arXiv subjects

P. Ubertini

Publications and source records attributed to P. Ubertini.

223 records · Page 13Linked to original sources

arXiv1999

Long term BeppoSAX-WFC monitoring of GRS 1758-258

Following a series of deep observations of the Galactic Bulge region performed with the Wide Field Cameras on board BeppoSAX, it has been possible to study throughout a long time period a number of persistent X-ray sources. We report on the behaviour of GRS 1758-258, an X-ray binary suggested as a black hole candidate on the basis of its timing and spectral characteristics. The source has been monitored for almost 2 years since August 1996; preliminary results of the data analysis are shown hereafter.

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arXiv1999

The INTEGRAL Core Observing Programme

The Core Programme of the INTEGRAL mission is defined as the portion of the scientific programme covering the guaranteed time observations for the INTEGRAL Science Working Team. This paper describes the current status of the Core Programme preparations and summarizes the key elements of the observing programme.

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arXiv1998

X-ray bursts from 1RXS J170930.2-263927 = XTE J1709-267

We report evidence of type-I X-ray bursting activity from the transient 1RXS J170930.2-263927 when it was in outburst early 1997. This identifies the source as a probable low-mass X-ray binary containing a neutron star. The error boxes of the detected bursts and of the persistent emission, as obtained with the Wide Field Cameras on board the BeppoSAX satellite, rule out an association with the proposed radio counterpart (Hjellming & Rupen 1997).

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arXiv1998

Discovery of SAX J1753.5-2349 and SAX J1806.5-2215: two X-ray bursters without detectable steady emission

We report the discovery with BeppoSAX-WFC of two new X-ray sources that were only seen during bursts: SAX J1753.5-2349 and SAX J1806.5-2215. For both sources, no steady emission was detected above an upper limit of 5 mCrab (2 to 8 keV) for 3 10**5 s around the burst events. The single burst detected from SAX J1753.5-2349 shows spectral softening and a black body color temperature of 2.0 keV. Following the analogy with bursts in other sources the burst very likely originates in a thermonuclear flash on a neutron star. The first of two burst detected from SAX J1806.5-2215 does not show spectral softening and cannot be confirmed as a thermonuclear flash.

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arXiv1998

Discovery of the X-ray transient SAX J1808.4-3658, a likely low mass X-ray binary

We report the discovery of a fairly bright transient during observations with the Wide Field Cameras on board the BeppoSAX satellite in September 1996. It was detected at a peak intensity of 0.1 Crab (2 to 10 keV) and lasted between 6 and 40 days above a detection threshold of 2 mCrab. Two very bright type I X-ray bursts were detected from this transient in the same observations. These almost certainly identify this X-ray transient as a low-mass X-ray binary with a neutron star as compact component. The double-peaked time history of both bursts at high energies suggests a peak luminosity close to the Eddington limit. Assuming this to be true implies a distance to this object of 4 kpc.

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arXiv1998

Proposed (to) EXIST: Hard X-ray Imaging All Sky Survey/Monitor

The hard x-ray (10-600 keV) sky is inherently time variable and yet has neither been surveyed nor monitored with a sensitive imaging telescope. The Energetic X-ray Imaging Survey Telescope (EXIST) is a mission concept, proposed for MIDEX, which would conduct the first imaging all-sky hard x-ray survey as well as provide a sensitive all sky monitor. With $\sim 60%$ sky coverage each orbit, and full sky coverage each 50 days, hard x-ray studies of gamma-ray bursts, AGN, galactic transients, x-ray binaries and accretion-powered pulsars can be conducted over a wide range of timescales. We summarize the scientific objectives of EXIST for both the survey and monitoring objectives. We describe the mission concept and the instrumentation approach, which would incorporate a large area array of Cd-Zn-Te (CZT) detectors, as well as some of our ongoing development of CZT array detectors.

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arXiv1997

Detection of X-ray bursts in the globular cluster NGC 6652

Two type I X-ray bursts were detected from a position consistent with the transient X-ray source in NGC 6652 with the Wide Field Camera of the BeppoSAX satellite, strongly suggesting that this transient is a neutron star. Our detection brings to ten the number of X-ray sources in globular clusters in which a type I X-ray burst has been seen, out of twelve known bright sources. The statistical evidence that the fraction of low-mass X-ray binaries which contain a black hole accretor is smaller in globular clusters than in the galactic disk is suggestive, but as yet not compelling.

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