arXiv subjects

W. T. Vestrand

Publications and source records attributed to W. T. Vestrand.

At least 19 recordsLinked to original sources

arXiv2022

The Synchrotron Low-Energy Spectrum Arising from the Cooling of Electrons in Gamma-Ray Bursts

This work is a continuation of a previous effort (Panaitescu 2019) to study the cooling of relativistic electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses, with application to the spectra and light-curves of the synchrotron Gamma-Ray Burst produced by such cooling electrons. Here, we derive the low-energy slope b_LE of GRB pulse-integrated spectrum and quantify the implications of the measured distribution of b_LE. If the magnetic field lives longer than it takes the cooling GRB electrons to radiate below 1-10 keV, then radiative cooling processes of power P(gamma) ~ gamma^n with n geq 2, i.e. synchrotron and inverse-Compton (iC) through Thomson scatterings, lead to a soft low-energy spectral slope b_LE leq -1/2 of the GRB pulse-integrated spectrum F_eps ~ eps^{b_LE} below the peak-energy E_p, irrespective of the duration of electron injection t_i. IC-cooling dominated by scatterings at the Thomson--Klein-Nishina transition of synchrotron photons below E_p has an index n = 2/3 -> 1 and yield harder integrated spectra with b_LE in [0,1/6], while adiabatic electron-cooling leads to a soft slope b_LE = -3/4. Radiative processes that produce soft integrated spectra can accommodate the harder slopes measured by CGRO/BATSE and Fermi/GBM only if the magnetic field life-time t_B is shorter than the time during which the typical GRB electrons cool to radiate below 1-10 keV, which is less than (at most) ten radiative cooling timescales t_rad of the typical GRB electron. In this case, there is a one-to-one correspondence between t_B and b_LE. To account for low-energy slopes b_LE > -3/4, adiabatic electron-cooling requires a similar restriction on t_B. In this case, the diversity of slopes arises mostly from how the electron-injection rate varies with time and not from the magnetic field timescale.

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arXiv2022

Properties of the Prompt Optical Counterpart Arising from the Cooling of Electrons in Gamma-Ray Bursts

This work extends a contemporaneous effort (Panaitescu & Vestrand 2022) to study the properties of the lower-energy counterpart synchrotron emission produced by the cooling of relativistic Gamma-Ray Burst (GRB) electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses. We derive the major characteristics (pulse duration, lag-time after burst, brightness relative to the burst) of the Prompt Optical Counterpart (POC) accompanying GRBs. Depending on the magnetic field life-time, duration of electron injection, and electron transit-time Dto from hard X-ray (GRB) to optical emitting energies, a (true) POC may appear during the GRB pulse (of duration dtg) or after (delayed OC). The signature of counterparts arising from the cooling of GRB electrons is that true POC pulses (Dto < dtg) last as long as the corresponding GRB pulse (dto ~ dtg) while delayed OC pulses (Dto > dtg) last as long as the transit-time (dto ~ Dto). If OC variability can be measured, then another signature for this OC mechanism is that the GRB variability is "passed" only to POCs but is lost for delayed OCs. Within the GRB electron cooling model for counterparts, POCs should be on average dimmer than delayed one (which is found to be consistent with the data), and harder GRB low-energy slopes bLE should be associated more often with the dimmer POCs The range of low-energy slopes bLE in [-1/2,1/3] produced by electron cooling and the average burst brightness of 1 mJy (with 1 dex dispersion) imply that POCs of hard GRBs can be dimmer than R=20 and difficult to detect by robotic telescopes (unless there is another mechanism that overshines the emission from cooling electrons) and that the POCs of soft GRBs can be brighter than R=10, i.e. as bright as the Optical Flashes (OFs) seen for several bursts.

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arXiv2014

"Self-absorbed" GeV light-curves of Gamma-Ray Burst afterglows

We investigate the effect that the absorption of high-energy (above 100 MeV) photons produced in GRB afterglow shocks has on the light-curves and spectra of Fermi-LAT afterglows. Afterglows produced by the interaction of a relativistic outflow with a wind-like medium peak when the blast-wave deceleration sets in, and the afterglow spectrum could be hardening before that peak, as the optical thickness to pair-formation is decreasing. In contrast, in afterglows produced in the interaction with a homogeneous medium, the optical thickness to pair-formation should increase and yield a light-curve peak when it reaches unity, followed by a fast light-curve decay, accompanied by a spectral softening. If energy is injected in the blast-wave, then the accelerated increase of the optical thickness yields a convex afterglow light-curve. Other features, such as a double-peak light-curve or a broad hump, can arise from the evolution of the optical thickness to photon-photon absorption. Fast decays and convex light-curves are seen in a few LAT afterglows, but the expected spectral softening is rarely seen in (and difficult to measure with) LAT observations. Furthermore, for the effects of photon-photon attenuation to shape the high-energy afterglow light-curve without attenuating it too much, the ejecta initial Lorentz factor must be in a relatively narrow range (50-200), which reduces the chance of observing those effects.

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arXiv2013

An external-shock model for GRB afterglow 130427A

The complex multiwavelength emission of GRB afterglow 130427A (monitored in the radio up to 10 days, in the optical and X-ray until 50 days, and at GeV energies until 1 day) can be accounted for by a hybrid reverse-forward shock synchrotron model, with inverse-Compton emerging only above a few GeV. The high ratio of the early optical to late radio flux requires that the ambient medium is a wind and that the forward-shock synchrotron spectrum peaks in the optical at about 10 ks. The latter has two consequences: the wind must be very tenuous and the optical emission before 10 ks must arise from the reverse-shock, as suggested also by the bright optical flash that Raptor has monitored during the prompt emission phase (<100 s). The VLA radio emission is from the reverse-shock, the Swift X-ray emission is mostly from the forward-shock, but the both shocks give comparable contributions to the Fermi GeV emission. The weak wind implies a large blast-wave radius (8 t_{day}^{1/2} pc), which requires a very tenuous circumstellar medium, suggesting that the massive stellar progenitor of GRB 130427A resided in a super-bubble.

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arXiv2013

The Bright Optical flash and Afterglow from the Gamma-Ray Burst GRB 130427A

The optical light that is generated simultaneously with the x-rays and gamma-rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse to form black holes. We report on the bright optical flash and fading afterglow from the powerful burst GRB 130427A and present a comparison with the properties of the gamma-ray emission that show correlation of the optical and >100 MeV photon flux light curves during the first 7,000 seconds. We attribute this correlation to co-generation in an external shock. The simultaneous, multi-color, optical observations are best explained at early times by reverse shock emission generated in the relativistic burst ejecta as it collides with surrounding material and at late times by a forward shock traversing the circumburst environment. The link between optical afterglow and >100 MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying GRB emission at GeV/TeV energies.

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arXiv2013

Peaks of optical and X-ray afterglow light-curves

The peaks of 30 optical afterglows and 14 X-ray light-curves display a good anticorrelation of the peak flux with the peak epoch: F_p ~ t_p^{-2.0} in the optical, F_p ~ t_p^{-1.6} in the X-ray, the distributions of the peak epochs being consistent with each other. We investigate the ability of two forward-shock models for afterglow light-curve peaks -- an observer location outside the initial jet aperture and the onset of the forward-shock deceleration -- to account for those peak correlations. For both models, the slope of the F_p - t_p relation depends only on the slope of the afterglow spectrum. We find that only a conical jet seen off-aperture and interacting with a wind-like medium can account for both the X-ray peak relation, given the average X-ray spectral slope beta_x = 1.0, and for the larger slope of the optical peak relation. However, any conclusion about the origin of the peak flux - peak epoch correlation is, at best, tentative, because the current sample of X-ray peaks is too small to allow a reliable measurement of the F_p - t_p relation slope and because more than one mechanism and/or one afterglow parameter may be driving that correlation.

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arXiv2012

The possible ubiquity of energy injection in Gamma-Ray Burst afterglows

Since its launch in 2004, the Swift satellite has monitored the X-ray afterglows of several hundred Gamma-Ray Bursts, and revealed that their X-ray light-curves are more complex than previously thought, exhibiting up to three power-law segments. Energy injection into the relativistic blast-wave energizing the burst ambient medium has been proposed most often to be the reason for the X-ray afterglow complexity. We examine 117 light-curve breaks of 98 Swift X-ray afterglows, selected for their high-quality monitoring and well-constrained flux decay rates. Thirty percent of afterglows have a break that can be an adiabatic jet-break, in the sense that there is one variant of the forward-shock emission from a collimated outflow model that can account for both the pre- and post-break flux power-law decay indices, given the measured X-ray spectral slope. If allowance is made for a steady energy injection into the forward-shock, then another 56 percent of X-ray afterglows have a light-curve break that can be explained with a jet-break. The remaining 12 percent that are not jet-breaks, as well as the existence of two breaks in 19 afterglows (out of which only one can be a jet-break), suggest that some X-ray breaks arise from a sudden change in the rate at which energy is added to the blast-wave, and it may well be that a larger fraction of X-ray light-curve breaks are generated by that mechanism. To test the above two mechanisms for afterglow light-curve breaks, we derive comprehensive analytical results for the dynamics of outflows undergoing energy injection and for their light-curves, including closure relations for inverse-Compton afterglows and for the emission from spreading jets interacting with an wind-like ambient medium.

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arXiv2012

Time-dependent excitation and ionization modelling of absorption-line variability due to GRB 080310

We model the time-variable absorption of FeII, FeIII, SiII, CII and CrII detected in UVES spectra of GRB 080310, with the afterglow radiation exciting and ionizing the interstellar medium in the host galaxy at a redshift of z=2.42743. To estimate the rest-frame afterglow brightness as a function of time, we use a combination of the optical VRI photometry obtained by the RAPTOR-T telescope array -- which are presented in this paper -- and Swift's X-Ray Telescope observations. Excitation alone, which has been successfully applied for a handful of other GRBs, fails to describe the observed column-density evolution in the case of GRB 080310. Inclusion of ionization is required to explain the column-density decrease of all observed FeII levels (including the ground state 6D9/2) and increase of the FeIII 7S3 level. The large population of ions in this latter level (up to 10% of all FeIII) can only be explained through ionization of FeII, whereby a large fraction of the ionized FeII ions -- we calculate 31% using the Flexible Atomic (FAC) and Cowan codes -- initially populate the 7S3 level of FeIII rather than the ground state. This channel for producing a significant FeIII 7S3 level population may be relevant for other objects in which absorption lines from this level -- the UV34 triplet -- are observed, such as BAL quasars and Eta Carinae. This provides conclusive evidence for time-variable ionization in the circumburst medium, which to date has not been convincingly detected. However, the best-fit distance of the neutral absorbing cloud to the GRB is 200--400 pc, i.e. similar to GRB-absorber distance estimates for GRBs without any evidence for ionization. We find that the presence of time-varying ionization in GRB 080310 is likely due to a combination of the super-solar iron abundance ([Fe/H]=+0.2) and the low HI column density (log N(HI)=18.7). [abridged]

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arXiv2011

Optical afterglows of Gamma-Ray Bursts: peaks, plateaus, and possibilities

The optical light-curves of GRB afterglows display either peaks or plateaus. We identify 16 afterglows of the former type, 17 of the latter, and 4 with broad peaks, that could be of either type. The optical energy release of these two classes is similar and is correlated with the GRB output, the correlation being stronger for peaky afterglows, which suggests that the burst and afterglow emissions of peaky afterglows are from the same relativistic ejecta and that the optical emission of afterglows with plateaus arises more often from ejecta that did not produce the burst emission. Consequently, we propose that peaky optical afterglows are from impulsive ejecta releases and that plateau optical afterglows originate from long-lived engines, the break in the optical light-curve (peak or plateau end) marking the onset of the entire outflow deceleration. In the peak luminosity--peak time plane, the distribution of peaky afterglows displays an edge with L_p \propto t_p^{-3}, which we attribute to variations (among afterglows) in the ambient medium density. The fluxes and epochs of optical plateau breaks follow a L_b \propto t_b^{-1} anticorrelation. Sixty percent of 25 afterglows that were well-monitored in the optical and X-rays show light-curves with comparable power-law decays indices and achromatic breaks. The other 40 percent display three types of decoupled behaviours: i) chromatic optical light-curve breaks (perhaps due to the peak of the synchrotron spectrum crossing the optical), ii) X-ray flux decays faster than in the optical (suggesting that the X-ray emission is from local inverse-Compton scattering), and iii) chromatic X-ray light-curve breaks (indicating that the X-ray emission is from external up-scattering).

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arXiv2010

SDWFS-MT-1: A Self-Obscured Luminous Supernova at z~0.2

We report the discovery of a six-month-long mid-infrared transient, SDWFS-MT-1 (aka SN 2007va), in the Spitzer Deep, Wide-Field Survey of the NOAO Deep Wide-Field Survey Bootes field. The transient, located in a z=0.19 low luminosity (M_[4.5]~-18.6 mag, L/L_MilkyWay~0.01) metal-poor (12+log(O/H)~7.8) irregular galaxy, peaked at a mid-infrared absolute magnitude of M_[4.5]~-24.2 in the 4.5 micron Spitzer/IRAC band and emitted a total energy of at least 10^51 ergs. The optical emission was likely fainter than the mid-infrared, although our constraints on the optical emission are poor because the transient peaked when the source was "behind" the Sun. The Spitzer data are consistent with emission by a modified black body with a temperature of ~1350 K. We rule out a number of scenarios for the origin of the transient such as a Galactic star, AGN activity, GRB, tidal disruption of a star by a black hole and gravitational lensing. The most plausible scenario is a supernova exploding inside a massive, optically thick circumstellar medium, composed of multiple shells of previously ejected material. If the proposed scenario is correct, then a significant fraction (~10%) of the most luminous supernova may be self-enshrouded by dust not only before but also after the supernova occurs. The spectral energy distribution of the progenitor of such a supernova would be a slightly cooler version of eta Carina, peaking at 20-30 microns.

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arXiv2010

GRB 081008: from burst to afterglow and the transition phase in between

We present a multi-wavelength study of GRB 081008, at redshift 1.967, by Swift, ROTSE-III and GROND. Compared to other Swift GRBs, GRB 081008 has a typical gamma-ray isotropic equivalent energy output (10^53 erg) during the prompt phase, and displayed two temporally separated clusters of pulses. The early X-ray emission seen by the Swift/XRT was dominated by the softening tail of the prompt emission, producing multiple flares during and after the Swift/BAT detections. Optical observations that started shortly after the first active phase of gamma-ray emission showed two consecutive peaks. We interpret the first optical peak as the onset of the afterglow associated with the early burst activities. A second optical peak, coincident with the later gamma-ray pulses, imposes a small modification to the otherwise smooth lightcurve and thus suggests a minimal contribution from a probable internal component. We suggest the early optical variability may be from continuous energy injection into the forward shock front by later shells producing the second epoch of burst activities. These early observations thus provide a potential probe for the transition from prompt to the afterglow phase. The later lightcurve of GRB 081008 displays a smooth steepening in all optical bands and X-ray. The temporal break is consistent with being achromatic at the observed wavelengths. Our broad energy coverage shortly after the break constrains a spectral break within optical. However, the evolution of the break frequency is not observed. We discuss the plausible interpretations of this behavior.

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arXiv2010

Evidence for Supernova-Synthesised Dust from the Rising Afterglow of GRB 071025 at z~5

We present observations and analysis of the broadband afterglow of Swift GRB 071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour observations begin at ~30 s after the GRB trigger in the host frame and during the initial rising phase of the afterglow. We associate the light curve peak at 580 s in the observer frame with the formation of the forward shock, giving an estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy distribution (even in regions not affected by the Lyman-alpha break) provides secure evidence of a large dust column. However, the inferred extinction curve shows a prominent flat component between 2000-3000 Angstroms in the rest-frame, inconsistent with any locally observed template but well-fit by models of dust formed by supernovae. Time-dependent fits to the extinction profile reveal no evidence of dust destruction and limit the decrease in the extinction column to Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations provide evidence of a transition in dust properties at z~5, in agreement with studies of high-z quasars, and suggest that SN-formed dust continues to dominate the opacity of typical galaxies at this redshift.

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arXiv2009

Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows

We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the ROTSE-III telescope network from March 2005 through June 2007. This set is comprised of 12 afterglows with early optical and Swift/XRT observations, with a median ROTSE-III response time of 45 s after the start of gamma-ray emission (8 s after the GCN notice time). These afterglows span four 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<Gamma_0<1000, consistent with expectations.

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arXiv2008

Gamma-Ray Burst at the extreme: "the naked-eye burst" GRB 080319B

On 19 March 2008, the northern sky was the stage of a spectacular optical transient that for a few seconds remained visible to the naked eye. The transient was associated with GRB 080319B, a gamma-ray burst at a luminosity distance of about 6 Gpc (standard cosmology), making it the most luminous optical object ever recorded by human kind. We present comprehensive sky monitoring and multi-color optical follow-up observations of GRB 080319B collected by the RAPTOR telescope network covering the development of the explosion and the afterglow before, during, and after the burst. The extremely bright prompt optical emission revealed features that are normally not detectable. The optical and gamma-ray variability during the explosion are correlated, but the optical flux is much greater than can be reconciled with single emission mechanism and a flat gamma-ray spectrum. This extreme optical behavior is best understood as synchrotron self-Compton model (SSC). After a gradual onset of the gamma-ray emission, there is an abrupt rise of the prompt optical flux suggesting that variable self-absorption dominates the early optical light curve. Our simultaneous multi-color optical light curves following the flash show spectral evolution consistent with a rapidly decaying red component due to large angle emission and the emergence of a blue forward shock component from interaction with the surrounding environment. While providing little support for the reverse shock that dominates the early afterglow, these observations strengthen the case for the universal role of the SSC mechanism in generating gamma-ray bursts.

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arXiv2008

Taxonomy of GRB optical light-curves: identification of a salient class of early afterglows

The temporal behaviour of the early optical emission from Gamma-Ray Burst afterglows can be divided in four classes: fast-rising with an early peak, slow-rising with a late peak, flat plateaus, and rapid decays since first measurement. The fast-rising optical afterglows display correlations among peak flux, peak epoch, and post-peak power-law decay index that can be explained with a structured outflow seen off-axis, but the shock origin (reverse or forward) of the optical emission cannot be determined. The afterglows with plateaus and slow-rises may be accommodated by the same model, if observer location offsets are larger than for the fast-rising afterglows, or could be due to a long-lived injection of energy and/or ejecta in the blast-wave. If better calibrated with more afterglows, the peak flux-peak epoch relation exhibited by the fast and slow-rising optical light-curves could provide a way to use this type of afterglows as standard candles.

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arXiv2007

The Dark Side of ROTSE-III Prompt GRB Observations

We present several cases of optical observations during gamma-ray bursts (GRBs) which resulted in prompt limits but no detection of optical emission. These limits constrain the prompt optical flux densities and the optical brightness relative to the gamma-ray emission. The derived constraints fall within the range of properties observed in GRBs with prompt optical detections, though at the faint end of optical/gamma flux ratios. The presently accessible prompt optical limits do not require a different set of intrinsic or environmental GRB properties, relative to the events with prompt optical detections.

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arXiv2007

Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages

We report on follow-up observations of the GRB 060927 using the ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at lambda ~ 8070 A produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A which we interpret as SiII at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a column density with log(N_HI/cm^-2) ~ 22.5. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from z>6 should be relatively easy to detect from the ground, but rapid NIR monitoring is necessary to ensure that they are found; ii) The presence of large HI column densities in some GRBs host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at z>5 and therefore the type of galaxies responsible for the reionization of the universe.

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arXiv2007

The Troublesome Broadband Evolution of GRB 061126: Does a Grey Burst Imply Grey Dust?

We report on observations of a gamma-ray burst (GRB 061126) with an extremely bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow is already fading as a power law 22 seconds after the trigger, with no detectable prompt contribution in our first exposure, which was coincident with a large prompt-emission gamma-ray pulse. The optical--infrared photometric spectral energy distribution is an excellent fit to a power law, but it exhibits a moderate red-to-blue evolution in the spectral index at about 500 s after the burst. This color change is contemporaneous with a switch from a relatively fast decay to slower decay. The rapidly decaying early afterglow is broadly consistent with synchrotron emission from a reverse shock, but a bright forward-shock component predicted by the intermediate- to late-time X-ray observations under the assumptions of standard afterglow models is not observed. Indeed, despite its remarkable early-time brightness, this burst would qualify as a dark burst at later times on the basis of its nearly flat optical-to-X-ray spectral index. Our photometric spectral energy distribution provides no evidence of host-galaxy extinction, requiring either large quantities of grey dust in the host system (at redshift 1.1588 +/- 0.0006, based upon our late-time Keck spectroscopy) or separate physical origins for the X-ray and optical afterglows.

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