L. Foschini

IBIS/PICsIT in-flight performances

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. PICsIT operates in the gamma-ray energy range between 175 keV and 10 MeV, with a typical energy resolution of 10% at 1 MeV, and an angular resolution of 12 arcmin within a100 square degree field of view, with the possibility to locate intense point sources in the MeV region at the few arcmin level. PICsIT is based upon a modular array of 4096 independent CsI(Tl) pixels,0.70 cm 2 in cross-section and 3 cm thick. In this work, the PICsIT on-board data handling and science operative modes are described. This work presents the in-flight performances in terms of background count spectra, sensitivity limit, and imaging capabilities.

XMM–Newton observations of the ultraluminous nuclear X-ray source in M 33

We present observations with XMM-Newton of M 33 X-8, the ultraluminous X-ray source (L0.5−10 keV ≈ 2×10 39 erg/s) closest to the centre of the galaxy. The best-fit model is similar to the typical model of Galactic black holes in very high state. Comparison with previous observations indicates that the source is still in a very high state after about 20 years of observations. No state transition has been observed even during the present set of XMM-Newton observations. We estimate the lower limit of the mass of the black hole >6 M� , but with proper parameters taking into account different effects, the best estimate becomes 12 M� . Our analysis favours the hypothesis that M 33 X-8 is a stellar mass black hole candidate, in agreement with the findings of other authors. In addition, we propose a different model where the high luminosity of the source is likely to be due to orientation effects of the accretion disc and anisotropies in the Comptonized emission.

INTEGRAL observations of the field of the BL lacertae object S5 0716+714

We have performed observations of the blazar S5 0716+714 with INTEGRAL on 2-6 April 2004. In the first months of 2004, the source had increased steadily in optical brightness and had undergone two outbursts. During the latter outburst, that occurred in March, it reached the extreme level of R = 12.1 mag, which triggered our INTEGRAL program. The target has been detected with IBIS/ISGRI up to 60 keV, with a flux of ∼3 × 10 −11 erg s −1 cm −2 in the 30-60 keV interval, a factor of ∼2 higher than observed by the BeppoSAX PDS in October 2000. In the field of S5 0716+714 we have also detected the Flat Spectrum Radio Quasar S5 0836+710 and the two Seyfert galaxies Mkn 3 and Mkn 6. Their IBIS/ISGRI spectra are rather flat, albeit consistent with those measured by BeppoSAX. In the spectrum of Mkn 3 we find some evidence of a break between ∼60 and ∼100 keV, reminiscent of the high energy cut-offs observed in other Seyfert galaxies. This is the first report of INTEGRAL spectra of weak Active Galactic Nuclei.

BL Lac identification for the ultraluminous X-ray source observed in the direction of NGC 4698

We report the identification of the optical and radio counterparts of the ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC 4698-ULX1). The optical spectrum taken with the VLT yields a redshift of z= 0:43, which implies that the ULX is not associated with the nearby galaxy NGC 4698. The spectral energy distribution calculated from the available data indicates that the source is likely to be a BL Lac object. The possible synchrotron peak at X-ray energies suggests that this source may be a-ray emitter.

Yet another galaxy identification for an ultraluminous X-ray source

We report on the identification of the optical and infrared counterparts of the ultraluminous X-ray source (ULX) XMMU J121214.5+131248 (NGC4168-ULX1). The optical spectrum yields a redshift of z = 0.217, which implies that the ULX is not associated with the nearby galaxy NGC 4168, but rather with a background object. Optical spectral line ratios and the spectral energy distribution constructed from the available data indicate that the source is likely a starburst nucleus.

In-flight calibrations of IBIS/PICsIT

PICsIT (Pixellated Imaging CaeSium Iodide Telescope) is the high energy detector of the IBIS telescope on-board the INTEGRAL satellite. It consists of 4096 independent detection units,0.7 cm 2 in cross-section, operating in the energy range between 175 keV and 10 MeV. The intrinsically low signal to noise ratio in the gamma-ray astronomy domain implies very long observations, lasting 10 5 -10 6 s. Moreover, the image formation principle on which PICsIT works is that of coded imaging in which the entire detection plane contributes to each decoded sky pixel. For these two main reasons, the monitoring, and possible correction, of the spatial and temporal non-uniformity of pixel performances, expecially in terms of gain and energy resolution, is of paramount importance. The IBIS on-board 22 Na calibration source allows the calibration of each pixel at an accuracy of<0.5% by integrating the data from a few revolutions at constant temperature. The two calibration lines, at 511 and 1275 keV, allow also the measurement and monitoring of the PICsIT energy resolution which proves to be very stable at19% and9% (FWHM) respectively, and consistent with the values expected analytical predictions checked against pre-launch tests.

GRB 021125 : The first GRB imaged by INTEGRAL

In the late afternoon of November 25th, 2002 a gamma-ray burst (GRB) was detected in the partially coded field of view (about 7:3 from the centre) of the imager IBIS on board the INTEGRAL satellite. The instruments on-board INTEGRAL allowed, for the first time, the observation of the prompt gamma-ray emission over a broad energy band from 15 to 500 keV. GRB 021125 lasted24 s with a mean flux of 5.0 photons cm 2 s 1 in the 20-500 keV energy band, and a fluence of 4:810 5 erg cm 2 in the same energy band. Here we report the analysis of the data from the imager IBIS and the spectrometer

INTEGRAL observation of 3EG J1736-2908

The possible identification by INTEGRAL of the EGRET source 3EG J1736-2908 with the active galactic nucleus GRS 1734-292 is discussed. The latter was discovered in 1990 and later identified with a Seyfert 1 galaxy. At the time of the compilation of the 3rd EGRET Catalog, it was not considered as a possible counterpart of the source 3EG J1736-2908, which remained unidentified. A detailed multiwavelength study of the EGRET error circle is presented, by including archival radio, soft- and hard-X observations, suggesting that GRS1734-292 could be a likely counterpart of 3EG J1736-2908, even though this poses very interesting questions about the production mechanisms of γ-rays with energies greater than 100 MeV.

The first XMM-Newton study of two Narrow-Line Seyfert 1 galaxies discovered in the Sloan Digital Sky Survey

The Early Data Release of the Sloan Digital Sky Survey (SDSS) contains 150 Narrow-Line Seyfert 1 (NLS1) galaxies, most of them previously unknown. We present here the study of the X-ray emission from two of these active galaxies (SDSS J030639.57 + 000343.2 and SDSS J141519.50−003021.6), based upon XMM-Newton observations. The spectral and timing characteristics of the two sources are presented and compared to the typical properties of known NLS1 galaxies. We found that these two NLS1 are within the dispersion range of the typical values of this class of AGN, although with some interesting features that deserve further studies.

Serpens X-1 observed by INTEGRAL

Here we report results of an INTEGRAL-AO1 observation of the X-ray burst and atoll source Ser X-1 performed in May 2003. The object was observed for a total on-source time of 400 ks but nearly 8 ◦ off-axis due to its amalgamation with an observation of SS 433, the pointing target source. Ser X-1 has been clearly detected up to 30 keV with unprecedented positional accuracy for high-energy emission. The 20-30 keV light curve showed substantial variability during the observation. Comparison with previous observations indicates that the source was in its high (banana) state and displayed a soft spectrum during the INTEGRAL pointing. A (non simultaneous) radio-to-γ-rays broad-band spectral energy distribution is also presented for the first time and discussed.