Afterglow Spectra Research Articles

Comparison study of the luminescent properties of the white-light long afterglow phosphors: Ca xMgSi 2O 5+x:Dy 3+( x=1, 2, 3)

A series of Dy 3+-doped calcium magnesium silicate phosphors: CaMgSi 2O 6:Dy 3+, Ca 2MgSi 2O 7:Dy 3+, and Ca 3MgSi 2O 8:Dy 3+ with white long-lasting afterglow were prepared and investigated. The characteristic intra-configurational 4f emissions of Dy 3+ were observed in the emission spectra as well as the afterglow spectra under ultraviolet excitation. The combination of the 480 nm blue emission corresponding to the 4F 9/2→ 6H 15/2 transition and the 575 nm yellow emission corresponding to the 4F 9/2→ 6H 13/2 transition yielded white-light emission. The white-coloured afterglow emission can last more than 1 h for most of the samples under study. The concentration dependence of the ratio of the yellow emission intensity with blue emission intensity was also examined and found to be varied for the different hosts. The thermoluminescence spectra above room temperature are employed for the discussion of the origin of the traps and the mechanism of the persistent luminescence.

PROBING COSMIC DUST OF THE EARLY UNIVERSE THROUGH HIGH-REDSHIFT GAMMA-RAY BURSTS

We explore the extinction properties of the dust in the distant universe through the afterglows of high-redshifted GRBs based on the Drude model which, unlike previous studies, does not require a prior assumption of template extinction laws. We select GRB070802 at z~2.45 (which shows clear evidence for the 2175\AA extinction bump) and GRB050904 at z~6.29, the 2nd most distant GRB observed to date. We fit their afterglow spectra to determine the extinction of their host galaxies. We find that (1) their extinction curves differ substantially from that of the Milky Way, the Small and Large Magellanic Clouds (which were widely adopted as template extinction laws in literature); (2) the 2175\AA extinction feature appears to be also present in GRB050904 at z~6.29; and (3) there does not appear to show strong evidence for a dependence of dust extinction on redshifts. The inferred extinction curves are closely reproduced in terms of a mixture of amorphous silicate and graphite, both of which are expected supernova condensates and have been identified in primitive meteorites as presolar grains originating from supernovae (which are considered as the main source of dust at high-z).

GRB 071003: Broadband Follow‐up Observations of a Very Bright Gamma‐Ray Burst in a Galactic Halo

The optical afterglow of long-duration GRB 071003 is among the brightest yet to be detected from any GRB, with R ≈ 12 mag in KAIT observations starting 42 s after the GRB trigger, including filtered detections during prompt emission. However, our high-S/N afterglow spectrum displays only extremely weak absorption lines at what we argue is the host redshift of z = 1.60435, in contrast to the three other, much stronger Mg II absorption systems observed at lower redshifts. Together with Keck adaptive optics observations, which fail to reveal a host galaxy coincident with the burst position, our observations suggest a halo progenitor and offer a cautionary tale about the use of Mg II for GRB redshift determination. We present early- through late-time observations spanning the electromagnetic spectrum, constrain the connection between the prompt emission and early variations in the light curve (we observe no correlation), and discuss possible origins for an unusual, marked rebrightening that occurs a few hours after the burst: likely either a late-time refreshed shock or a wide-angle secondary jet. Analysis of the late-time afterglow is most consistent with a wind environment, suggesting a massive star progenitor. Together with GRB 070125, this may indicate that a small but significant portion of star formation in the early universe occurred far outside what we consider a normal galactic disk.

Probing the complex environments of GRB host galaxies and intervening systems: high resolution spectroscopy of GRB050922C

Aims. The aim of this paper is to investigate the environment of gamma ray bursts (GRBs) and the interstellar matter of their host galaxies. Methods. We use to this purpose high resolution spectroscopic observations of the afterglow of GRB050922C, obtained with UVES/VLT∼ 3.5 hours after the GRB event. Results. We found that, as for most high resolution spectra of GRBs, the spectrum of the afterglow of GRB050922C is complex. At least seven components contribute to the main absorption system at z = 2.1992. The detection of lines of neutral elements like MgI and the detection of fine-structure levels of the ions FeII, S iII and CII allows us to separate components in the GRB ISM along the line of sight. Moreover, in addition to the main system, we have analyzed the five intervening systems between z = 2.077 and z = 1.5664 identified along the GRB line of sight. Conclusions. GRB afterglow spectra are very complex, but full of information. This can be used to disentangle the contribution of the different parts of the GRB host galaxy and to study their properties. Our metallicity estimates agree with the scenario of GRBs exploding in low metallicity galaxies

A Survey for NvAbsorption atz ≈ zGRBin GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star

We survey N V absorption in the afterglow spectra of long-duration gamma-ray bursts (GRBs) with the intent to study highly ionized gas in the galaxies hosting these events. We identify a high incidence (6/7) of spectra exhibiting N V gas with z ≈ zGRB, and the majority show large column densities N(N+ 4) 1014 cm −2. With one exception, the observed line profiles are kinematically cold; i.e., they are narrow and have small velocity offset (δ v 20 km s−1) from absorption lines associated with neutral gas. In addition, the N V absorption has similar velocity to that of the UV-pumped fine-structure lines, indicating that these high ions are located within ≈1 kpc of the GRB afterglow. These characteristics are unlike those for N V gas detected in the halo/disk of the Milky Way or along sight lines through high-z damped Lyα systems but resemble the narrow absorption line systems associated with quasars and some high-z starbursts. We demonstrate that GRB afterglows photoionize nitrogen to N+4 at r ≈ 10 pc. This process can produce N V absorption with characteristics resembling the majority of our sample, and we argue that it is the principal mechanism for N+4 along GRB sight lines. Therefore, the observations provide a snapshot of the physical conditions at this distance. In this scenario, the observations imply that the progenitor's stellar wind is confined to r 103 cm −3) environments, typical of molecular clouds. The observations, therefore, primarily constrain the physical conditions—metallicity, density, velocity fields—of the gas within the (former) molecular cloud region surrounding the GRB.

Robust photometric redshift determinations of gamma-ray burst afterglows at z ≳ 2

Theory suggests that about 10% of Swift-detected gamma-ray bursts (GRBs) will originate at redshifts greater than 5 yet a number of high redshift candidates may be left unconfirmed due to the lack of measured redshifts. Here we introduce our code, GRBz, a method of simultaneous multi-parameter fitting of GRB afterglow optical and near infrared, spectral energy distributions. It allows for early determinations of the photometric redshift, spectral index and host extinction to be made. We assume that GRB afterglow spectra are well represented by a power-law decay and model the effects of absorption due to the Lyman forest and host extinction. We use a genetic algorithm-based routine to simultaneously fit the parameters of interest, and a Monte Carlo error analysis. We use GRBs of previously determined spectroscopic redshifts to prove our method, while also introducing new near infrared data of GRB 990510 which further constrains the value of the host extinction. Our method is effective in estimating the photometric redshift of GRBs, relatively unbiased by assumptions of the afterglow spectral index or the host galaxy extinction. Monte Carlo error analysis is required as the method of error estimate based on the optimum population of the genetic algorithm underestimates errors significantly.

Shock Vorticity Generation from Accelerated Ion Streaming in the Precursor of Ultrarelativistic Gamma‐Ray Burst External Shocks

We investigate the interaction of nonthermal ions (protons and nuclei) accelerated in an ultrarelativistic blastwave with the pre-existing magnetic field of the medium into which the blastwave propagates. While particle acceleration processes such as diffusive shock acceleration can accelerate ions and electrons, the accelerated electrons suffer larger radiative losses. Under certain conditions, the ions can attain higher energies and reach farther ahead of the shock than the electrons, and so the nonthermal particles can be partially charge-separated. To compensate for the charge separation, the upstream plasma develops a return current, which, as it flows across the magnetic field, drives transverse acceleration of the upstream plasma and a growth of density contrast in the shock upstream. If the density contrast is strong by the time the fluid is shocked, vorticity is generated at the shock transition. The resulting turbulence can amplify the post-shock magnetic field to the levels inferred from gamma-ray burst afterglow spectra and light curves. Therefore, since the upstream inhomogeneities are induced by the ions accelerated in the shock, they are generic even if the blastwave propagates into a medium of uniform density. We speculate about the global structure of the shock precursor, and delineate several distinct physical regimes that are classified by an increasing distance from the shock and, correspondingly, a decreasing density of nonthermal particles that reach that distance.

The host of GRB 060206: kinematics of a distant galaxy

Context. GRB afterglow spectra are sensitive probes of interstellar matter along the line-of-sight in their host galaxies, as well as in intervening galaxies. The rapid fading of GRBs makes it very difficult to obtain spectra of sufficient resolution and S/ Nt o allow for these kinds of studies. Aims. We investigate the state and properties of the interstellar medium in the host of GRB 060206 at z = 4.048 with a detailed study of groundstate and finestructure absorption lines in an early afterglow spectrum. This allows us to derive conclusions on the nature and origin of the absorbing structures and their connection to the host galaxy and/or the GRB. Methods. We used early (starting 1.6 h after the burst) WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 detecting a range of metal absorption lines and their finestructure transitions. Additional information is provided by the afterglow lightcurve. The resolution and wavelength range of the spectra and the bright afterglow have facilitated a detailed study and fitting of the absorption line systems in order to derive column densities. We also used deep imaging to detect the host galaxy and probe the nature of an intervening system at z = 1.48 seen in absorption in the afterglow spectra. Results. We detect four discrete velocity systems in the resonant metal absorption lines, best explained by shells within and/or around the host created by starburst winds. The finestructure lines have no less than three components with strengths decreasing from the redmost components. We therefore suggest that the finestructure lines are best explained as being produced by UV pumping from which follows that the redmost component is the one closest to the burst where N v was detected as well. The host is detected in deep HST imaging with F814WAB = 27.48 ± 0.19 mag and a 3σ upper limit of H = 20.6 mag (Vega) is achieved. A candidate counterpart for the intervening absorption system is detected as well, which is quite exceptional for an absorber in the sightline towards a GRB afterglow. The intervening system shows no temporal evolution as claimed by Hao et al. (2007, ApJ, 659, 99), which we prove from our WHT spectra taken before and Subaru spectra taken during those observations.

Probing the neutral fraction of the IGM with GRBs during the epoch of reionization

We show that near-infrared observations of the red side of the Ly-alpha line from a single gamma ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's redshift to better than ~0.3. Some GRB sight-lines will encounter more neutral hydrogen than others at fixed x_H owing to the patchiness of reionisation. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on x_H from the z = 6.3 burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Ly-alpha absorbers is the same as measured at lower redshifts, a GRB from an epoch when x_H = 0.5 can be used to detect a partly neutral IGM at 98% confidence level 10% of the time (and, for an observation with three times the sensitivity, 30% of the time).

Low-Mass and Metal-Poor Gamma-Ray Burst Host Galaxies

AbstractGamma-ray bursts (GRBs) are cosmologically distributed, very energetic and very transient sources detected in the γ-ray domain. The identification of their x-ray and optical afterglows allowed so far the redshift measurement of 150 events, from z = 0.01 to z = 6.29. For about half of them, we have some knowledge of the properties of the parent galaxy. At high redshift (z > 2), absorption lines in the afterglow spectra give information on the cold interstellar medium in the host. At low redshift (z < 1.0) multi-band optical-NIR photometry and integrated spectroscopy reveal the GRB host general properties. A redshift evolution of metallicity is not noticeable in the whole sample. The typical value is a few times lower than solar. The mean host stellar mass is similar to that of the Large Magellanic Cloud, but the mean star formation rate is five times higher. GRBs are discovered with γ-ray, not optical or NIR, instruments. Their hosts do not suffer from the same selection biases of typical galaxy surveys. Therefore, they might represent a fair sample of the most common galaxies that existed in the past history of the universe, and can be used to better understand galaxy formation and evolution.

The Luminosity Function of Long Gamma-Ray Burst and their rate at z ≥ 6

AbstractWe compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low–metallicity environments. We then test model predictions against the Swift 3-year data, showing that scenario i) is robustly ruled out. Moreover, we show that the number of bright GRBs detected by Swift suggests that GRBs should have experienced some sort of luminosity evolution with redshift, being more luminous in the past. Finally we propose to use the observations of the afterglow spectrum of GRBs at z ≥ 5.5 to constrain the reionization history, and then applied our method to the case of GRB 050904.

Line Searches in Swift X-Ray Spectra

Prior to the launch of the Swift mission several X-ray line detections were reported in Gamma Ray Burst afterglow spectra. To date, these pre-Swift era results have not been conclusively confirmed. The most contentious issue in this area is the choice of statistical method used to evaluate the significance of these features. In this paper we compare three different methods already extant in the literature for assessing the significance of possible line features and discuss their relative advantages and disadvantages. The methods are demonstrated by application to observations of 40 bursts from the archive of Swift XRT at early times (< few ks post trigger in the rest frame of the burst). Based on this thorough analysis we found no strong evidence for emission lines. For each of the three methods we have determined detection limits for emission line strengths in bursts with spectral parameters typical of the Swift-era sample. We also discuss the effects of the current calibration status on emission line detection.

The Molecular Hydrogen Deficit in Gamma‐Ray Burst Afterglows

Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveals the absence of molecular hydrogen absorption lines, a surprising result in light of their large neutral hydrogen column densities and the detection of H2 in similar, more local star-forming regions such as 30 Doradus in the Large Magellanic Cloud (LMC). Observational evidence further indicates that the bulk of the neutral hydrogen column in these sight lines lies more than 100 pc from the GRB progenitor and that H2 was likely absent prior to the burst, suggesting that direct flux from the star, FUV background fields, or both suppressed its formation. We present one-dimensional radiation hydrodynamical models of GRB host galaxy environments, including self-consistent radiative transfer of both ionizing and Lyman-Werner (L-W) photons, nine-species primordial chemistry with dust formation of H2, and dust extinction of UV photons. We find that a single GRB progenitor is sufficient to ionize neutral hydrogen to distances of 50-100 pc, but that a galactic L-W background is required to dissociate molecular hydrogen in the ambient ISM. Intensities of 0.1-100 times the Galactic mean are necessary to destroy H2 in the cloud, depending on its density and metallicity. The minimum radii at which neutral hydrogen will be found in afterglow spectra is insensitive to the mass of the progenitor or the initial mass function (IMF) of its cluster, if present.

Spectral Evolution of GRB060904A Observed with Swift and Suzaku- Possibility of Inefficient Electron Acceleration

We observed an X-ray afterglow of GRB060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the Swift/BAT and Swift/XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma =$ 1.51$^{+0.04}_{-0.03}$ to $\Gamma =$ 5.30$^{+0.69}_{-0.59}$ within a few hundred seconds. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron-acceleration and synchrotron-radiation processes. We then applied an alternative broken power-law with exponential cutoff (BPEC) model. It is valid to consider the exponential shape is equivalent to a synchrotron cutoff. Since the spectral cutoff appears in the soft X-ray band, we conclude that the electron acceleration must be inefficient in the internal shocks of GRB060904A. These cutoff spectra suddenly disappeared at the end of the prompt tail. After that, typical afterglow spectra with $\Gamma =$ 2.0 have been continuously and preciously monitored by both Swift/XRT and Suzaku/XIS. We could successfully trace the temporal history of two characteristic break energies that have a time dependence of $\propto t^{-3}$-$t^{-4}$, while the following afterglow spectra are quite stable. This fact indicates that the emitting material of the prompt tail is due to completely different dynamics from the shallow decay component. Therefore, the emission sites of two distinct phenomena obviously differ from each other.

White-light long-lasting phosphor Sr 2SiO 4:Pr 3+

A white-emitting phosphor Sr 2SiO 4: Pr 3+ was synthesized through a solid-state reaction, and characterized by XRD, scanning electron microscopy (SEM), fluorescence spectrophotometer and thermo luminescence (TL) meter. Its emission spectra is composed of bluish purple (peaking at 390 nm), green (peaking at 535 nm) and red (peaking at 604 nm) light emission. They originate from the transitions of 4f → 5d, 3P 0 → 3H 5 and 1D 2 → 3H 4 of Pr 3+. The afterglow emission spectrum is similar to the emission spectra. And the afterglow can last over 40 min in darkness. The TL curve shows that there is only one thermo luminescence band peak at about 376.480 K, which is responsible for the long-lasting emission.

The circumstellar medium around a rapidly rotating, chemically homogeneously evolving, possible gamma-ray burst progenitor

Context. Rapidly rotating, chemically homogeneously evolving massive stars are considered to be progenitors of long gamma-ray bursts. Aims. We present numerical simulations of the evolution of the circumstellar medium around a rapidly rotating 20 Mstar at a metal- licity of Z = 0.001. Its rotation is fast enough to produce quasi-chemically homogeneous evolution. While conventionally, a star of 20 Mwould not evolve into a Wolf-Rayet stage, the considered model evolves from the main sequence directly to the helium main sequence. Methods. We use the time-dependent wind parameters, such as mass loss rate, wind velocity and rotation-induced wind anisotropy from the evolution model as input for a 2D hydrodynamical simulation. Results. While the outer edge of the pressure-driven circumstellar bubble is spherical, the circumstellar medium close to the star shows strong non-spherical features during and after the periods of near-critical rotation. Conclusions. We conclude that the circumstellar medium around rapidly rotating massive stars differs considerably from the sur- rounding material of non-rotating stars of similar mass. Multiple blue-shifted high velocity absorption components in gamma-ray burst afterglow spectra are predicted. As a consequence of near critical rotation and short stellar evolution time scales during the last few thousand years of the star's life, we find a strong deviation of the circumstellar density profile in the polar direction from the 1/R 2 density profile normally associated with stellar winds close to the star.

Probing the Interstellar Medium and Stellar Environments of Long-Duration GRBs

AbstractWe review the properties of the gas surrounding high-redshift gamma-ray bursts (GRBs) assessed through the analysis of damped Lyman-alpha systems (DLAs) identified in their afterglow spectra. These GRB-DLAs are characterized by large H I column densities with a median of N(H I) = 1021.7 cm−2, no molecular gas signatures, metallicities ranging from 1/100 to nearly solar with a median exceeding 1/10 solar, and no anomalous abundance patterns. The detection of the atomic Mg lines and the time-variability of the fine-structure lines demonstrates that the majority of the neutral gas along the GRB sightlines is located between 50 pc and a few kpc from the GRB. This implies that this gas is presumably associated with the ambient interstellar medium of the host galaxy and that the derived properties from low-ionization lines do not directly constrain the local environment of the GRB progenitor. The highly ionized gas, traced by N V lines, which could result from a pre-existing H II region produced by the GRB progenitor and neighboring OB stars, appears on the other hand to be very local to the GRB at about 10 pc, yielding a snapshot of the medium's physical conditions at this radius.

Intervening Metal Systems in GRB and QSO Sight Lines: The Mgiiand CivQuestion

Prochter and coworkers recently found that the number density of strong intervening 0.5 < z < 2 Mg II absorbers detected in gamma-ray burst (GRB) afterglow spectra is nearly 4 times larger than those in QSO spectra. We have conducted a similar study using C IV absorbers. Our C IV sample, consisting of a total of 19 systems, is drawn from three high-resolution and high to moderate signal-to-noise ratio VLT UVES spectra of three long-duration GRB afterglows, covering the redshift interval 1.6 < z < 3.1. The column density distribution and number density of this sample do not show any statistical difference from the same quantities measured in QSO spectra. We discuss several possibilities for the discrepancy between C IV and Mg II absorbers and conclude that a higher dust extinction in the Mg II QSO samples studied up to now would give the most straightforward solution. However, this effect is only important for the strong Mg II absorbers. Regardless of the reasons for this discrepancy, this result confirms once more that GRBs can be used to detect a side of the universe that was unknown before, not necessarily connected with GRBs themselves, providing an alternative and fundamental investigative tool of the cosmic evolution of the universe.

GLASTProspects forSwift‐Era Afterglows

We calculate the GeV spectra of GRB afterglows produced by inverse Compton scattering of the sub-MeV emission of these objects. We improve on earlier treatments by using refined afterglow parameters and new model developments motivated by recent Swift observations. We present time-dependent GeV spectra for standard, constant parameter models, as well as for models with energy injection and with time-varying parameters, for a range of burst parameters. We evaluate the limiting redshift to which such afterglows can be detected by the GLAST LAT, as well as AGILE.

A New Constraint on the Escape Fraction in Distant Galaxies Using γ-Ray Burst Afterglow Spectroscopy

We describe a new method to measure the escape fraction fesc of ionizing radiation from distant star-forming galaxies using the afterglow spectra of long-duration gamma-ray bursts (GRBs). Optical spectra of GRB afterglows allow us to evaluate the optical depth of the host ISM, according to the neutral hydrogen column density N(HI) observed along the sightlines toward the star-forming regions where the GRBs are found. Different from previous effort in searching for faint, transmitted Lyman continuum photons, our method is not subject to background subtraction uncertainties and does not require prior knowledge of either the spectral shape of the host galaxy population or the IGM Lya forest absorption along these GRB sightlines. Because most GRBs occur in sub-L_* galaxies, our study also offers the first constraint on fesc for distant low-mass galaxies that dominate the cosmic luminosity density. We have compiled a sample of 27 GRBs at redshift z>2 for which the underlying N(HI) in the host ISM are known. These GRBs together offer a statistical sampling of the integrated optical depth to ionizing photons along random sightlines from star-forming regions in the host galaxies, and allow us to estimate the mean escape fraction <fesc> averaged over different viewing angles. We find <fesc>=0.02\pm 0.02 and place a 95% c.l. upper limit <fesc> <= 0.075 for these hosts. We discuss possible biases of our approach and implications of the result. Finally, we propose to extend this technique for measuring <fesc> at z~0.2 using spectra of core-collapse supernovae.