Emission Signature

We analyze in detail the hydrodynamics and afterglow emission of an ultrarelativistic blast wave when it expands in a density-jump medium. Such a medium is likely to appear in the vicinity of gamma-ray bursts (GRBs) associated with massive stars. The interaction of the blast wave with this medium is described through a reverse shock and a forward shock. We show that the reverse shock is initially relativistic if the factor of a density jump (α) is much larger than 21 and Newtonian if 1 < α ≪ 21. We also calculate light curves of the afterglow emission during the interaction if the reverse shock is relativistic and find that the optical flux density initially decays abruptly, then rises rapidly, and finally fades based on a power law, which could be followed by an abrupt decay when the reverse shock has just crossed the originally swept-up matter. Therefore, one property of an afterglow occurring in a large density-jump medium is an abrupt drop followed by a bump in the light curve and thus provides a probe of circumburst environments. In addition, this property may not only account for the optical afterglows of GRB 970508 and GRB 000301C but may also explain the X-ray afterglow of GRB 981226.