arXiv subjects

R. L. C. Starling

Publications and source records attributed to R. L. C. Starling.

At least 73 records · Page 4Linked to original sources

arXiv2014

GRB 130925A: an ultra-long Gamma Ray Burst with a dust-echo afterglow, and implications for the origin of the ultra-long GRBs

GRB 130925A was an unusual GRB, consisting of 3 distinct episodes of high-energy emission spanning $\sim$20 ks, making it a member of the proposed category of `ultra-long' bursts. It was also unusual in that its late-time X-ray emission observed by Swift was very soft, and showed a strong hard-to-soft spectral evolution with time. This evolution, rarely seen in GRB afterglows, can be well modelled as the dust-scattered echo of the prompt emission, with stringent limits on the contribution from the normal afterglow (i.e. external shock) emission. We consider and reject the possibility that GRB 130925A was some form of tidal disruption event, and instead show that if the circumburst density around GRB 130925A is low, the long duration of the burst and faint external shock emission are naturally explained. Indeed, we suggest that the ultra-long GRBs as a class can be explained as those with low circumburst densities, such that the deceleration time (at which point the material ejected from the nascent black hole is decelerated by the circumburst medium) is $\sim$20 ks, as opposed to a few hundred seconds for the normal long GRBs. The increased deceleration radius means that more of the ejected shells can interact before reaching the external shock, naturally explaining both the increased duration of GRB 130925A, the duration of its prompt pulses, and the fainter-than-normal afterglow.

astro-ph.HE↗

arXiv2014

Diversity of GRB energetics vs. SN homogeneity: supernova 2013cq associated with the gamma-ray burst 130427A

Long-duration gamma-ray bursts (GRBs) have been found to be associated with broad-lined type-Ic supernovae (SNe), but only a handful of cases have been studied in detail. Prompted by the discovery of the exceptionally bright, nearby GRB130427A (redshift z=0.3399), we aim at characterising the properties of its associated SN2013cq. This is the first opportunity to test directly the progenitors of high-luminosity GRBs. We monitored the field of the Swift long duration GRB130427A using the 3.6-m TNG and the 8.2-m VLT during the time interval between 3.6 and 51.6 days after the burst. Photometric and spectroscopic observations revealed the presence of the type Ic SN2013cq. Spectroscopic analysis suggests that SN2013cq resembles two previous GRB-SNe, SN1998bw and SN2010bh associated with GRB980425 and XRF100316D, respectively. The bolometric light curve of SN2013cq, which is significantly affected by the host galaxy contribution, is systematically more luminous than that of SN2010bh ($\sim$ 2 mag at peak), but is consistent with SN1998bw. The comparison with the light curve model of another GRB-connected SN2003dh, indicates that SN2013cq is consistent with the model when brightened by 20%. This suggests a synthesised radioactive $^{56}$Ni mass of $\sim 0.4 M_\odot$. GRB130427A/SN2013cq is the first case of low-z GRB-SN connection where the GRB energetics are extreme ($E_{\rm γ, iso} \sim 10^{54}$ erg). We show that the maximum luminosities attained by SNe associated with GRBs span a very narrow range, but those associated with XRFs are significantly less luminous. On the other hand the isotropic energies of the accompanying GRBs span 6 orders of magnitude (10$^{48}$ erg $< E_{\rm γ, iso} <$ 10$^{54}$ erg), although this range is reduced when corrected for jet collimation. The GRB total radiated energy is in fact a small fraction of the SN energy budget.

astro-ph.HE↗

arXiv2013

Calibration of X-ray absorption in our Galaxy

Prediction of the soft X-ray absorption along lines of sight through our Galaxy is crucial for understanding the spectra of extragalactic sources, but requires a good estimate of the foreground column density of photoelectric absorbing species. Assuming uniform elemental abundances this reduces to having a good estimate of the total hydrogen column density, N(Htot)=N(HI)+2N(H2). The atomic component, N(HI), is reliably provided using the mapped 21 cm radio emission but estimating the molecular hydrogen column density, N(H2), expected for any particular direction, is difficult. The X-ray afterglows of GRBs are ideal sources to probe X-ray absorption in our Galaxy because they are extragalactic, numerous, bright, have simple spectra and occur randomly across the entire sky. We describe an empirical method, utilizing 493 afterglows detected by the Swift XRT, to determine N(Htot) through the Milky Way which provides an improved estimate of the X-ray absorption in our Galaxy and thereby leads to more reliable measurements of the intrinsic X-ray absorption and, potentially, other spectral parameters, for extragalactic X-ray sources. We derive a simple function, dependent on the product of the atomic hydrogen column density, N(HI), and dust extinction, E(B-V), which describes the variation of the molecular hydrogen column density, N(H2), of our Galaxy, over the sky. Using the resulting N(Htot) we show that the dust-to-hydrogen ratio is correlated with the carbon monoxide emission and use this ratio to estimate the fraction of material which forms interstellar dust grains. Our resulting recipe represents a significant revision in Galactic absorption compared to previous standard methods, particularly at low Galactic latitudes.

astro-ph.HE↗

arXiv2013

X-ray absorption evolution in Gamma-Ray Bursts: intergalactic medium or evolutionary signature of their host galaxies?

The intrinsic X-ray emission of Gamma-Ray Bursts (GRBs) is often found to be absorbed over and above the column density through our own galaxy. The extra component is usually assumed to be due to absorbing gas lying within the host galaxy of the GRB itself. There is an apparent correlation between the equivalent column density of hydrogen, N(H,intrinsic) (assuming it to be at the GRB redshift), and redshift, z, with the few z>6 GRBs showing the greatest intrinsic column densities. We investigate the N(H,intrinsic) - z relation using a large sample of Swift GRBs, as well as active galactic nuclei (AGN) and quasar samples, paying particular attention to the spectral energy distributions of the two highest redshift GRBs. Various possible sample biases and systematics that might produce such a correlation are considered, and we conclude that the correlation is very likely to be real. This may indicate either an evolutionary effect in the host galaxy properties, or a contribution from gas along the line-of-sight, in the diffuse intergalactic medium (IGM) or intervening absorbing clouds. Employing a more realistic model for IGM absorption than in previous works, we find that this may explain much of the observed opacity at z>~3 providing it is not too hot, likely between 10^5 K and 10^6.5 K, and moderately metal enriched, Z~0.2 Z_sun. This material could therefore constitute the Warm Hot Intergalactic Medium. However, a comparable level of absorption is also expected from the cumulative effect of intervening cold gas clouds, and given current uncertainties it is not possible to say which, if either, dominates. At lower redshifts, we conclude that gas in the host galaxies must be the dominant contributor to the observed X-ray absorption.

astro-ph.HE↗

arXiv2013

A new population of ultra-long duration gamma-ray bursts

We present comprehensive multiwavelength observations of three gamma-ray bursts (GRBs) with durations of several thousand seconds. We demonstrate that these events are extragalactic transients; in particular we resolve the long-standing conundrum of the distance of GRB 101225A (the "Christmas-day burst"), finding it to have a redshift z=0.847, and showing that two apparently similar events (GRB 111209A and GRB 121027A) lie at z=0.677 and z=1.773 respectively. The systems show extremely unusual X-ray and optical lightcurves, very different from classical GRBs, with long lasting highly variable X-ray emission and optical light curves that exhibit little correlation with the behaviour seen in the X-ray. Their host galaxies are faint, compact, and highly star forming dwarf galaxies, typical of "blue compact galaxies". We propose that these bursts are the prototypes of a hitherto largely unrecognized population of ultra-long GRBs, that while observationally difficult to detect may be astrophysically relatively common. The long durations may naturally be explained by the engine driven explosions of stars of much larger radii than normally considered for GRB progenitors which are thought to have compact Wolf-Rayet progenitor stars. However, we cannot unambiguously identify supernova signatures within their light curves or spectra. We also consider the alternative possibility that they arise from the tidal disruption of stars by supermassive black holes.

astro-ph.HE↗

arXiv2013

High energy emission from transients

Cosmic explosions dissipate energy into their surroundings on a very wide range of time-scales: producing shock waves and associated particle acceleration. The historical culprits for the acceleration of the bulk of Galactic cosmic rays are supernova remnants: explosions on ~10000 year time-scales. Increasingly however, time-variable emission points to rapid and efficient particle acceleration in a range of different astrophysical systems. Gamma-ray bursts have the shortest time-scales, with inferred bulk Lorentz factors of ~1000 and photons emitted beyond 100 GeV, but active galaxies, pulsar wind nebulae and colliding stellar winds are all now associated with time-variable emission at ~TeV energies. Cosmic photons and neutrinos at these energies offer a powerful probe of the underlying physical mechanisms of cosmic explosions, and a tool for exploring fundamental physics with these systems. Here we discuss the motivations for high-energy observations of transients, the current experimental situation, and the prospects for the next decade, with particular reference to the major next-generation high-energy observatory CTA.

astro-ph.HE↗

arXiv2012

The Optical SN 2012bz Associated with the Long GRB 120422A

The association of Type Ic SNe with long-duration GRBs is well established. We endeavor, through accurate ground-based observational campaigns, to characterize these SNe at increasingly high redshifts. We obtained a series of optical photometric and spectroscopic observations of the Type Ic SN2012bz associated with the Swift long-duration GRB120422A (z=0.283) using the 3.6-m TNG and the 8.2-m VLT telescopes. The peak times of the light curves of SN2012bz in various optical filters differ, with the B-band and i'-band light curves reaching maximum at ~9 and ~23 rest-frame days, respectively. The bolometric light curve has been derived from individual bands photometric measurements, but no correction for the unknown contribution in the near-infrared (probably around 10-15%) has been applied. Therefore, the present light curve should be considered as a lower limit to the actual UV-optical-IR bolometric light curve. This pseudo-bolometric curve reaches its maximum (Mbol = -18.56 +/- 0.06) at 13 +/- 1 rest-frame days; it is similar in shape and luminosity to the bolometric light curves of the SNe associated with z<0.2 GRBs and more luminous than those of SNe associated with XRFs. A comparison with the model generated for the bolometric light curve of SN2003dh suggests that SN2012bz produced only about 15% less 56Ni than SN2003dh, about 0.35 Msol. Similarly the VLT spectra of SN2012bz, after correction for Galactic extinction and for the contribution of the host galaxy, suggest comparable explosion parameters with those observed in SN2003dh (EK~3.5 x 10^52 erg, Mej~7 Msol) and a similar progenitor mass (~25-40 Msol). GRB120422A is consistent with the Epeak-Eiso and the EX,iso-Egamma,iso-E_peak relations. GRB120422A/SN2012bz shows the GRB-SN connection at the highest redshift so far accurately monitored both photometrically and spectroscopically.

astro-ph.HE↗

arXiv2012

A search for thermal X-ray signatures in Gamma-Ray Bursts II: The Swift sample

In several gamma-ray bursts (GRBs) excess emission, in addition to the standard synchrotron afterglow spectrum, has been discovered in the early time X-ray observations. It has been proposed that this excess comes from black body emission, which may be related to the shock break-out of a supernova in the GRBs progenitor star. This hypothesis is supported by the discovery of excess emission in several GRBs with an associated supernova. Using mock spectra we show that it is only likely to detect such a component, similar to the one proposed in GRB 101219B, at low redshift and in low absorption environments. We also perform a systematic search for black body components in all the GRBs observed with the Swift satellite and find six bursts (GRB 061021, 061110A, 081109, 090814A, 100621A and 110715A) with possible black body components. Under the assumption that their excess emission is due to a black body component we present radii, temperatures and luminosities of the emitting components. We also show that detection of black body components only is possible in a fraction of the Swift bursts.

astro-ph.HE↗

arXiv2012

A search for thermal X-ray signatures in Gamma-Ray Bursts I: Swift bursts with optical supernovae

The X-ray spectra of Gamma-Ray Bursts can generally be described by an absorbed power law. The landmark discovery of thermal X-ray emission in addition to the power law in the unusual GRB 060218, followed by a similar discovery in GRB 100316D, showed that during the first thousand seconds after trigger the soft X-ray spectra can be complex. Both the origin and prevalence of such spectral components still evade understanding, particularly after the discovery of thermal X-ray emission in the classical GRB 090618. Possibly most importantly, these three objects are all associated with optical supernovae, begging the question of whether the thermal X-ray components could be a result of the GRB-SN connection, possibly in the shock breakout. We therefore performed a search for blackbody components in the early Swift X-ray spectra of 11 GRBs that have or may have associated optical supernovae, accurately recovering the thermal components reported in the literature for GRBs 060218, 090618 and 100316D. We present the discovery of a cooling blackbody in GRB 101219B/SN2010ma, and in four further GRB-SNe we find an improvement in the fit with a blackbody which we deem possible blackbody candidates due to case-specific caveats. All the possible new blackbody components we report lie at the high end of the luminosity and radius distribution. GRB 101219B appears to bridge the gap between the low-luminosity and the classical GRB-SNe with thermal emission, and following the blackbody evolution we derive an expansion velocity for this source of order 0.4c. We discuss potential origins for the thermal X-ray emission in our sample, including a cocoon model which we find can accommodate the more extreme physical parameters implied by many of our model fits.

astro-ph.HE↗

arXiv2012

Star formation in the early universe: beyond the tip of the iceberg

We present late-time Hubble Space Telescope imaging of the fields of six Swift GRBs lying at 5.0 90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy luminosity function, and the supply of ionizing photons in the early universe.

astro-ph.CO↗

arXiv2012

The optically unbiased GRB host (TOUGH) survey. III. Redshift distribution

We present 10 new gamma-ray burst (GRB) redshifts and another five redshift limits based on host galaxy spectroscopy obtained as part of a large program conducted at the Very Large Telescope (VLT). The redshifts span the range 0.345 < z < 2.54. Three of our measurements revise incorrect values from the literature. The homogeneous host sample researched here consists of 69 hosts that originally had a redshift completeness of 55% (with 38 out of 69 hosts having redshifts considered secure). Our project, including VLT/X-shooter observations reported elsewhere, increases this fraction to 77% (53/69), making the survey the most comprehensive in terms of redshift completeness of any sample to the full Swift depth, analyzed to date. We present the cumulative redshift distribution and derive a conservative, yet small, associated uncertainty. We constrain the fraction of Swift GRBs at high redshift to a maximum of 14% (5%) for z > 6 (z > 7). The mean redshift of the host sample is assessed to be > 2.2, with the 10 new redshifts reducing it significantly. Using this more complete sample, we confirm previous findings that the GRB rate at high redshift (z > 3) appears to be in excess of predictions based on assumptions that it should follow conventional determinations of the star formation history of the universe, combined with an estimate of its likely metallicity dependence. This suggests that either star formation at high redshifts has been significantly underestimated, for example due to a dominant contribution from faint, undetected galaxies, or that GRB production is enhanced in the conditions of early star formation, beyond that usually ascribed to lower metallicity.

astro-ph.CO↗

arXiv2012

Detailed optical and near-infrared polarimetry, spectroscopy and broadband photometry of the afterglow of GRB 091018: Polarisation evolution

[Abridged] A number of phenomena have been observed in GRB afterglows that defy explanation by simple versions of the standard fireball model, leading to a variety of new models. Polarimetry can be a major independent diagnostic of afterglow physics, probing the magnetic field properties and internal structure of the GRB jets. In this paper we present the first high quality multi-night polarimetric light curve of a Swift GRB afterglow, aimed at providing a well calibrated dataset of a typical afterglow to serve as a benchmark system for modelling afterglow polarisation behaviour. In particular, our dataset of the afterglow of GRB 091018 (at redshift z=0.971) comprises optical linear polarimetry (R band, 0.13 - 2.3 days after burst); circular polarimetry (R band) and near-infrared linear polarimetry (Ks band). We add to that high quality optical and near-infrared broadband light curves and spectral energy distributions as well as afterglow spectroscopy. The linear polarisation varies between 0 and 3%, with both long and short time scale variability visible. We find an achromatic break in the afterglow light curve, which corresponds to features in the polarimetric curve. We find that the data can be reproduced by jet break models only if an additional polarised component of unknown nature is present in the polarimetric curve. We probe the ordered magnetic field component in the afterglow through our deep circular polarimetry, finding P_circ < 0.15% (2 sigma), the deepest limit yet for a GRB afterglow, suggesting ordered fields are weak, if at all present. Our simultaneous R and Ks band polarimetry shows that dust induced polarisation in the host galaxy is likely negligible.

astro-ph.CO↗

arXiv2012

Swift follow-up observations of unclassified ASCA sources

We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and plane surveys. Our short pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of ~1E-13 erg cm-2 s-1, which translates into a luminosity of ~1E33 erg s-1 for an assumed distance of D=8 kpc. The brightest source in our sample reaches a maximum 0.3-10 keV luminosity of ~2E36 (D/8kpc)^2 erg s-1 during our observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19 were not detected during our program. Since we are probing the faint end of the ASCA source populations, we expect a large fraction of the non-detections to be due to the Eddington bias. This is strengthened by the fact that we find the observed XRT count rates to be predominantly lower than expected based on the reported ASCA intensities. Investigation of the ASCA properties and long-term X-ray variability leads us to conclude that the non-detections likely include two spurious ASCA detections and three objects that are variable or transient X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies of ~2-4", which significantly improves upon their ASCA uncertainties of ~1'. We use the X-ray spectra and variability to characterise these objects. Most appear to be faint, persistent X-ray emitters that have highly absorbed spectra. Based on their X-ray properties we identify three accreting compact objects: one confirmed (AX J1754.2-2754) and one candidate (AX J1538.3-5541) X-ray binary, and one possible magnetically accreting white dwarf (AX J1620.1-5002). Furthermore, we use the improved positions of XRT-detected sources to search for counterparts in simultaneously obtained Swift/UVOT images and possible associations with cataloged sources at various wavelengths. This reveals three possible main sequence stars amongst our sample.

astro-ph.HE↗

arXiv2011

Polarimetry of the transient relativistic jet of GRB 110328 / Swift J164449.3+573451

We present deep infrared (Ks band) imaging polarimetry and radio (1.4 and 4.8 GHz) polarimetry of the enigmatic transient Swift J164449.3+573451. This source appears to be a short lived jet phenomenon in a galaxy at redshift z = 0.354, activated by a sudden mass accretion onto the central massive black hole, possibly caused by the tidal disruption of a star. We aim to find evidence for this scenario through linear polarimetry, as linear polarisation is a sensitive probe of jet physics, source geometry and the various mechanisms giving rise to the observed radiation. We find a formal Ks band polarisation measurement of P_lin = 7.4 +/- 3.5 % (including systematic errors). Our radio observations show continuing brightening of the source, which allows sensitive searches for linear polarisation as a function of time. We find no evidence of linear polarisation at radio wavelengths of 1.4 GHz and 4.8 GHz at any epoch, with the most sensitive 3 sigma limits as deep as 2.1%. These upper limits are in agreement with expectations from scenarios in which the radio emission is produced by the interaction of a relativistic jet with a dense circumsource medium. We further demonstrate how the polarisation properties can be used to derive properties of the jet in Swift J164449.3+573451, exploiting the similarities between this source and the afterglows of gamma-ray bursts.

astro-ph.CO↗

arXiv2011

Recovering Swift-XRT energy resolution through CCD charge trap mapping

The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch.

astro-ph.IM↗

arXiv2011

GRB 070125 and the environments of spectral-line poor afterglow absorbers

GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the GRB environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the NIR-to-X-ray spectral energy distribution (SED) and studying the high signal-to-noise VLT/FORS afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the UV, low equivalent hydrogen column density and little reddening caused by a LMC- or SMC-type extinction curve. From the weak MgII absorption at z=1.5477 in the spectrum, we derived logN(MgII)=12.96+0.13-0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely an LLS with a 0.03Z\odot<Z<1.3Z\odot metallicity. The comparison with other GRB absorbers places GRB070125 at the low end of the absorption line equivalent width distribution, confirming that weak spectral features and spectral-line poor absorbers are not so uncommon in afterglow spectra. Moreover, we show that the effect of photo-ionization on the gas surrounding the GRB, combined with a low N(HI) along a short segment of the line of sight within the host galaxy, can explain the lack of spectral features in GRB070125. Finally, the non-detection of an underlying galaxy is consistent with a faint GRB host galaxy, well within the GRB host brightness distribution. Thus, the possibility that GRB070125 is simply located in the outskirts of a gas-rich, massive star-forming region inside its small and faint host galaxy seems more likely than a gas-poor, halo environment origin.

astro-ph.CO↗

arXiv2011

An extremely luminous panchromatic outburst from the nucleus of a distant galaxy

Variable X-ray and gamma-ray emission is characteristic of the most extreme physical processes in the Universe, and studying the sources of these energetic photons has been a major driver in astronomy for the past 50 years. Here we present multiwavelength observations of a unique gamma-ray selected transient, discovered by Swift, which was accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z=0.3534. Its high-energy emission has lasted much longer than any gamma-ray burst, while its peak luminosity was about 100 times higher than the brightest active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a new, rare mechanism associated with a massive black hole in the nucleus of a galaxy.

astro-ph.HE↗

arXiv2010

A tale of two GRB-SNe at a common redshift of z = 0.54

We present ground-based and HST optical observations of the optical transients (OTs) of long-duration Gamma Ray Bursts (GRBs) 060729 and 090618, both at a redshift of z = 0.54. For GRB 060729, bumps are seen in the optical light curves (LCs), and the late-time broadband spectral energy distributions (SEDs) of the OT resemble those of local type Ic supernovae (SNe). For GRB 090618, the dense sampling of our optical observations has allowed us to detect well-defined bumps in the optical LCs, as well as a change in colour, that are indicative of light coming from a core-collapse SN. The accompanying SNe for both events are individually compared with SN1998bw, a known GRB-supernova, and SN1994I, a typical type Ic supernova without a known GRB counterpart, and in both cases the brightness and temporal evolution more closely resemble SN1998bw. We also exploit our extensive optical and radio data for GRB 090618, as well as the publicly-available SWIFT -XRT data, and discuss the properties of the afterglow at early times. In the context of a simple jet-like model, the afterglow of GRB 090618 is best explained by the presence of a jet-break at t-to > 0.5 days. We then compare the rest-frame, peak V -band absolute magnitudes of all of the GRB and X-Ray Flash (XRF)-associated SNe with a large sample of local type Ibc SNe, concluding that, when host extinction is considered, the peak magnitudes of the GRB/XRF-SNe cannot be distinguished from the peak magnitudes of non-GRB/XRF SNe.

astro-ph.CO↗