D. C. Hannikainen

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.

The 1998 outburst of xte j1550-564: a model based on multiwavelength observations

The 1998 September outburst of the black hole X-ray binary XTE J1550-564 was monitored at X-ray, optical, and radio wavelengths. We divide the outburst sequence into five phases and discuss their multiwavelength properties. The outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a longer timescale. We suggest that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is determined by the distribution of specific angular momentum in the accreting matter. The companion in XTE J1550-564 is likely to be an active magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. We suggest that its magnetic field can create a magnetic bag capable of confining gas inside the Roche lobe of the primary. The impulsive rise in the hard X-rays is explained by the inflow of material with low angular momentum onto the black hole, on a free-fall timescale, when the magnetic confinement breaks down. At the same time, high angular momentum matter, transferred via ordinary Roche lobe overflow, is responsible for the formation of a disk.

INTEGRAL SPI limits on electron-positron annihilation radiation from the galactic plane

The center of our Galaxy is a known strong source of electron-positron 511 keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched in 2002 October, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high-resolution, coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution, and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic plane. No positive annihilation flux was detected outside of the central region ( l > 40°) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511 keV flux.

XTE J1550–564: a superluminal ejection during the September 1998 outburst

In 1998 September, the X-ray transient XTE J1550–564 underwent amajor outburst in soft and hard X-rays, followed by a radio flare. Australian Long Baseline Array imagesobtained shortly after the peak in the radio flare showed evolving structure.The components observed have an apparent separation velocity of >2c.

An ultracompact x-ray binary in the Globular Cluster M15 (NGC 7078)

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) wave band. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semiamplitude, and we clearly detect a period of 22.5806+/-0.0002 minutes. We have carried out extensive Monte Carlo simulations, which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV-bright (FUV~=17 mag) and is characterized by an extremely blue spectral energy distribution (Flambda~lambda-2.0). We also find evidence for an excess of flux between 1500 and 1600 A and probably between 1600 and 2000 A that might be due to C IV 1550 A and He II 1640 A emission lines. We also show that M15 X-2s X-ray luminosity can be powered by accretion at the rate expected for mass transfer driven by gravitational waves at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is ~34°. We conclude that all observational characteristics of M15 X-2 are consistent with its being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.

Simultaneous multi-wavelength observations of GRS 1915+105

We present the result of multi-wavelength observations of the microquasar GRS 1915+105 in a plateau state with a luminosity of

The hardness–intensity diagram of Cygnus X-3: revisiting the radio/X-ray states

{\sim}7.5\times10^{38}

Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations

 erg s -1 (~40%

Spectral variability in Cygnus X-3

L_\mathrm{Edd}

The nature of the hard state of Cygnus X‐3

), conducted simultaneously with the INTEGRAL and RXTE satellites, the ESO/NTT, the Ryle Telescope, the NRAO VLA and VLBA, in 2003 April 2–3. For the first time were observed concurrently in GRS 1915+105 all of the following properties: a strong steady optically thick radio emission corresponding to a powerful compact jet resolved with the VLBA, bright near-IR emission, a strong QPO at 2.5 Hz in the X-rays and a power law dominated spectrum without any cutoff in the 3–400 keV range.