arXiv subjects

M. Cocchi

Publications and source records attributed to M. Cocchi.

45 records · Page 3Linked to original sources

arXiv1999

Discovery of type-I X-ray bursts from GRS 1741.9-2853

For the first time X-ray bursts have been detected from a sky position consistent with the one of GRS 1741.9-2853, a GRANAT transient source located only ~10' from the Galactic Centre. A total of 3 bursts have been observed by the Wide Field Cameras telescopes on board BeppoSAX during a monitoring observation of the Sgr A region in August-September 1996. The characteristics of the events are consistent with type-I bursts, thus identifying the source as a likely low-mass X-ray binary containing a neutron star. Evidence of photospheric radius expansion due to super-Eddington luminosity is present in one of the observed bursts, thus leading to an estimate og the source distance (~8 kpc).

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arXiv1999

A new X-ray outburst in the globular cluster NGC 6440: SAX J1748.9-2021

For the second time in 27 years a bright transient X-ray source has been detected coincident with the globular cluster NGC 6440. It was found to be active in August, 1998, with the Wide Field Camera and the narrow field instruments on the BeppoSAX spacecraft, and with the All-Sky Monitor and the Proportional Counter Array on the RossiXTE spacecraft. Four X-ray bursts were detected, at least one of which shows the characteristics of a thermonuclear flash on a neutron star, in analogy with some ~20 optically identified low-mass X-ray binaries. The broad-band spectrum is hard as is common among low-mass X-ray binaries of lower luminosity (>~10^37 erg/s) and can be explained by a Comptonized model. During the burst the >30 keV emission brightened, consistent with part of the burst emission being Compton up scattered within ~10^11 cm.

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arXiv1999

Bursts from GS 1826-238: a clocked thermonuclear flashes generator

The transient X-ray source GS 1826-238 was monitored during five different observing periods between August 1996 and October 1998 with the BeppoSAX Wide Field Camera instrument in the framework of a deep observation of the Galactic Center region. A first detection of X-ray bursts from this source occurred, ruling out its previously suggested black hole candidacy and strongly suggesting the compact object to be a weakly magnetized neutron star. During the 2.5 years of monitoring, corresponding to ~2.0 Ms observing time, 70 bursts were detected from the source. We here report a quasi-periodicity of 5.76 h in the burst occurrence time which is present during all observations. This is the first example of quasi-periodic bursting over a period of years. It is in line with the history of a rather constant flux since the turn on in 1988 and points to a very stable accretion.

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arXiv1999

SAX J1810.8-2609: an X-ray bursting transient

We report on the BeppoSAX/WFC observation of a strong type-I X-ray burst from the recently discovered transient source SAX J1810.8-2609. The observed event showed evidence of photospheric radius expansion due to super-Eddington burst luminosity. Burst detection identifies the source as a probable low-mass X-ray binary harbouring a neutron star.

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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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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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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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