arXiv subjects

R. Willingale

Publications and source records attributed to R. Willingale.

66 records · Page 4Linked to original sources

arXiv2005

The dust-scattered X-ray halo around Swift GRB 050724

This paper discusses the X-ray halo around the Swift gamma-ray burst GRB 050724 (z=0.258), detected by the Swift X-Ray Telescope. The halo, which forms a ring around the fading X-ray source, expands to a radius of 200" within 8 ks of the burst exactly as expected for small-angle X-ray scattering by Galactic dust along the line of sight to a cosmologically distant GRB. The expansion curve and radial profile of the halo constrain the scattering dust to be concentrated at a distance of D = 139 +/- 9 pc (from Earth) in a cloud/sheet of thickness delta-D < 22 pc. The halo was observed only out to scattering angles of 200", for which the scattering is dominated by the largest grains, with a maximum size estimated to be a_max ~ 0.4-0.5 um. The scattering-to-extinction ratio was estimated to be tau_scat/A_V > 0.022; this is a lower limit to the true value because contribution from smaller grains, which scatter to larger angles, was not directly observed. The line-of-sight to the GRB passes close to the Ophiuchus molecular cloud complex, which provides a plausible site for the scattering dust.

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arXiv2005

The soft X-ray blast in the apparently sub-luminous GRB 031203

GRB031203 was a very low apparent luminosity gamma-ray burst (GRB). It was also the first GRB with a dust-scattered X-ray halo. The observation of the halo allowed us to infer the presence of a large soft X-ray fluence in the total burst output. It has, however, also been claimed that GRB031203 was intrinsically sub-energetic, representative of a class of spectrally hard, low-energy bursts quite different from other GRBs. Reanalysis of the available data confirms our original finding that GRB031203 had a very large soft X-ray component, the time of which can be constrained to within a few minutes after the burst, implying that while GRB031203 did indeed have a very low apparent luminosity, it was also very soft. Notions propagated in the literature regarding the uncertainties in the determination of the soft X-ray fluence from the halo data and on the available constraints from the hard X-ray data are addressed: the properties of the scattering dust along the line of sight (grain sizes, precise location and the geometry) are determined directly from the high quality X-ray data so that there is little uncertainty about the scatterer; constraints on the X-ray lightcurve from the Integral spacecraft at the time of the soft X-ray blast are not complete because of a slew in the spacecraft pointing shortly after the burst. Claims that GRB031203 was intrinsically under-energetic and that it represents a deviation from the luminosity-peak energy relation do not appear to be substantiated by the data, regardless of whether the soft X-ray component is declared part of the prompt emission or the afterglow. We conclude that the difference between the soft and hard X-ray spectra from XMM-Newton and Integral indicate that a second soft pulse probably occurred in this burst as has been observed in other GRBs, notably GRB050502B.

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arXiv2004

A practical system for X-ray Interferometry

X-ray interferometry has the potential to provide imaging at ultra high angular resolutions of 100 micro arc seconds or better. However, designing a practical interferometer which fits within a reasonable envelope and that has sufficient collecting area to deliver such a performance is a challenge. A simple system which can be built using current X-ray optics capabilities and existing detector technology is described. The complete instrument would be ~20 m long and ~2 m in diameter. Simulations demonstrate that it has the sensitivity to provide high quality X-ray interferometric imaging of a large number of available targets.

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arXiv2004

A very low luminosity X-ray flash: XMM-Newton observations of GRB 031203

GRB031203 was observed by XMM-Newton twice, first with an observation beginning 6 hours after the burst, and again after 3 days. The afterglow had average 0.2-10.0keV fluxes for the first and second observations of 4.2+/-0.1x10^-13 and 1.8+/-0.1x10^-13 erg/cm^2/s respectively, decaying very slowly according to a power-law with an index of -0.55+/-0.05. The prompt soft X-ray flux, inferred from a detection of the dust echo of the prompt emission, strongly implies that this burst is very soft and should be classified as an X-ray flash (XRF) and further, implies a steep temporal slope (<~-1.7) between the prompt and afterglow phases or in the early afterglow, very different from the later afterglow decay slope. A power-law (Gamma=1.90+/-0.05) with absorption at a level consistent with the Galactic foreground absorption fits the afterglow spectrum well. A bright, low-redshift (z=0.105) galaxy lies within 0.5 arcsec of the X-ray position and is likely to be the GRB host. At this redshift, GRB031203 is the closest GRB or XRF known after GRB980425. It has a very low equivalent isotropic gamma-ray energy in the burst (~3x10^49 erg) and X-ray luminosity in the afterglow (9x10^42 erg/s at 10 hours), 3-4 orders of magnitude less than typical bursts, though higher than either the faint XRF020903 or GRB980425. The rapid initial decline and subsequent very slow fading of the X-ray afterglow is also similar to that observed in GRB980425, indicating that GRB031203 may be representative of low luminosity bursts.

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arXiv2004

The discovery of an evolving dust scattered X-ray halo around GRB 031203

We report the first detection of a time-dependent, dust-scattered X-ray halo around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six hours after the burst. The halo appeared as concentric ring-like structures centered on the GRB location. The radii of these structures increased with time as t^{1/2}, consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.

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arXiv2003

Gamma-Ray Bursts observed by XMM-Newton

Analysis of observations with XMM-Newton have made a significant contribution to the study of Gamma-ray Burst (GRB) X-ray afterglows. The effective area, bandpass and resolution of the EPIC instrument permit the study of a wide variety of spectral features. In particular, strong, time-dependent, soft X-ray emission lines have been discovered in some bursts. The emission mechanism and energy source for these lines pose major problems for the current generation of GRB models. Other GRBs have intrinsic absorption, possibly related to the environment around the progenitor, or possible iron emission lines similar to those seen in GRBs observed with BeppoSAX. Further XMM-Newton observations of GRBs discovered by the Swift satellite should help unlock the origin of the GRB phenomenon over the next few years.

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arXiv2003

The X-ray, optical and radio evolution of the GRB030329 afterglow and the associated SN2003dh

Extensive X-ray, optical and radio observations of the bright afterglow of the Gamma Ray Burst GRB 030329 are used to construct the multi-frequency evolution of the event. The data are fitted using the standard fireball shock model to provide estimates of the initial energy, epsilon = 6.8 x 10^52 ergs sr^-1, the density of the ambient medium, n_0 = 1 cm^-3, the electron and magnetic energy density fractions, epsilon_e = 0.24 & epsilon_B = 0.0017, the power law index of the relativistic electron spectrum, p = 2.25, and the opening angle of the jet, theta_j = 3 degrees. Deviations from the standard model seen in the optical and radio are most likely attributable to the concurrent hypernova SN2003dh. Peaks at 0.23 and 1.7 days in the R-band are much brighter than expected from a standard SN, and there is a large radio excess over the expected afterglow flux for t>2 days. No deviation from the best-fit afterglow model is seen in the X-ray decline, indicating that the excess optical and radio flux from 1-5 days arises from a later injection of slower electrons by the central engine.

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arXiv2002

The mass and energy budget of Cassiopeia A

Further analysis of X-ray spectroscopy results recently obtained from the MOS CCD cameras on-board XMM-Newton provides a detailed description of the hot and cool X-ray emitting plasma in Cas A. Measurement of the Doppler broadening of the X-ray lines is consistent with the expected ion velocities, ~1500 km/s along the line of sight, in the post shock plasma. Assuming a constant total pressure throughout the remnant we estimate the total remnant mass as 10 Msun and the total thermal energy as 7E43 J. We derive the differential mass distribution as a function of ionisation age for both X-ray emitting components. This distribution is consistent with a hot component dominated by swept up mass heated by the primary shock and a cool component which are ablated clumpy ejecta material which were and are still being heated by interaction with the preheated swept up material. We calculate a balanced mass and energy budget for the supernova explosion giving 1E44 J in ejected mass; approximately 0.4 Msun of the ejecta were diffuse with an initial rms velocity of 15000 km/s while the remaining ~1.8 Msun were clumpy with an initial rms velocity of ~2400 km/s. Using the Doppler velocity measurements of the X-ray spectral lines we can project the mass into spherical coordinates about the remnant. This provides quantitative evidence for mass and energy beaming in the supernova explosion. The mass and energy occupy less than 4.5 sr (<40 % of the available solid angle) around the remnant and 64 % of the mass occurs in two jets within 45 degrees of a jet axis. We calculate a swept up mass of 7.9 Msun in the emitting plasma and estimate that the total mass lost from the progenitor prior to the explosion could be as high as ~20 Msun.

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arXiv2001

X-ray spectral imaging and Doppler mapping of Cassiopeia A

A detailed X-ray spectral analysis of Cas A using a deep exposure from the EPIC-MOS cameras on-board XMM-Newton is presented. Spectral fitting was performed on a 15x15 grid of 20"x20" pixels using a two component non-equilibrium ionisation model (NEI) giving maps of ionisation age, temperature, interstellar column density, abundances and Doppler velocities. The abundances of Si, S, Ar and Ca are strongly correlated. The abundance ratios are consistent with the nucleosynthesis yield from the collapse of a 12 Msun progenitor. The abundance ratios Ne/Si, Mg/Si, Fe/Si and Ni/Si are very variable and distinctly different from S/Si, Ar/Si and Ca/Si, in line with the current explosive nucleosynthesis models. The ionisation age and temperature of both NEI components varies considerably over the remnant. Accurate determination of these parameters yield reliable Doppler velocities for both components. The data are consistent with a plasma velocity of 2600 km/s at the shock radius of 153" implying a primary shock velocity of 4000+/-500 km/s. The Si-K and S-K line emission from the cool component is confined to a relatively narrow shell with radius 100-150". This component is almost certainly ejecta material which has been heated by a combination of the reverse shock and heating of ejecta clumps as they plough through the medium which has been pre-heated by the primary shock. The Fe-K line emission is expanding faster and spans a radius range 110-170". The bulk of the Fe emission is confined to two large clumps and it is likely that these too are the result of ablation from ejecta bullets rather swept up circumstellar medium.

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arXiv2000

The Extended X-ray Halo of the Crab-like SNR G21.5-0.9

Recent XMM-Newton observations reveal an extended (150") low-surface brightness X-ray halo in the supernova remnant G21.5-0.9. The near circular symmetry, the lack of any limb brightening and the non-thermal spectral form, all favour an interpretation of this outer halo as an extension of the central synchrotron nebula rather than as a shell formed by the supernova blast wave and ejecta. The X-ray spectrum of the nebula exhibits a marked spectral softening with radius, with the power-law spectral index varying from Gamma = 1.63 +/- 0.04 in the core to Gamma = 2.45 +/- 0.06 at the edge of the halo. Similar spectral trends are seen in other Crab-like remnants and reflect the impact of the synchrotron radiation losses on very high energy electrons as they diffuse out from the inner nebula. A preliminary timing analysis provides no evidence for any pulsed X-ray emission from the core of G21.5-0.9.

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arXiv2000

Cassiopeia A: on the origin of the hard X-ray continuum and the implication of the observed O VIII Ly-alpha/Ly-beta distribution

We present the first results on the hard X-ray continuum image (up to 15 keV) of the supernova remnant Cas A measured with the EPIC cameras onboard XMM-Newton. The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like the bright X-ray knots and filaments or the primary blast wave, but is spread over the whole remnant with a rather flat hardness ratio of the 8-10 and 10-15 keV energy bands. This result does not support an interpretation of the hard X-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hard X-ray emission close to the primary shock front is expected. In fact a weak rim of emission near the primary shock front is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. The equivalent width of the Fe-K line blend varies by more than an order of magnitude over the remnant, it is hard to explain this as Fe-emission from the reverse shock heated ejecta given the ejecta temperature and the age of the remnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the first time, to extract monochromatic images in several highly ionised element species with high spectral resolution. We present here a preliminary result on the measurement of the O VIII Ly-alpha and Ly-beta brightness distribution and brightness ratios. The large observed decrease of the Ly-alpha/Ly-beta ratio going from the N to the SE can be explained by small-scale (10 arcsec) variations in the N_H column over the remnant and the potential presence of resonance scattering of the O VIII Ly-alpha photons in the limb brightened shell.

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arXiv1997

Final performances of the X-ray Mirrors of the JET-X telescope

The Joint European Telesscope (JET-X) is one of the core scientific instruments of the SPECTRUM-X-Gamma mission. JET-X was designed to study X-ray emissions in the band 0.3-10 KeV. Its angular resolution is better than 20 arcsec; the values of the effective areas are 161 cm2 at 1.5 KeV and 69 cm2 at 8 Kev (for one telescope). The value of the HEW circle in function of the off-axis angle is presented; it is found to be below 20 arcsec at 1.5 Kev and around 23 arcsec at 8 KeV up to an off-axis angle of 12 arcmin.

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