arXiv subjects

D. Lazzati

Publications and source records attributed to D. Lazzati.

77 records · Page 5Linked to original sources

arXiv2000

The discovery of the optical/IR counterpart of the 12s transient X-ray pulsar GS 0834-43

We report the discovery of the optical/infra-red counterpart of the 12.3s transient X-ray pulsar GS0834-43. We re-analysed archival ROSAT PSPC observations of GS0834-43, obtaining two new refined positions, about 14" and 18" away from the previously published one, and a new spin period measurement. Within the new error circles we found a relatively faint (V=20.1) early type reddened star (V-R=2.24). The optical spectrum shows a strong Halpha emission line. The IR observations of the field confirm the presence of an IR excess for the Halpha-emitting star (K'=11.4, J-K'=1.94) which is likely surrounded by a conspicuous circumstellar envelope. Spectroscopic and photometric data indicate a B0-2 V-IIIe spectral-type star located at a distance of 3-5kpc and confirm the Be-star/X-ray binary nature of GS0834-43.

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arXiv1999

ESO deep observations of the optical afterglow of GRB990510

We present the results of optical observations of the GRB990510 field carried out at different epochs from European Southern Observatory (ESO) telescopes. Deep observations, down to a limiting magnitude of about 27 and 24 in the Bessel-R and Gunn-I band, respectively, were obtained between May 16 and 18 from the ESO NTT-SUSI2 telescope and on May 20 from the ESO 3.6 m (EFOSC2) telescope. These observations, together with other published photometric data, allowed to monitor the faint tail of the decaying Optical Transient (OT) associated to the GRB990510. We discuss the light curves in the different filters (V, R and I) in the light of the recently proposed decay laws. No obvious host associated to the GRB990510 optical afterglow was found in the R and I band images. By comparing the light curves with respect to the theoretical colors of galaxies with different morphology we derived a lower limit of R about 26.6 for the host galaxy.

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arXiv1999

GRB 990510: linearly polarized radiation from a fireball

Models for gamma-ray burst afterglows envisage an hyper-relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed ~18.5 hr after the event and linear polarization in the R band was measured at a level of 1.7 +/- 0.2 %. This is the first detection of linear polarization in the optical afterglow of a gamma-ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.

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arXiv1998

Redshift determination in the X-ray band of gamma-ray bursts

If gamma-ray bursts originate in dense stellar forming regions, the interstellar material can imprint detectable absorption features on the observed X-ray spectrum. Such features can be detected by existing and planned X-ray satellites, as long as the X-ray afterglow is observed after a few minutes from the burst. If the column density of the interstellar material exceeds ~10^{23} cm^{-2} there exists the possibility to detect the K_alpha fluorescent iron line, which should be visible for more than one year, long after the X-ray afterglow continuum has faded away. Detection of these X-ray features will make possible the determination of the redshift of gamma-ray bursts even when their optical afterglow is severely dimmed by extinction.

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arXiv1997

Search and analysis of small scale structures in two X-ray clusters of galaxies

We present a refinement of the wavelet analysis technique for the detection and characterisation of small scale features embedded in a strongly varying background. This technique handles with particular care the side effects of non-orthogonality in the wavelet space which can cause spurious detections and lead to a biased estimate of source parameters. This novel technique is applied to two ROSAT PSPC pointed observations of nearby clusters of galaxies, A1367 and A194. We find evidence that the case of A1367 is not unique and that galaxy-scale X-ray emission could be a quite common property of clusters of galaxies. We detect 28 sources in the field of A1367 and 26 in the field of A194. Since these numbers are significantly larger than those expected from the $\log N - \log S$ relation in the field, most of the sources are expected to be associated with the cluster itself and indeed several identifications with galaxies are possible. In addition, CCD observations have revealed that two X-ray sources in the field of A194, classified as extended by the multi-scale analysis, are very likely associated with two background galaxy clusters at intermediate redshift.

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