D. R. Willis

INTEGRAL observations of recurrent fast X-ray transient sources

Fast X-ray Transients (FXTs) are believed to be non-recurrent bright X-ray sources lasting less than a day and occuring at serendipitous positions, they can best be detected and discovered by instruments having a sufficiently wide field of view and high sensitivity. The IBIS/ISGRI instrument onboard INTEGRAL is particularly suited to detect new or already known fast X-ray transient sources. We report on IBIS/ISGRI detection of newly discovered outbursts of three fast transient sources located at low Galactic latitude: SAX J1818.6−1703; IGR J16479−4514; IGR J17391−302/XTE J1739−302. The reported results confirm and strengthen the very fast transient nature of these sources, given that all their newly detected outbursts have a duration less than ∼3 h. Additionally, they provide the first evidence for a possible recurrent fast transient behaviour as all three sources were detected in outburst by ISGRI more than once during the last 2 years.

Cosmic X-ray background and Earth albedo Spectra with Swift/BAT

We use Swift/BAT Earth occultation data at different geomagnetic latitudes to derive a sensitive measurement of the Cosmic X-ray background (CXB) and of the Earth albedo emission in the 15–200keV band. We compare our CXB spectrum with recent (INTEGRAL, BeppoSAX) and past results (HEAO-1) and find good agreement. Using an independent measurement of the CXB spectrum we are able to confirm our results. This study shows that the BAT CXB spectrum has a normalization � 8 ± 3% larger than the HEAO-1 measurement. The BAT accurate Earth albedo spectrum can be used to predict the level of photon background for satellites in low Earth and mid inclination orbits.

A description of sources detected by INTEGRAL during the first 4 years of observations

Context. In its first 4 years of observing the sky above 20 keV, INTEGRAL-ISGRI has detected 500 sources, around half of which are new or unknown at these energies. Follow-up observations at other wavelengths revealed that some of these sources feature unusually large column densities, long pulsations, and other interes ting characteristics. Aims. We investigate where new and previously-known sources detected by ISGRI fit in the parameter space of high-energy object s, and we use the parameters to test correlations expected from theoretical predictions. For example, the influence of the l ocal absorbing matter on periodic modulations is studied for Galactic High-Mass X-ray Binaries (HMXBs) with OB supergiant and Be companions. We examine the spatial distribution of different types of sources in the Milky Way using various projections of the Galactic plane, in order to highlight signatures of stellar evolution and to sp eculate on the origin of the group of sources whose classifica tions are still uncertain. Methods. Parameters that are available in the literature, such as pos itions, photoelectric absorption ( NH), spin and orbital periods, and distances or redshifts, were collected for all sources d etected by ISGRI. These values and their references are provided online. Results. ISGRI has detected similar numbers of X-ray Binaries and Active Galactic Nuclei (AGN). The former group contains new members of the class of HMXBs with supergiant stellar companions. Usually, this type of object presents strong intrinsi c absorption which leads to a peak emission in an energy range that ISGRI is ideally suited to detect. Thanks to these additional system s, we are able to show that HMXBs are generally segregated in plots of intrinsic NH versus the orbital period of the system and versus the spin period of the pulsar, based on whether the companion is a Be or an OB supergiant star. We also find a tentative but expected an ticorrelation between NH and the orbital period, and a possible and unexpected correlation between the NH and the spin period. While only a handful of new Low-Mass X-ray Binaries (LMXBs) have been discovered, there are many sources that remain unclassifi ed and they appear to follow a spatial distribution typical of Gala ctic sources (especially LMXBs) rather than extragalactic sources.

Evidence of polarisation in the prompt gamma-ray emission from GRB 930131 and GRB 960924

The true nature of the progenitor to GRBs remains elusive; one characteristic that would constrain our understanding of the GRB mechanism considerably is gamma-ray polarimetry measurements of the initial burst flux. We present a method that interprets the prompt GRB flux as it Compton scatters off the Earths atmosphere, based on detailed modelling of both the Earths atmosphere and the orbiting detectors. The BATSE mission aboard the CGRO monitored the whole sky in the 20keV-1 MeV energy band continuously from April 1991 until June 2000. We present the BATSE Albedo Polarimetry System (BAPS), and show that GRB 930131 and GRB 960924 provide evidence of polarisation in their prompt flux that is consistent with degrees of polarisation of Π > 35% and n > 50% respectively. While the evidence of polarisation is strong, the method is unable to strongly constrain the degree of polarisation beyond a systematics based estimation. Hence the implications on GRB theory are unclear, and further measurements essential.

Global characteristics of the first IBIS/ISGRI catalogue sources: unveiling a murky episode of binary star evolution

INTEGRAL is the first gamma-ray astronomy mission with a sufficient sensitivity and angular resolution combination appropriate to the detection and identification of considerable numbers of gamma-ray emitting sources. The large field of view (∼30 ◦ zero response FWHM) enables INTEGRAL to survey the galactic plane on a regular (∼weekly) basis as part of the core programme. The first source catalogue, based on the 1st year of core programme data (∼5 Ms) has been completed and published (Bird et al. 2004, ApJ, 607, L33). It contained 123 γ-ray sources (24 HMXB, 54 LMXB, 28 “unknown”, plus 17 others) – sufficient numbers for a reasonable statistical analysis of their global properties. These were located to a positional accuracy of typically 0.72 arcmin. The detection of previously unknown γ-ray emitting sources generally exhibiting high intrinsic absorption, which do not have readily identifiable counterparts at other wavelengths, is intriguing. The substantial fraction (roughly 20% of the total number) of unclassified γ-ray sources suggests they must constitute a significant family of objects. In this paper we review the global characteristics of the known galactic sources as well as the unclassified objects with the twin aims of investigating how the unclassified set may fit into stellar evolution and improving our understanding of known X-ray binary systems through the non-thermal γ-ray channel. In the context of the known systems we are very conscious that they constitute a γ-ray selected set, and may exhibit subtle generic differences to the rest of the class. We present Log(N)–Log(S ) distributions, angular distributions, and for systems with reliable distance estimates the spatial distributions within the Galaxy and luminosity functions. For the unknown sources, this statistical analysis has shown that they are most likely to be HMXBs containing a highly magnetised neutron star. The lack of X-ray counterparts for these sources indicates a high degree of intrinsic obscuration.

The Modelling of Background Noise in Astronomical Gamma Ray Telescopes

Gamma ray photons interact with matter through a wide variety of complex physical mechanisms, which can be readily imitated by other particle processes. Unfortunately since γ-ray telescopes are obliged to function in a hostile radiation environment above the earths atmosphere the net result is low signal to noise observations and a corresponding loss in sensitivity. Consequently, understanding the generation of the systematic background noise is crucial if the full performance of a γ-ray instrument is to be realised. In the past it was not possible to reliably estimate the background levels in a fully quantitative manner; semi-empirical methods were employed. Although the basic underlying sources of the background noise were reasonably well understood, and the spectral intensities of the associated particles were reasonably well known, it was not possible to associate resultant noise components with the input source of that noise with any real degree of accuracy. The advent of sophisticated and accurate computer programmes capable of dependably representing the requisite particle physics processes and interactions coupled with the advances made in the context of high power/low cost computers has revolutionised the situation. The so-called ‘mass modelling’ technique is a truly physics-based approach, which takes the input particle spectra of the local radiation environment together with a computer representation of the mechanical structure and chemical composition of the instrumentation and associated spacecraft to trace the trajectories and interactions of all the incident particles throughout the system. All energy deposits from the various interactions and likewise those from the prompt and delayed secondary products are also accurately recorded.Subsequent energy discriminators and time coincidences can be applied to the event arrays with additional software to simulate on-board electronics systems. Internal spectral counting rates may be readily derived and analysed in terms of a wide variety of desired purposes. The impact of an accurate physics-based mass modelling technique has been to expand application of the procedure to effectively all the main aspects of a space γ-ray astronomy mission: instrument design; internal counting rates and spectral sensitivity estimates; optimisation of the design of the on-board processing electronics; operational planning and mission optimisation; estimation of radiation damage and its limitation; calibration planning and interpretation; the production of accurate instrumental response matrices; data analysis software; normalisation of astronomical results across instrument and instrumental degradation boundaries; and data archiving.

The INTEGRAL Mass Model – TIMM

The INTEGRAL Mass Model (TIMM) was started in 1995 and aimed to create a detailed geometrical model of the whole INTEGRAL satellite on computer. In parallel, a comprehensive Monte Carlo simulation code (called GGOD) has been developed. The mass model and the Monte Carlo code together enable the in-flight operation of INTEGRAL to be simulated at the individual event level. Thus TIMM can be used to provide an independent evaluation of the performance of the individual instruments, to study the interference and complementarity between instruments, to generate test data for software development, and as a powerful tool for post-launch diagnosis. In this paper TIMM is briefly reviewed, some examples from ground calibration are presented, and preliminary comparison to flight data is shown. The future use of TIMM to flat field flight data is also briefly discussed.

Detection and analysis of a new INTEGRAL hard X-ray transient, IGR J17285-2922

IASF/CNR Section of Bologna, Via P. Gobetti 101, 40129 Bologna, Italythe date of receipt and acceptance should be inserted laterAbstract.We present the transient nature of INTEGRAL source IGR J17285–2922, identified from a single periodof activity during an IBIS/ISGRI Galactic Centre Deep Exposure in September 2003. The source has a maximumdetection significance of 14σ in the 20–100 keV energy range and exhibits a flux of 6.5 mCrab before it moves outof the ISGRI field of view. The source is visible to at least 150 keV and its spectrum can be fit with a power lawslope of Γ = 2.1±0.17; a more physical model could not be fit due to poor statistics. Detected characteristics areconsistent with the source being a Galactic low-mass X-ray binary harbouring a black hole or neutron star.Key words. gamma-rays:observations–surveys, X-rays:individuals:IGR J17285-2922–X-rays:binaries

The IBIS/ISGRI Survey of the Galactic Plane - Global Characteristics of the Gamma-Ray Sources

INTEGRAL is the first gamma-ray astronomy mission with a sufficient sensitivity and angular resolution combination appropriate to the detection and identification of considerable numbers of gamma-ray emitting sources. The large field of view enables INTEGRAL to survey the Galactic Plane on a regular (

The Second IBIS/ISGRI Soft Gamma-Ray Survey Catalog

\sim