Michael R. Goad

Bright X-ray Flares in Gamma-Ray Burst Afterglows

Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Gamma-Ray Bursts: Restarting the Engine

Recent gamma-ray burst observations have revealed late-time, highly energetic events that deviate from the simplest expectations of the standard fireball picture. Instead, they may indicate that the central engine is active or restarted at late times. We suggest that fragmentation and subsequent accretion during the collapse of a rapidly rotating stellar core offers a natural mechanism for this.

Very Early Optical Afterglows of Gamma-Ray Bursts: Evidence for Relative Paucity of Detection

Very early observations with the Swift satellite of γ-ray burst (GRB) afterglows reveal that the optical component is not detected in a large number of cases. This is in contrast to the bright optical flashes previously discovered in some GRBs (e.g., GRB 990123 and GRB 021211). Comparisons of the X-ray afterglow flux to the optical afterglow flux and prompt γ-ray fluence is used to quantify the seemingly deficient optical, and in some cases X-ray, light at these early epochs. This comparison reveals that some of these bursts appear to have higher than normal γ-ray efficiencies. We discuss possible mechanisms and their feasibility for explaining the apparent lack of early optical emission. The mechanisms considered include, foreground extinction, circumburst absorption, Lyα blanketing and absorption due to high-redshift, low-density environments, rapid temporal decay, and intrinsic weakness of the reverse shock. Of these, foreground extinction, circumburst absorption, and high redshift provide the best explanations for most of the nondetections in our sample. There is tentative evidence of suppression of the strong reverse shock emission. This could be because of a Poynting flux-dominated flow or a pure nonrelativistic hydrodynamic reverse shock.

The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus.

We have investigated the observational characteristics of a class of broad emission line region (BLR) geometries that connect the outer accretion disc with the inner edge of the dusty toroidal obscuring region (TOR). We suggest that the BLR consists of photoionised gas of densities which allow for efficient cooling by UV/optical emission lines and of incident continuum fluxes which discourage the formation of grains, and that such gas occupies the range of distance and scale height between the continuumemitting accretion disc and the dusty TOR. As a first approximation, we assume a population of clouds illuminated by ionising photons from the central source, with the scale height of the illuminated clouds growing with increasing radial distance, forming an effective surface of a ”bowl”. Observer lines of sight which peer into the bowl lead to a Type 1 Active Galactic Nuclei (AGN) spectrum. We assume the gas dynamics are dominated by gravity, and we include in this model the effects of transverse Doppler shift, gravitational redshift and scale-height dependent macro-turbulence. Our simple model reproduces many of the commonly observed phenomena associated with the central regions of AGN, including : (i) the shorter than expected continuum–dust delays (geometry), (ii) the absence of response in the core of the optical recombination lines on short timescales (geometry/photoionisation), (iii) an enhanced red-wing response on short timescales (GR and TDS), (iv) the observed differences between the delays for high- and low-ionisation lines (photoionisation), (v) identifying one of the possible primary contributors to the observed line widths for near face-on systems even for purely transverse motion (GR and TDS), (vi) a mechanism responsible for producing Lorentzian profiles (especially in the Balmer and Mg ii emission-lines) in low inclination systems (turbulence), (vii) the absence of significant continuum–emission-line delays between the line wings and line core (turbulence; such time-delays are weak for virialised motion, and turbulence serves to reduce any differences which may be present), (viii) associating the boundary between population A and population B sources (Sulentic et al. 2000) as the cross-over between inclination dependent (population A) and inclination independent (population B) line profiles (GR+TDS), (ix) provides a partial explanation of the differences between the emission-line profiles, here explained in terms of their line formation radius (photoionisation and/or turbulence), and (x) the unexpectedly high (but necessary) covering fractions (geometry). A key motivation of this work was to reveal the physical underpinnings of the reported measurements of supermassive black hole (SMBH) masses and their uncertainties. We have driven our model with simulated continuum light-curves in order to determine the virial scale factor f, from measurements of the simulated continuum– emission-line delay, and the width (fwhm, σl) and shape (fwhm/σl) of the rms and mean line profiles for the energetically more important broad UV and optical recombination lines used in SMBH mass determinations. We thus attempt to illuminate the physical dependencies of the empirically determined value of f. We find that SMBH masses derived from measurements of the fwhm of the mean and rms profiles show the closest correspondence between the emission lines in a single object, even though the emission line fwhm is a more biased mass indicator with respect to inclination. The predicted large discrepancies in the SMBH mass estimates between emission lines at low inclination, as derived using σl, we suggest may be used as a means of identifying near face-on systems. Our general results do not depend on specific choices in the simplifying assumptions, but are in fact generic properties of BLR geometries with axial symmetry that span a substantial range in radially-increasing scale height supported by turbulence, which then merge into the inner dusty TOR.

A deep XMM-Newton observation of the ultraluminous X-ray source Holmberg II X-1: the case against a 1000-M⊙ black hole

We present results from a 112 ks long look by XMM-Newton at the ultraluminous X-ray source (ULX) Holmberg II X-1, long thought to be the one of best candidates for the missing class of intermediate mass black holes (IMBHs). Our data comprises the first high quality XMM-Newton/RGS spectrum of an ULX, and an XMM-Newton/EPIC spectrum with unprecedented signal-to-noise. A detailed timing analysis shows that any variability on time-scales of minutes to hours is very weak (< few per cent fractional rms), though larger amplitude variations on much shorter time-scales could be hidden by photon counting statistics. This result suggests that if Ho II X-1 harbours an IMBH, then we are observing this source in a highly unusual and atypical state when compared with the known variability behaviour of other accreting systems of large mass. Moreover unlike Galactic X-ray binaries, our spectral analysis indicates the possible presence of an optically-thick low temperature corona. Taken together our timing and spectral analysis suggests that the compact companion is most-likely a high luminosity analogue of black hole binary systems similar to GRS 1915+105, the Galactic microquasar, harbouring a compact object of mass no greater than 100 solar masses.

The identification of an optical counterpart to the super-Eddington X-ray source, NGC 5204 X-1

We report the identification of a possible optical counterpart to the super-Eddington X-ray source NGC 5204 X-1. New CHANDRA data shows that the X-ray source is point-like, with a luminosity of 5.2× 10 39 erg s 1 (0.5 - 8 keV). It displays mediumand long-term X-ray variability in observations spanning a period of 20 years. The accurate CHANDRA position allows us to identify a blue optical continuum source (mv = 19.7) at the position of NGC 5204 X-1, using newly-obtained optical data from the INTEGRAL instrument on the William Herschel Telescope. The X-ray and optical source properties are consistent with the scenario in which we are observing the beamed X-ray emission of a high-mass X-ray binary in NGC 5204, composed of an O star with either a black hole or neutron star companion.

The Next Generation Transit Survey (NGTS)

The Next Generation Transit Survey (NGTS) is a new ground-based sky survey designed to find transiting Neptunes and super-Earths. By covering at least sixteen times the sky area of Kepler , we will find small planets around stars that are sufficiently bright for radial velocity confirmation, mass determination and atmospheric characterisation. The NGTS instrument will consist of an array of twelve independently pointed 20 cm telescopes fitted with red-sensitive CCD cameras. It will be constructed at the ESO Paranal Observatory, thereby benefiting from the very best photometric conditions as well as follow up synergy with the VLT and E-ELT. Our design has been verified through the operation of two prototype instruments, demonstrating white noise characteristics to sub-mmag photometric precision. Detailed simulations show that about thirty bright super-Earths and up to two hundred Neptunes could be discovered. Our science operations are due to begin in 2014.

Swift XRT Observations of the Afterglow of GRB 050319

Swift discovered the high-redshift GRB 050319 with the Burst Alert Telescope (BAT) and began observing with its narrow-field instruments only 225 s after the burst onset. The afterglow X-ray emission was monitored by the XRT up to 28 days after the burst. The light curve shows a decay with three different phases, each characterized by a distinct slope: an initial steep decay with a power-law index of ~5.5, a second phase characterized by a flat decay slope of ~0.54, and a third phase with a decay slope of ~1.14. During the first phase the spectral energy distribution is softer than in the following two phases, and the photon index is consistent with the GRB prompt spectrum. The extrapolation of the BAT light curve to the XRT band suggests that the initial fast-decaying phase of the XRT afterglow might be the low-energy tail of the prompt emission. The second break in the afterglow light curve occurs about 27,000 s after the burst. The spectral energy distribution before and after the second break does not change, and it can be tentatively interpreted as a jet break or the end of a delayed or continuous energy injection phase.

The nature of the close magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7★

Submitted for publication in Monthly Notices of the Royal Astronomical Society by the Royal Astronomical Society and Blackwell Publishing.