arXiv subjects

P. Ubertini

Publications and source records attributed to P. Ubertini.

At least 181 records · Page 10Linked to original sources

arXiv2005

The 1 - 50 keV spectral and timing analysis of IGR J18027-2016: an eclipsing, high mass X-ray binary

We report the association of the INTEGRAL source IGR J18027-2016 with the BeppoSAX source SAX J1802.7-2017. IGR J18027-2016 is seen to be a weak, persistent source by the IBIS/ISGRI instrument on board INTEGRAL with an average source count rate of 0.55 counts s^-1 (~6.1 mCrab) in the 20-40 keV band. Timing analysis performed on the ISGRI data identifies an orbital period of 4.5696 +/- 0.0009 days and gives an ephemeris of mid-eclipse as, T{mid} = 52931.37 +/- 0.04 MJD. Re-analysis of archival BeppoSAX data has provided a mass function for the donor star, f(m) = 16 +/- 1 M{sun} and a projected semimajor axis of a{x}sin{i} = 68 +/- 1 lt-s. We conclude that the donor is an OB-supergiant with a mass of 18.8-29.3 M{sun} and a radius of 15.0-23.4 R{sun}. Spectra obtained by XMM-Newton and ISGRI indicate a high hydrogen column density of NH = 6.8 x 10^22 cm^-2, which suggests intrinsic absorption. The source appears to be a high mass X-ray binary with the neutron star emitting X-rays through wind-fed accretion while in an eclipsing orbit around an OB-supergiant.

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arXiv2005

Using the ROSAT Bright Source Catalogue to find Counterparts for IBIS/ISGRI Survey Sources

The IBIS/ISGRI first year galactic plane survey has produced a catalogue containing 123 hard X-ray sources visible down to a flux limit of a few milliCrabs. The point source location accuracy of typically 1-3 arcminutes has allowed the counterparts for 95 of these sources to be found at other wavelengths. In order to identify the remaining 28 objects, we have cross-correlated the ISGRI catalogue with the ROSAT All Sky Survey Bright Source Catalogue. In this way, for ISGRI sources which have a counterpart in soft X-rays, we can use the, much smaller, ROSAT error box to search for identifications. As expected, we find a strong correlation between the two catalogues and calculate that there are 75 associations with the number expected by chance to be almost zero. Of these 75 sources, ten are in the list of unidentified objects. Using the ROSAT error boxes we provide tentative associations for 8 of these.

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arXiv2005

Discovery of the INTEGRAL X/Gamma-ray transient IGR J00291+5934: a Comptonised accreting ms pulsar ?

We report the discovery of a high-energy transient with the IBIS/ISGRI detector on board the INTEGRAL observatory. The source, namely IGR J00291+5934, was first detected on 2nd December 2004 in the routine monitoring of the IBIS/ISGRI 20--60 keV images. The observations were conducted during Galactic Plane Scans, which are a key part of the INTEGRAL Core Programme observations. After verifying the basic source behaviour, the discovery was announced on 3rd December. The transient shows a hard Comptonised spectrum, with peak energy release at about 20 keV and a total luminosity of ~ 0.9E36 erg/s in the 5--100 keV range, assuming a distance of 3 kpc. Following the INTEGRAL announcement of the discovery of IGR J00291+5934, a number of observations were made by other instruments. We summarise the results of those observations and, together with the INTEGRAL data, identifiy IGR J00291+5934 as the 6th member of a class of accreting X-ray millisecond pulsars.

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arXiv2005

INTEGRAL/IBIS search for e-e+ annihilation radiation from the Galactic Center Region

Electron-positron annihilation radiation from the Galactic Center region has been detected since the seventies, but its astrophysical origin is still a topic of a scientific debate. We have analyzed data of the gamma-ray imager IBIS/ISGRI onboard of ESA's INTEGRAL platform in the e$^{-}$e$^{+}$ line. During the first year of the missions Galactic Center Deep Exposure no evidence for point sources at 511 keV has been found in the ISGRI data; the $2 σ$ upper limit for resolved single point sources is estimated to be $1.6\times 10^{-4} ph cm^{-2} s^{-1}$.

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arXiv2004

GRB 040403: a faint X-ray rich Gamma-ray Burst discovered by INTEGRAL

GRB 040403 is one of the faintest gamma-ray bursts for which a rapid and accurate localization has been obtained. Here we report on the gamma-ray properties of this burst, based on observations with the IBIS instrument aboard INTEGRAL, and the results of searches for its optical afterglow. The steep spectrum (power law photon index = 1.9 in the 20-200 keV range) implies that GRB 040403 is most likely an X-ray rich burst. Our optical limit of R > 24.2 at 16.5 hours after the burst, indicates a rather faint afterglow, similar to those seen in other relatively soft and faint bursts.

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arXiv2004

3-200 keV spectral states and variability of the INTEGRAL Black Hole binary IGR J17464-3213

On March 2003, IBIS, the gamma-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be a HEAO-1 transient, H1743-322. In this paper we report on the high energy behaviour of this BHC studied with the three main instruments onboard INTEGRAL. The data, collected with unprecedented sensitivity in the hard X-Ray range, show a quite hard Comptonised emission from 3 keV up to 150 keV during the rising part of the source outburst, with no thermal emission detectable. A few days later, a prominent soft disk multicolour component appears, with the hard tail luminosity almost unchanged: 10-9 erg*cm-2*s-1. Two months later, during a second monitoring campaign near the end of the outburst, the observed disk component was unchanged. Conversely, the Comptonised emission from the central-hot part of the disk reduced by a factor of 10. We present here its long term behaviour in different energy ranges and the combined JEM-X, SPI and IBIS wide band spectral evolution of this source.

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arXiv2004

1E 1740.7-2942: temporal and spectral evolution from INTEGRAL and RXTE observations

We present results of the monitoring of the black hole candidate 1E 1740.7-2942 with INTEGRAL, in combination with simultaneous observations by RXTE. We concentrate on broad-band spectra from INTEGRAL/IBIS and RXTE/PCA instruments. During our observations, the source spent most of its time in the canonical low/hard state with the measured flux variation within a factor of two. In 2003 September the flux started to decline and in 2004 February it was below the sensitivity level of the INTEGRAL and RXTE instruments. Notably, during the decline phase the spectrum changed, becoming soft and typical of black hole binaries in the intermediate/soft state.

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arXiv2004

Spectral states and transient behaviour of a sample of X-ray bursters observed by BeppoSax

During observation campaigns of the Galactic Bulge region, BeppoSAX detected a total of 21 new X-ray bursters in about 6 years of operation. These sources are mostly transient and often feature a hard X-ray spectrum, extending up to ~ 200 keV. A hard spectrum is generally found in weak, relatively short outbursts. On the other hand two sources, SAX J1747.0-2853 and SAX J1750.8-2900, have been seen with soft X-ray transient behaviour. In both low/hard and high/soft state, a thermal component is observed at energies below \~10 keV and the spectra are compatible with the same model consisting of a blackbody or disk blackbody plus a hard or Comptonized component. Light curves are characterized by either weak, short flares or longer, high luminosity eruptions with exponential decay.

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arXiv2004

High-energy observations of the state transition of the X-ray nova and black hole candidate XTE J1720-318

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton and RXTE. The source, which underwent an X-ray outburst in 2003 January, was observed in February in a spectral state dominated by a soft component with a weak high-energy tail. The XMM-Newton data provided a high column density Nh of 1.2*e22 cm^{-2} which suggests that the source lies at the Galactic Centre distance. The simultaneous RXTE and INTEGRAL Target of Opportunity observations allowed us to measure the weak and steep tail, typical of a black-hole binary in the so-called High/Soft State. We then followed the evolution of the source outburst over several months using the INTEGRAL Galactic Centre survey observations. The source became active again at the end of March: it showed a clear transition towards a much harder state, and then decayed to a quiescent state after April. In the hard state, the source was detected up to 200 keV with a power law index of 1.9 and a peak luminosity of 7*e36 erg s^{-1} in the 20-200 keV band, for an assumed distance of 8 kpc. We conclude that XTE J1720-318 is indeed a new member of the black hole X-ray novae class which populate our galactic bulge and we discuss its properties in the frame of the spectral models used for transient black hole binaries.

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arXiv2004

The INTEGRAL View of the Galactic Nucleus

We present the preliminary results of the observational campaign performed in 2003 to study the Galactic Nucleus with INTEGRAL. The mosaicked images obtained with the IBIS/ISGRI coded aperture instrument in the energy range above 20 keV, give a yet unseen view of the high-energy sources of this region in hard X and gamma-rays, with an angular resolution of 12'. We report on the discovery of a source, IGR J17456-2901, compatible with the instrument's point spread function and coincident with the Galactic Nucleus Sgr A* to within 0.9'. The source is visible up to 60-80 keV with a 20-100 keV luminosity at 8 kpc of 3 x 10E35 erg/s. Although we cannot unequivocally associate the new INTEGRAL source to the Galactic Nucleus, this is the first report of significant hard X-ray emission from within the inner 10' of the Galaxy and a contribution from the galactic center supermassive black hole itself cannot be excluded. Here we discuss the results obtained and the perspectives for future observations of the Galactic Nucleus with INTEGRAL and other observatories

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arXiv2004

Detection of hard X-ray emission from the Galactic nuclear region with INTEGRAL

This letter presents the first results of an observational campaign to study the Galactic Centre with INTEGRAL. The mosaicked images obtained with the IBIS/ISGRI coded aperture instrument in the energy ranges 20-40 and 40-100 keV, give a yet unseen view of the high-energy sources of this region in hard X- and gamma-rays with an angular resolution of 12 arcmin (FWHM). We report on the discovery of a source, IGR J17456-2901, coincident with the Galactic Nucleus SgrA* to within 0.9 arcmin. Located at R.A.(J2000.0) = 17h45m38.5s, Dec.(J2000.0) = -29:01:15, the source is visible up to about 100 keV with a 20-100 keV luminosity at 8 kpc of (2.89 +/- 0.41) x 10^35 ergs/s. Although the new INTEGRAL source cannot unequivocally be associated to the Galactic Nucleus, this is the first report of significant hard X-ray emission from within the inner 10 arcmin of the Galaxy and a contribution from the galactic supermassive black hole itself cannot be excluded.

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arXiv2004

INTEGRAL monitoring of the Black Hole candidate 1E 1740.7-2942

The brightest persistent Galactic black hole candidate close to the Galactic Centre, 1E 1740.7-2942, has long been observed with INTEGRAL. In this paper, we report on the long-term hard X-ray monitoring obtained during the first year of observations as part of the Galactic Centre Deep Exposure. We discuss the temporal and spectral behaviours in different energy bands up to 250 keV, as well as the hardness-flux correlations.

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arXiv2004

The INTEGRAL view of the Soft Gamma-ray Repeater SGR 1806-20

We present the results obtained by INTEGRAL on the Soft-Gamma Ray Repeater SGR 1806-20. In particular we report on the temporal and spectral properties of the bursts detected during a moderately active period of the source in September and October 2003 and on the search for quiescent emission.

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arXiv2004

From BeppoSAX to INTEGRAL: PDS observations of hard X-ray sources detected in the IBIS survey

The first IBIS galactic plane survey has provided a list of high energy emitting objects above 20 keV; these sources have been detected mostly in the crowded region of the Galactic Centre and partly along the Galactic Plane. In order to validate the detection procedure, to help in the identification process and to study the nature of these IBIS sources, this list has been cross correlated with the data archive of the PDS instrument on BeppoSAX, which operated in a similar energy band and with a similar sensitivity. We discover a number of associations whose detailed analysis will be particularly useful for the survey work. Also, thanks to the imaging capability of IBIS/ISGRI, objects which could not be studied by the PDS due to contamination from nearby sources can now be associated with a definite source or sources.

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arXiv2004

X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2--10 keV fluxes between 0.1 and 5.3 X 10^-9 erg/s/cm^2. The equivalent luminosity range is 6 X 10^35 to 2 X 10^37 erg/s. The 0.3--10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe-K emission line at 6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column density in the line of sight is (8.8+/-0.5) X 10^22 cm^-2. The most conspicuous spectral changes in this model are represented by variations of the temperature and radius of the inner edge of the accretion disk, and a jump of the equivalent width of the Fe-K line in one observation. Furthermore, 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close to outbursts. We derive a distance of 7.5+/-1.3 kpc which is consistent with previous determinations. Our failure to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to quiescence between outbursts and is a true transient.

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arXiv2003

Two spectral states of the transient X-ray burster SAX J1747.0-2853

The neutron star binary SAX J1747.0-2853, located in the Galactic Center region at about 0.5 deg from Sgr A* and at a distance of 9 kpc, has been observed in outburst four times (1998, 1999, 2000 and 2001) by BeppoSAX and RossiXTE. At the time of its discovery in 1998 the source was observed in a low/hard state, showing a hard tail with a high energy cutoff of 70 keV. About two years later the source reappeared about one order of magnitude brighter in the X-rays (0.5-10 keV) and with a significantly steeper spectrum. As was the case for the low state, the data could be fitted by an input model based on two continuum primary components: a) a soft thermal excess, which is ~4 times more luminous than the one found in hard state; b) a non-thermal component which is compatible with either a power-law or a comptonization spectrum. The soft component is equally well described by pure blackbody or multi-color disk emission, with significantly higher temperature than observed in low state (~1.3 vs. the ~0.5 keV assuming pure blackbody). For this model, the flux of the non-thermal component below ~10 keV is a significant fraction of the total X-ray flux, i.e. greater than ~50% in the 2-10 keV band.

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arXiv2003

In flight calibration of the ISGRI camera

ISGRI, the IBIS low energy camera (15 keV - 1 MeV) on board INTEGRAL, is the first large CdTe gamma-ray imager in orbit. We present here an overview of the ISGRI in-flight calibrations performed during the first months after launch. We discuss the stability of the camera as well as the CdTe pixels response under cosmic radiation. The energy calibra tions were done using lead and tungsten fluorescence lines and the $\mathrm{^{22}Na}$ calibration unit. Thermal effects and charge correction algorithm are discussed, and the resulting energy resolution is presented. The ISGRI background spatial and spectral non-uniformity is also described, and some image correction results are presented.

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arXiv2003

INTEGRAL discovery of a bright highly obscured galactic X-ray binary source IGR J16318-4848

INTEGRAL regularly scans the Galactic plane to search for new objects and in particular for absorbed sources with the bulk of their emission above 10-20 keV. The first new INTEGRAL source was discovered on 2003 January 29, 0.5 degree from the Galactic plane and was further observed in the X-rays with XMM-Newton. This source, IGR J16318-4848, is intrinsically strongly absorbed by cold matter and displays exceptionally strong fluorescence emission lines. The likely infrared/optical counterpart indicates that IGR J16318-4848 is probably a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton thick environment. Strongly absorbed sources, not detected in previous surveys, could contribute significantly to the Galactic hard X-ray background between 10 and 200 keV.

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