ESA Science & Technology - Publication Archive
Context INTEGRAL has two sensitive gamma-ray instruments that have detected and localised 47 gamma-ray bursts (GRBs) from its launch in October 2002 up to July 2007.
Aims. We present the spectral, spatial, and temporal properties of the bursts in the INTEGRAL GRB catalogue using data from the imager, IBIS, and spectrometer, SPI.
Methods. Spectral properties of the GRBs are determined using power-law and, where appropriate, Band model and quasithermal model fits to the prompt emission. Spectral lags, i.e. the time delay in the arrival of low-energy gamma-rays with respect to high-energy gamma-rays, are measured for 31 of the GRBs.
Results. The photon index distribution of power-law fits to the prompt emission spectra is presented and is consistent with that obtained by Swift. The peak flux distribution shows that INTEGRAL detects proportionally more weak GRBs than Swift because of its higher sensitivity in a smaller field of view. The all-sky rate of GRBs above ~0.15 ph/cm²/s is ~ 1400 per year in the fully coded field of view of IBIS. Two groups are identified in the spectral lag distribution of INTEGRAL GRBs, one with short lags <0.75s (between 25-50 keV and 50-300 keV) and one with long lags >0.75s . Most of the long-lag GRBs are inferred to have low redshifts because of their long spectral lags, their tendency to have low peak energies, and their faint optical and X-ray afterglows. They are mainly observed in the direction of the supergalactic plane with a quadrupole moment of Q=-0.225 +/- 0.090 and hence reflect the local large-scale structure of the Universe.
Aims. We present the first albedo determination of 2867 Steins, the asteroid target of the Rosetta space mission together with 21 Lutetia.
Methods.The data were obtained in polarimetric mode at the ESO-VLT telescope with the FORS1 instrument in the V and R filters. Observations were carried out from June to August 2005 covering the phase angle range from 10.3 degrees to 28.3 degrees, allowing the determination of the asteroid albedo by the well known experimental relationship between the albedo and the slope of the polarimetric curve at the inversion angle.
Results. The measured polarization values of Steins are small, confirming an E-type classification for this asteroid, as already suggested from its spectral properties. The inversion angle of the polarization curve in the V and R filters is respectively of 17.3±1.5 degrees and 18.4±1.0 degrees, and the corresponding slope parameter is of 0.037±0.003%/deg and 0.032±0.003%/deg. On the basis of its polarimetric slope value, we have derived an albedo of 0.45±0.1, that gives an estimated diameter of 4.6 km, assuming an absolute V magnitude of 13.18 mag.
Methods. We correlated extended X-ray sources from the second XMM-Newton source catalogue (2XMM) with the SDSS in order to identify new clusters of galaxies. Distant cluster candidates in empty SDSS fields were imaged in the r- and z-bands with the Large Binocular Telescope. We extracted the X-ray spectra of the cluster candidates and fitted thermal plasma models to the data.
Results. We determined the redshift 0.99 ± 0.03 for 2XMM J083026.2+524133 from its X-ray spectrum. With a bolometric luminosity of 1.8 × 1045 erg s-1 this is the most X-ray luminous cluster at redshifts z>1. We measured a gas temperature of 8.2 ± 0.9 keV and estimate a cluster mass M500 = 5.6 × 1014 MSun. The optical imaging revealed a rich cluster of galaxies.