arXiv subjects

R. L. C. Starling

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

At least 91 records · Page 5Linked to original sources

arXiv2010

Swift follow-up of unidentified X-ray sources in the XMM-Newton Slew Survey

We present deep Swift follow-up observations of a sample of 94 unidentified X-ray sources from the XMM-Newton Slew Survey. The X-ray Telescope on-board Swift detected 29% of the sample sources; the flux limits for undetected sources suggests the bulk of the Slew Survey sources are drawn from one or more transient populations. We report revised X-ray positions for the XRT-detected sources, with typical uncertainties of 2.9", reducing the number of catalogued optical matches to just a single source in most cases. We characterise the sources detected by Swift through their X-ray spectra and variability and via UVOT photometry and catalogued nIR, optical and radio observations. Six sources can be associated with known objects and 8 may be associated with unidentified ROSAT sources within the 3-sigma error radii of our revised X-ray positions. We find 10 of the 30 XRT-detected sources are clearly stellar in nature, including one periodic variable star and 2 high proper motion stars. For 11 sources we propose an AGN classification, among which 4 are detected with BAT and 3 have redshifts spanning z = 0.2 - 0.9 obtained from the literature or from optical spectroscopy presented here. The 67 Slew Survey sources we do not detect with Swift are studied via their characteristics in the Slew Survey and by comparison with the XRT and BAT detected population. We suggest that these are mostly if not all extragalactic, though unlikely to be highly absorbed sources in the X-rays such as Compton thick AGN. A large number of these are highly variable soft X-ray sources. A small fraction of mainly hard-band detections may be spurious. This follow-up programme brings us a step further to completing the identifications of a substantial sample of XMM-Newton Slew Survey sources, important for understanding the nature of the transient sky and allowing flux-limited samples to be constructed.

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arXiv2010

Detailed Radio View on Two Stellar Explosions and Their Host Galaxy: XRF080109/SN2008D and SN2007uy in NGC2770

The galaxy NGC2770 hosted two core-collapse supernova explosions, SN2008D and SN2007uy, within 10 days of each other and 9 years after the first supernova of the same type, SN1999eh, was found in that galaxy. In particular SN2008D attracted a lot of attention due to the detection of an X-ray outburst, which has been hypothesized to be caused by either a (mildly) relativistic jet or the supernova shock breakout. We present an extensive study of the radio emission from SN2008D and SN2007uy: flux measurements with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope, covering ~600 days with observing frequencies ranging from 325 MHz to 8.4 GHz. The results of two epochs of global Very Long Baseline Interferometry observations are also discussed. We have examined the molecular gas in the host galaxy NGC2770 with the Arizona Radio Observatory 12-m telescope, and present the implications of our observations for the star formation and seemingly high SN rate in this galaxy. Furthermore, we discuss the near-future observing possibilities of the two SNe and their host galaxy at low radio frequencies with the Low Frequency Array.

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arXiv2010

Discovery of the nearby long, soft GRB 100316D with an associated supernova

We report the Swift discovery of nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low redshift host galaxy and associated supernova. We derive the redshift of the event to be z = 0.0591 +/- 0.0001 and provide accurate astrometry for the GRB-SN. We study the extremely unusual prompt emission with time-resolved gamma-ray to X-ray spectroscopy, and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini South, VLT and HST observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB--SN connection can be gleaned.

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arXiv2010

The Swift Burst Analyser I: BAT and XRT spectral and flux evolution of Gamma Ray Bursts

Context: Gamma Ray Burst models predict the broadband spectral evolution and the temporal evolution of the energy flux. In contrast, standard data analysis tools and data repositories provide count-rate data, or use single flux conversion factors for all of the data, neglecting spectral evolution. Aims: To produce Swift BAT and XRT light curves in flux units, where the spectral evolution is accounted for. Methods: We have developed software to use the hardness ratio information to track spectral evolution of GRBs, and thus to convert the count-rate light curves from the BAT and XRT instruments on Swift into accurate, evolution-aware flux light curves. Results: The Swift Burst Analyser website (http://www.swift.ac.uk/burst_analyser) contains BAT, XRT and combined BAT-XRT flux light curves in three energy regimes for all GRBs observed by the Swift satellite. These light curves are automatically built and updated when data become available, are presented in graphical and plain-text format, and are available for download and use in research.

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arXiv2009

Swift observations of the X-ray and UV evolution of V2491 Cyg (Nova Cyg 2008 No. 2)

We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the Super-Soft Source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light-curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic white dwarf, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence timescale estimates.

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arXiv2009

Can X-ray emission powered by a spinning-down magnetar explain some GRB light curve features?

Long duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly-rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray emission arises from internal shocks in the jet or magnetised outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetised millisecond pulsar, or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray lightcurves of all GRBs observed by the Swift satellite we identified a subset of bursts which have a feature in their light curves which we call an internal plateau -- unusually constant emission followed by a steep decay -- which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength and find that they are consistent with the most extreme predicted values for magnetars.

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arXiv2009

Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution

GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ROTSE and a host of other telescopes and was detected in the radio by the VLA. The redshift of z= 3.355 +/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasithermal model shifting from ~60 keV to ~3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this originated from the afterglow component, not the pulsed/flaring prompt emission. Broadband optical and X-ray coverage of the afterglow at the start of the final X-ray decay (~8 ks) reveals a spectral break between the optical and X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there is no evidence for a jet break in the afterglow up to six days following the burst.

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arXiv2009

Constraining the energy budget of GRB 080721

We follow the bright, highly energetic afterglow of Swift-discovered GRB 080721 at z=2.591 out to 36 days or 3e6 s since the trigger in the optical and X-ray bands. We do not detect a break in the late-time light curve inferring a limit on the opening angle of theta_j >= 7.3 deg and setting tight constraints on the total energy budget of the burst of E_gamma >= 9.9e51 erg within the fireball model. To obey the fireball model closure relations the GRB jet must be expanding into a homogeneous surrounding medium and likely lies behind a significant column of dust. The energy constraint we derive can be used as observational input for models of the progenitors of long gamma-ray bursts: we discuss how such high collimation-corrected energies could be accommodated with certain parameters of the standard massive star core-collapse models. We can, however, most probably rule out a magnetar progenitor for this GRB which would require 100% efficiency to reach the observed total energy.

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arXiv2009

A glimpse of the end of the dark ages: the gamma-ray burst of 23 April 2009 at redshift 8.3

It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant, source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place in the redshift range z=10.8 +/- 1.4, while observations of quasars and Lyman-alpha galaxies have shown that the process was essentially completed by z=6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare beyond z~7. Here we report the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07. This is well beyond the redshift of the most distant spectroscopically confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive stars were being produced, and dying as GRBs, ~625 million years after the Big Bang. In addition, the accurate position of the burst pinpoints the location of the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will constrain the evolving rate of star formation in the early universe, while rapid spectroscopy of their afterglows will allow direct exploration of the progress of reionization with cosmic time.

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arXiv2009

Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs

We present a homogeneous X-ray analysis of all 318 Gamma Ray Bursts detected by the X-ray Telescope on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2--3 we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4--6 we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

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arXiv2009

Testing the blast wave model with Swift GRBs

The complex structure of the light curves of Swift GRBs has made the identification of breaks, and the interpretation of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to identify breaks, which are possibly hidden, and to constrain the blast wave parameters; electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multi-wavelength light curves and X-ray spectra of our sample to the predictions of the blast wave model. We can successfully interpret all of the bursts in our sample of 10, except two, within this framework and we can estimate, with confidence, the electron energy distribution index for 6 of the sample. Furthermore we identify jet breaks in a number of the bursts. A statistical analysis of the distribution of p reveals that, even in the most conservative case of least scatter, the values are not consistent with a single, universal value. The values of k suggest that the circumburst density profiles are not drawn from only one of the constant density or wind-like media populations.

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arXiv2009

In search of progenitors for supernova-less GRBs 060505 and 060614: re-examination of their afterglows

GRB060505 and GRB060614 are nearby long-duration gamma-ray bursts (LGRBs) without accompanying supernovae (SNe) down to very strict limits. They thereby challenge the conventional LGRB-SN connection and naturally give rise to the question: are there other peculiar features in their afterglows which would help shed light on their progenitors? To answer this question, we combine new observational data with published data and investigate the multi-band temporal and spectral properties of the two afterglows. We find that both afterglows can be well interpreted within the framework of the jetted standard external shock wave model, and that the afterglow parameters for both bursts fall well within the range observed for other LGRBs. Hence, from the properties of the afterglows there is nothing to suggest that these bursts should have another progenitor than other LGRBs. Recently, Swift-discovered GRB080503 also has the spike + tail structure during its prompt gamma-ray emission seemingly similar to GRB060614. We analyse the prompt emission of this burst and find that this GRB is actually a hard-spike + hard-tail burst with a spectral lag of 0.8$\pm$0.4 s during its tail emission. Thus, the properties of the prompt emission of GRB060614 and GRB080503 are clearly different, motivating further thinking of GRB classification. Finally we note that, whereas the progenitor of the two SN-less bursts remains uncertain, the core-collapse origin for the SN-less bursts would be quite certain if a wind-like environment can be observationally established, e.g, from an optical decay faster than the X-ray decay in the afterglow's slow cooling phase.

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arXiv2009

The unusual X-ray light-curve of GRB 080307: the onset of the afterglow?

Swift-detected GRB 080307 showed an unusual smooth rise in its X-ray light-curve around 100 seconds after the burst, at the start of which the emission briefly softened. This `hump' has a longer duration than is normal for a flare at early times and does not demonstrate a typical flare profile. Using a two component power-law-to-exponential model, the rising emission can be modelled as the onset of the afterglow, something which is very rarely seen in Swift-X-ray light-curves. We cannot, however, rule out that the hump is a particularly slow early-time flare, or that it is caused by upscattered reverse shock electrons.

astro-ph.HE↗

arXiv2009

Swift GRBs and the blast wave model

The complex structure of the light curves of Swift GRBs has made their interpretation and that of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to constrain the blast wave parameters: electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multi-wavelength light curves and X-ray spectra of a Swift sample to the predictions of the blast wave model. We can successfully interpret all of the bursts in our sample of 10, except two, within the framework of the blast wave model, and we can estimate with confidence the electron energy distribution index for 6 of the sample. Furthermore we identify jet breaks in half of the bursts. A statistical analysis of the distribution of p reveals that, even in the most conservative case of least scatter, the values are not consistent with a single, universal value. The values of k suggest that the circumburst density profiles are not drawn from only one of the constant density or wind-like media populations.

astro-ph.HE↗

arXiv2008

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.

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arXiv2008

The 2008 May burst activation of SGR 1627-41

In May 2008 the soft gamma-ray repeater SGR 1627-41 resumed its bursting activity after nearly a decade of quiescence. After detection of a bright burst, Swift pointed its X-ray telescope in the direction of the source in less than five hours and followed it for over five weeks. In this paper we present an analysis of the data from these Swift observations and an XMM-Newton one performed when SGR 1627-41 was still in a quiescent state. The analysis of the bursts detected with Swift/BAT shows that their temporal and spectral properties are similar to those found in previous observations of SGR 1627-41 and other soft gamma-ray repeaters. The maximum peak luminosity of the bursts was about 2E+41 erg/s. Our data show that the outburst was accompanied by a fast flux enhancement and by a hardening of the spectrum with respect to the persistent emission.

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arXiv2008

On the nature of late X-ray flares in Swift Gamma-ray Bursts

Previously detected in only a few gamma-ray bursts (GRBs), X-ray flares are now observed in ~50% of Swift GRBs, though their origins remain unclear. Most flares are seen early on in the afterglow decay, while some bursts exhibit flares at late times of 10^4 to 10^5 seconds, which may have implications for flare models.We investigate whether a sample of late time (> 10^4s) flares are different from previous samples of early time flares, or whether they are merely examples on the tail of the early flare distribution. We examine the X-ray light curves of Swift bursts for late flares and compare the flare and underlying temporal power-law properties with those of early flares, and the values of these properties predicted by the blast wave model. The burst sample shows late flare properties consistent with those of early flares, where the majority of the flares can be explained by either internal or external shock, though in a few cases one origin is favoured over the other. The underlying power laws are mainly consistent with the normal decay phases of the afterglow. If confirmed by the ever growing sample of late time flares, this would imply that, in some cases, prolonged activity out to a day or a restarting of the central engine is required.

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arXiv2008

GRB 080319B: A Naked-Eye Stellar Blast from the Distant Universe

Long duration gamma-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of a massive star. Over the last forty years, our understanding of the GRB phenomenon has progressed dramatically; nevertheless, fortuitous circumstances occasionally arise that provide access to a regime not yet probed. GRB 080319B presented such an opportunity, with extraordinarily bright prompt optical emission that peaked at a visual magnitude of 5.3, making it briefly visible with the naked eye. It was captured in exquisite detail by wide-field telescopes, imaging the burst location from before the time of the explosion. The combination of these unique optical data with simultaneous gamma-ray observations provides powerful diagnostics of the detailed physics of this explosion within seconds of its formation. Here we show that the prompt optical and gamma-ray emissions from this event likely arise from different spectral components within the same physical region located at a large distance from the source, implying an extremely relativistic outflow. The chromatic behaviour of the broadband afterglow is consistent with viewing the GRB down the very narrow inner core of a two-component jet that is expanding into a wind-like environment consistent with the massive star origin of long GRBs. These circumstances can explain the extreme properties of this GRB.

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