Nigel P. Bannister

The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs

We present a multiwavelength analysis of Swift GRB 061007. The 2 m robotic Faulkes Telescope South began observing 137 s after the onset of the γ-ray emission, when the optical counterpart was already decaying from R ~ 10.3 mag, and continued observing for the next 5.5 hr. These observations begin during the final γ-ray flare and continue through and beyond a long, soft tail of γ-ray emission whose flux shows an underlying simple power-law decay identical to that seen at optical and X-ray wavelengths, with temporal slope α ~ 1.7 (F ∝ t-α). This remarkably simple decay in all of these bands is rare for Swift bursts, which often show much more complex light curves. We suggest the afterglow emission begins as early as 30-100 s and is contemporaneous with the ongoing variable prompt emission from the central engine, but originates from a physically distinct region dominated by the forward shock. The observed multiwavelength evolution of GRB 061007 is explained by an expanding fireball whose optical, X-ray, and late-time γ-ray emission is dominated by emission from a forward shock with typical synchrotron frequency, νm, that is already below the optical band as early as t = 137 s and a cooling frequency, νc, above the X-ray band to at least t = 105 s. In contrast, the typical frequency of the reverse shock lies in the radio band at early time. We suggest that the unexpected lack of bright optical flashes from the majority of Swift GRBs may be explained with a low νm originating from small microphysics parameters, e and B. Finally, the optical light curves imply a minimum jet opening angle θ = 4.7°, and no X-ray jet break before t ~ 106 s makes GRB 061007 a secure outlier to spectral energy correlations.

LOBSTER-ISS: an imaging x-ray all-sky monitor for the International Space Station

We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.

Heavy-element abundance patterns in hot DA white dwarfs

We present a series of systematic abundance measurements for 25 hot DA white dwarfs in the temperature range ∼20 000‐110 000 K, based on far-ultraviolet spectroscopy with the Space Telescope Imaging Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS) onboard Hubble Space Telescope, IUE and FUSE. Using our latest heavy-element blanketed non-local thermodynamic equilibrium (non-LTE) stellar atmosphere calculations we have addressed the heavy-element abundance patterns, making completely objective measurements of abundance values and upper limits using a χ 2 fitting technique to determine the uncertainties in the abundance measurements, which can be related to the formal upper limits in those stars

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.

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.

Circumstellar features in hot DA white dwarfs

We present a phenomenological study of highly ionised, non-photospheric absorption features in high spectral resolution vacuum ultraviolet spectra of 23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige 24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these features. We find four new objects with multiple components in one or more of the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD 2218+706. A fifth object, REJ 2156-546 also shows some evidence of multiple components, though further observations are required to confirm the detection. We discuss possible origins for these features including ionisation of the local interstellar environment, the presence of material inside the gravitational well of the white dwarf, mass loss in a stellar wind, and the existence of material in an ancient planetary nebula around the star. We propose ionisation of the local interstellar medium as the origin of these features in G191-B2B and REJ 1738+665, and demonstrate the need for higher resolution spectroscopy of the sample, to detect multiple ISM velocity components and to identify circumstellar features which may lie close to the photospheric velocity.

PG 1115+166 – a long‐period DA+DB binary

We present spectra of the DAB white dwarf PG 1115+166. Radial velocity measurements of the Balmer lines and the He i 6678 A line show that this is a binary white dwarf with a period of 30.09 d in which the Balmer lines move in anti-phase to the He i line, i.e. PG 1115+166 is a DA+DB binary. The minimum masses of the stars are MDA= 0.43 ± 0.15 M⊙ and MDB= 0.52 ± 0.12 M⊙. The separation of the stars is about 45 R⊙, which is much smaller than a typical asymptotic giant branch (AGB) progenitor of a white dwarf, implying that there has been at least one common envelope phase in this binary. Indeed, it is possible that this binary may have suffered up to three mass transfer episodes – two associated with the red giant phase prior to the formation of each white dwarf, and a third associated with the ‘born-again’ red giant phase of the DB white dwarf. PG 1115+166 has the longest orbital period of any post common envelope white dwarf–white dwarf binary found to date. Published models for the formation of white dwarf–white dwarf binaries do not predict any white dwarfs with the combination of a long orbital period and high mass found in PG 1115+166. We conclude that PG 1115+166 is a key object for testing models of binary star evolution, and it may also be a key object for our understanding of the formation of DB white dwarfs. We outline the observational tests which can be applied to scenarios for the formation of PG 1115+166 and apply them to the simplest case of a single common-envelope phase. This suggests that some part of the internal energy stored in the envelope of the AGB star, e.g. as ionized hydrogen, may have contributed to the ejection of the common envelope, but there are several unanswered questions concerning this simple scenario.

The ages of L dwarfs

We present a new method to derive the age of young (<0.7 Gyr) L dwarfs based on their near-infrared photometry, colours and distances. The method is based on samples of L dwarfs belonging to the Upper Sco association (5 Myr), the Alpha Per (85 Myr) and Pleiades (125 Myr) clusters, and the Ursa Major (400 Myr) and Hyades (625 Myr) moving groups. We apply our method to a number of interesting objects in the literature, including a known L dwarf binary, L dwarf companions and spectroscopic members of the young σ Orionis cluster.

Far-ultraviolet spectroscopy of the hot DA white dwarf WD 2218+706 (DeHt5) with STIS

We present the first evidence for the direct detection of nickel in the photosphere of the hot DO white dwarf REJ0503−289. While this element has been seen previously in the atmospheres of hot H-rich white dwarfs, this is one of the first similar discoveries in a He-rich object. Intriguingly, iron, which is observed to be more abundant than Ni in the hot DA stars, is not detected, the upper limit to its abundance (Fe/He= 10−6) implying a Fe/Ni ratio a factor 10 lower than seen in the H-rich objects (Ni/He= 10−5 for REJ0503−289). The abundance of nickel and various other elements heavier than He were determined from GHRS spectra. We used two completely independent sets of NLTE model atmospheres which both provide the same results. This not only reduces the possibility of systematic errors in our analysis but is also an important consistency check for both model atmosphere codes. We have also developed a more objective method of determining Teff and log g, from the He lines in the optical spectrum, in the form of a formal fitting of the line profiles to a grid of model spectra, an analogue of the standard procedure utilising the Balmer lines in DA white dwarfs. This gives the assigned uncertainties in Teff and log g a firm statistical basis and allows us to demonstrate that inclusion of elements heavier than H, He and C in the spectral calculations, exclusively considered in most published optical analyses, yields a systematic downward shift in the measured value of Teff .

High-Resolution Spectroscopy of G191-B2B in the Extreme-Ultraviolet

We report a high-resolution (R = 3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme-ultraviolet band 220-245 A. A low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.