arXiv subjects

F. Verbunt

Publications and source records attributed to F. Verbunt.

60 records · Page 4Linked to original sources

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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arXiv1998

High-Energy Observations of the Binary Millisecond Pulsar PSR J0218+4232

We report the detection of pulsed X-ray emission ($4.9σ$) from the binary millisecond pulsar PSR J0218+4232 in a 100 ks ROSAT HRI observation. The pulse profile shows a sharp main pulse and an indication for a second weaker pulse at $\sim$ 0.47 phase separation. The pulsed fraction is 37 \pm 13%. PSR J0218+4232 was several times in the field of view of the high-energy $γ$-ray telescope EGRET and a source positionally consistent with the pulsar was detected above 100 MeV. Spatial and timing analyses of EGRET data indicate that the source is probably multiple: Between 0.1 GeV and 1 GeV PSR J0218+4232 is the most likely counterpart, while the BL Lac 3C66A is the best candidate above 1 GeV. If part of the EGRET signal truly belongs to the pulsar, then this would be the first millisecond $γ$-ray pulsar.

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arXiv1998

X rays from old open clusters: M 67 and NGC 188

We have observed the old open clusters M 67 and NGC 188 with the ROSAT PSPC. In M 67 we detect a variety of X-ray sources. The X-ray emission by a cataclysmic variable, a single hot white dwarf, two contact binaries, and some RS CVn systems is as expected. The X-ray emission by two binaries located below the subgiant branch in the Hertzsprung Russell diagram of the cluster, by a circular binary with a cool white dwarf, and by two eccentric binaries with orbital period > 700 d is puzzling. Two members of NGC 188 are detected, including the FK Com type star D719. Another possible FK Com type star, probably not a member of NGC 188, is also detected.

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arXiv1998

Nine X ray sources in the globular cluster 47 Tucanae

We analyze ROSAT HRI observations obtained from 1992 to 1996 of the globular cluster 47 Tuc. Identifications of two X-ray sources with HD 2072 and with a galaxy, respectively, are used to obtain accurate (<2'') positions of the X-ray sources in the cluster. We find possible optical counterparts, including the blue objects V1 and V2, for three X-ray sources in the core of 47 Tuc, but note that the probability of chance positional coincidence is significant. One of the five sources previously reported by us to reside in the cluster core is found to be an artefact of misalignment between subsequent satellite pointings.

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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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arXiv1996

ROSAT HRI observations of the globular clusters M13 and M92

We report on 40 kiloseconds of ROSAT HRI observations of the globular cluster M92 (NGC 6341) and 20 kiloseconds of observations of the globular cluster M13 (NGC 6205). We find that the low-luminosity (10^{32.5} erg/sec at 7.5 kpc) source previously observed near the core of M92 with the ROSAT PSPC remains unresolved at HRI resolution; we can identify it with M92 with 99 per cent confidence. In M13 we find that the source seen with the ROSAT PSPC lies within the core and is possibly associated with the cluster (96 per cent confidence). We find probabilities of 99.8 per cent (M92) and 98 per cent (M13) for the presence of additional unresolved emission within these globular clusters. We interpret these results in light of current theories regarding the low-luminosity sources.

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