arXiv subjects

A. Beardmore

Publications and source records attributed to A. Beardmore.

At least 19 recordsLinked to original sources

arXiv2025

Einstein Probe Discovery of EP J182730.0-095633: A New Black Hole X-ray Binary Candidate in Faint Outburst?

Black hole X-ray binaries (candidates) currently identified in our galaxy are mainly transient sources, with the majority discovered through the detection of their X-ray outbursts. Among these, only four were found during faint outbursts exhibiting peak X-ray luminosities $L_{\rm X}\lesssim10^{36}~{\rm erg~s^{-1}}$, likely due to the previous lack of sensitive, wide-field monitoring instruments in the X-ray band. In this Letter, we present the discovery of an intriguing X-ray transient, EP J182730.0-095633, via the Einstein Probe (EP) and subsequent multi-wavelength follow-up studies. This transient, located on the Galactic plane, experienced a faint and brief X-ray outburst lasting about 20 days. Its X-ray spectrum is non-thermal and consistent with a power-law model with a nearly constant photon index of $Γ\sim2$ throughout the outburst. A long-lasting millihertz quasi-periodic oscillation (QPO) signal was detected in its X-ray light curve, centered around a frequency of $\sim0.04$ Hz. A transient near-infrared source was identified as its counterpart, although no optical emission was detectable, likely due to significant extinction. A radio counterpart was also observed, displaying an inverted radio spectrum with $α\sim0.45$. The X-ray spectral and temporal characteristics, along with the multi-wavelength properties, indicate that the source is a faint low-mass X-ray binary, with the compact object likely being a black hole. This work demonstrates the potential of the EP in discovering new X-ray binaries by capturing faint-level X-ray outbursts.

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arXiv2023

Soft X-ray and FUV observations of Nova Her 2021 (V1674~Her) with AstroSat

Nova Her 2021 or V1674 Her was one of the fastest novae to be observed so far. We report here the results from our timing and spectral studies of the source observed at multiple epochs with AstroSat. We report the detection of a periodicity in the source in soft X-rays at a period of 501.4--501.5 s which was detected with high significance after the peak of the super-soft phase, but was not detected in the far ultraviolet (FUV) band of AstroSat. The shape of the phase-folded X-ray light curves has varied significantly as the nova evolved. The phase-resolved spectral studies reveal the likely presence of various absorption features in the soft X-ray band of 0.5--2 keV, and suggest that the optical depth of these absorption features may be marginally dependent on the pulse phase. Strong emission lines from Si, N and O are detected in the FUV, and their strength declined continuously as the nova evolved and went through a bright X-ray state.

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arXiv2015

The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?

We present a wide dataset of gamma-ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814A the best.

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arXiv2014

The Microchannel X-ray Telescope for the Gamma-Ray Burst mission SVOM

We present the Microchannel X-ray Telescope, a new light and compact focussing telescope that will be flying on the Sino-French SVOM mission dedicated to Gamma-Ray Burst science. The MXT design is based on the coupling of square pore micro-channel plates with a low noise pnCCD. MXT will provide an effective area of about 50 cmsq, and its point spread function is expected to be better than 3.7 arc min (FWHM) on axis. The estimated sensitivity is adequate to detect all the afterglows of the SVOM GRBs, and to localize them to better then 60 arc sec after five minutes of observation.

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arXiv2014

Chandra Spectroscopy of MAXI J1305-704: Detection of an Infalling Black Hole Disk Wind?

We report on a Chandra/HETG X-ray spectrum of the black hole candidate MAXI J1305-704. A rich absorption complex is detected in the Fe L band, including density-sensitive lines from Fe XX, XXI, and XXII. Spectral analysis over three bands with photoionization models generally requires a gas density of n > 1 E+17 cm^-3. Assuming a luminosity of L = 1 E+37 erg/s, fits to the 10-14 A band constrain the absorbing gas to lie within r = 3.9(7) E+3 km from the central engine, or about r = 520 +/- 90 (M/5 Msun) r_g, where r_g = GM/c^2. At this distance from the compact object, gas in Keplerian orbits should have a gravitational red-shift of z = v/c ~ 3 +/- 1 E-3 (M/5 Msun), and any tenuous inflowing gas should have a free-fall velocity of v/c ~ 6 +/- 1 E-2 (M/5 Msun)^1/2. The best-fit single-zone photoionization models measure a red-shift of v/c = 2.6-3.2 E-3. Models with two zones provide significantly improved fits; the additional zone is measured to have a red-shift of v/c =4.6-4.9 E-2 (models including two zones suggest slightly different radii and may point to lower densities). Thus, the shifts are broadly consistent with the photoionization radius. The results may be explained in terms of a "failed wind" like those predicted in some numerical simulations. We discuss our results in the context of accretion flows across the mass scale, and the potential role of failed winds in black hole state transitions.

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arXiv2014

GRB 130427A: a Nearby Ordinary Monster

Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe. Because of its intrinsic luminosity ($L\sim 3 \times 10^{53}$ erg s$^{-1}$) and its relative proximity ($z=0.34$), GRB 130427A was a unique event that reached the highest fluence observed in the gamma-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs, suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies.

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arXiv2013

Obscuration effects in Super-Soft-Source X-ray spectra

Super-Soft-Source (SSS) X-ray spectra are blackbody-like spectra with effective temperatures ~3-7x10^5 K and luminosities of 10^{35-38} erg/s. SSS grating spectra display atmospheric absorption lines. Radiation transport atmosphere models can be used to derive physical parameters, but more sophisticated models are required. We bypass the complications of spectral models and concentrate on the data in a comparative, qualitative study. We inspect all available X-ray grating SSS spectra to determine systematic, model-independent trends. We use comparative plots of spectra of different systems to find common and different features. The results are interpreted in the context of system parameters obtained from the literature. We find two distinct types of SSS spectra which we name SSa and SSe. Their main observational characteristics are either clearly visible absorption lines or emission lines, respectively, while both types contain atmospheric continuum emission. SSe may be obscured SSa systems, which is supported by similarities between SSe and SSa with obscured and unobscured AGN, respectively. Further, we find all known or suspected high-inclination systems to emit permanently in an SSe state. Some sources are found to transition between SSa and SSe states, becoming SSe when fainter. SSS spectra are subject to various occultation processes. In Cal 87, the accretion disc blocks the central hot source when viewed edge on. In novae, the accretion disc may have been destroyed during the initial explosion but could have reformed by the time of the SSS phase. In addition, clumpy ejecta may lead to temporary obscuration events. The emission lines originate from reprocessed emission in the accretion disc, its wind or further out in clumpy ejecta while Thomson scattering allows continuum emission to be visible also during total obscuration of the central hot source.

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arXiv2012

Swift follow-up observations of candidate gravitational-wave transient events

We present the first multi-wavelength follow-up observations of two candidate gravitational-wave (GW) transient events recorded by LIGO and Virgo in their 2009-2010 science run. The events were selected with low latency by the network of GW detectors and their candidate sky locations were observed by the Swift observatory. Image transient detection was used to analyze the collected electromagnetic data, which were found to be consistent with background. Off-line analysis of the GW data alone has also established that the selected GW events show no evidence of an astrophysical origin; one of them is consistent with background and the other one was a test, part of a "blind injection challenge". With this work we demonstrate the feasibility of rapid follow-ups of GW transients and establish the sensitivity improvement joint electromagnetic and GW observations could bring. This is a first step toward an electromagnetic follow-up program in the regime of routine detections with the advanced GW instruments expected within this decade. In that regime multi-wavelength observations will play a significant role in completing the astrophysical identification of GW sources. We present the methods and results from this first combined analysis and discuss its implications in terms of sensitivity for the present and future instruments.

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arXiv2012

GRB100814A as a member of the growing set of bursts with sudden optical rebrightening

We present the gamma-ray, X-ray, optical and radio data for GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, a sudden and prominent rebrightening in the optical band occurs followed by a fast decay in both bands. This optical rebrightening is accompanied by possible chromatic variations. We discuss possible interpretations, such as double component scenarios and internal dissipation mechanism, with their virtues and drawbacks. We also compare GRB100814A with other Swift bursts that show optical rebrightenings with similar properties.

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arXiv2009

The 22-Month Swift-BAT All-Sky Hard X-ray Survey

We present the catalog of sources detected in the first 22 months of data from the hard X-ray survey (14--195 keV) conducted with the BAT coded mask imager on the \swift satellite. The catalog contains 461 sources detected above the 4.8 sigma level with BAT. High angular resolution X-ray data for every source from Swift XRT or archival data have allowed associations to be made with known counterparts in other wavelength bands for over 97% of the detections, including the discovery of ~30 galaxies previously unknown as AGN and several new Galactic sources. A total of 266 of the sources are associated with Seyfert galaxies (median redshift z ~ 0.03) or blazars, with the majority of the remaining sources associated with X-ray binaries in our Galaxy. This ongoing survey is the first uniform all sky hard X-ray survey since HEAO-1 in 1977. Since the publication of the 9-month BAT survey we have increased the number of energy channels from 4 to 8 and have substantially increased the number of sources with accurate average spectra. The BAT 22-month catalog is the product of the most sensitive all-sky survey in the hard X-ray band, with a detection sensitivity (4.8 sigma) of 2.2e-11 erg/cm2/s (1 mCrab) over most of the sky in the 14--195 keV band.

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arXiv2009

Swift observations of GW Lib: a unique insight into a rare outburst

The second known outburst of the WZ Sge type dwarf nova GW Lib was observed in April 2007. We have obtained unique multiwavelength data of this outburst which lasted ~ 26 days. AAVSO observers recorded the outburst in the optical, which was also monitored by WASP, with a peak V magnitude of ~ 8. The outburst was followed in the UV and X-ray wavelengths by the Swift UVOT and XRT telescopes. The X-ray flux at optical maximum was found to be three orders of magnitude above the pre-outburst quiescent level, whereas X-rays are normally suppressed during dwarf nova outbursts. A distinct supersoft X-ray component was also detected at optical maximum, which probably arises from an optically-thick boundary layer. Follow-up Swift observations taken one and two years after the outburst show that the post-outburst quiescent X-ray flux remains an order of magnitude higher than the pre-outburst flux. The long interoutburst timescale of GW Lib with no observed normal outbursts support the idea that the inner disc in GW Lib is evacuated or the disc viscosity is very low.

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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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arXiv2008

The Swift X-Ray Telescope: Status and Performance

We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing gamma-ray burst (GRB)afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.

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arXiv2008

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.

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arXiv2007

Swift multi-wavelength observations of the bright flaring burst GRB051117A

We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Burst 051117A as observed by Swift. The superb quality of the early X-ray light-curve and spectra of this source, one of the brightest seen by the X-ray Telescope at such early times, allows an unprecedented look at the spectral and temporal evolution of the prompt and early afterglow emission for this GRB and allows us to place stringent limits on the detection of lines. The X-ray light-curve at early times is characteristic of a shot-noise process, with individual shots well-modelled by a fast-rise and exponential decay spanning a broad range in rise-times and decay rates. A temporal spectral analysis of the early light-curve shows that the photon index and source intensity are highly correlated with the spectrum being significantly harder when brighter, consistent with the movement of the peak of the Band function to lower energies following individual flares. The high quality spectrum obtained from the first orbit of WT mode data, enables us to place a 3 sigma upper limit on the strength of any emission line features of EW < 15 eV, assuming a narrow emission-line of 100 eV at the peak of the effective area (abridged).

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arXiv2006

The early X-ray emission from GRBs

We present observations of the early X-ray emission for a sample of 40 gamma-ray bursts (GRBs) obtained using the Swift satellite for which the narrow-field instruments were pointed at the burst within 10 minutes of the trigger. Using data from the Burst Alert and X-Ray Telescopes, we show that the X-ray light curve can be well described by an exponential that relaxes into a power law, often with flares superimposed. The transition time between the exponential and the power law provides a physically defined timescale for the burst duration. In most bursts the power law breaks to a shallower decay within the first hour, and a late emission "hump" is observed which can last for many hours. In other GRBs the hump is weak or absent. The observed variety in the shape of the early X-ray light curve can be explained as a combination of three components: prompt emission from the central engine; afterglow; and the late hump. In this scenario, afterglow emission begins during or soon after the burst and the observed shape of the X-ray light curve depends on the relative strengths of the emission due to the central engine and that of the afterglow. There is a strong correlation such that those GRBs with stronger afterglow components have brighter early optical emission. The late emission hump can have a total fluence equivalent to that of the prompt phase. GRBs with the strongest late humps have weak or no X-ray flares.

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arXiv2006

GRB 050505: A high redshift burst discovered by Swift

We report the discovery and subsequent multi-wavelength afterglow behaviour of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505. This burst is the third most distant burst, measured by spectroscopic redshift, discovered after GRB 000131 (z = 4.50) and GRB 050904 (z = 6.29). GRB 050505 is a long GRB with a multipeaked gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4 keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 days, detecting two breaks in the light curve at 7.4(+/-1.5) ks and 58.0 (+9.9/-15.4) ks after the burst trigger. The power law decay slopes before, between and after these breaks were 0.25 (+0.16/-0.17), 1.17 (+0.08/-0.09) and 1.97 (+0.27/-0.28) respectively. The light curve can also be fit with a `smoothly broken' power law model with a break observed at ~ T+18.5 ks, with decay slopes of ~0.4 and ~1.8 before and after the break respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fit with a single power law of photon index ~1.90. This behaviour is expected for the cessation of continued energisation of the ISM shock followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution indeed shows the cooling frequency to be below the X-ray but above optical frequencies. The optical -- X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical/UV extinction, with E(B-V) ~0.10 for a SMC-like extinction curve.

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arXiv2005

Swift and XMM observations of the dark GRB 050326

We present Swift and XMM observations of GRB 050326, detected by Swift-BAT. The fluence was 7.7x10^-6 erg cm^-2 (20-150 keV), and its spectrum was hard, with a power law photon index 1.25. The afterglow light curve did not show any break nor flares between ~1 hr and ~6 d after the burst, and decayed with a slope 1.70. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame Hydrogen column density is significant, N_H_z > 4x10^21 cm^-2, and the redshift of the absorber is z > 1.5. There was good agreement between the Swift-XRT and XMM results. By comparing the prompt and afterglow fluxes, we found that an early break occurred before the XRT observation. The properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break constrains the jet angle to be >7 deg. Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was >3x10^51 erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical/UV wavelengths. Thus, this GRB was "truly dark", with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z>4.5 is required. (abridged)

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