Lorella Angelini

Aims.The aim of this paper is to study the Gamma Ray Burst (GRB) environment through the analysis of the optical absorption features due to the gas surrounding the GRB.Methods.To this purpose we analyze high resolution spectroscopic observations (–45 000, corresponding to 14 km s-1 at 4200 Å and 6.6 km s-1 at 9000 Å) of the optical afterglow of GRB050730, obtained with UVES@VLT ~4 h after the GRB trigger.Results.The spectrum shows that the ISM of the GRB host galaxy at is complex, with at least five components contributing to the main absorption system. We detect strong C II*, Si II*, O I* and Fe II* fine structure absorption lines associated to the second and third component.Conclusions.For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the longest wavelength, highest positive velocity shift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as C IV and O VI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10–100 times smaller than that of component 3. We evaluated the mean metallicity of the system obtaining values ≈10-2 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values. The mean [C/Fe] ratio agrees well with that expected by single star-formation event models. Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site.

We report the results of Swift X-Ray Telescope (XRT) observations of GRB 050318. This event triggered the Burst Alert Telescope (BAT) aboard Swift and was followed-up with XRT and UVOT for 11 consecutive orbits starting from 54 min after the trigger. A previously unknown fading X-ray source was detected and accurately monitored. The source was found to decrease in intensity with time and a clear temporal break occurring at ~18 000 s after the trigger was observed. The X-ray light curve was found to be consistent with a broken power-law with decay indices and before and after the break. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with energy index of . No evidence of spectral evolution was found. We compare these results with those obtained with UVOT and separately reported and refine the data analysis of BAT. We discuss our results in the framework of a collimated fireball model and a synchrotron radiation emission mechanism. Assuming the GRB redshift derived from the farthest optical absorption complex (), the event is fully consistent with the – correlation.

The X-ray binary pulsar GX 1+4 was observed with Ginga every year from 1987 to 1991 and with ASCA in 1994. During the Ginga observations, GX 1+4 was in the steady spin-down phase, although the X-ray flux was not steady. Assuming a distance of 10 kpc, the absorption-corrected X-ray luminosity decreased down to L_{2−20 keV}=2.7×10³⁶ ergs s^-1 in 1991, after the peak activity of L_{2−20 keV}=1.2×10³⁷ ergs s^-1 in 1989. On the other hand, the absorption column density showed a drastic increase over the Ginga observation series. It was less than 10²³ cm^-2 at the beginning of the series, and it reached a maximum of (1.4±0.2)×10²⁴ cm^-2 in 1991, indicating a rapid accumulation of matter in the vicinity of the source. The center energy and equivalent width of the iron line were consistent with emission by isotropically distributed cold matter. The ASCA observation was performed on 1994 September 15, a month before the transition into a spin-up phase. The source brightened again to L_{2−20 keV}=10³⁷ ergs s^-1. The absorption column density was observed to decrease for the first time to (2.08±0.02)×10²³ cm^-2. The ionization degree of iron in the absorbing matter was determined to be Fe I-Fe IV using the ratio of the line-center energy to the absorption-edge energy. The low ionization degree is consistent with an absorbing matter distribution extending ~10¹² cm from the source. We compared the results with optical observations and found that the optical data also supports the picture. Based on the geometrical model, possible causes of the bimodal behavior of the source are discussed.

The X-ray binary pulsar GX 1+4 was observed with Ginga every year from 1987 to 1991, and with ASCA in 1994. During the Ginga observations, GX 1+4 was in the steady spindown phase, while the X-ray flux was not steady. The X-ray luminosity decreased down to L = 2.7\times10^{36} erg s^{-1} in 1991, after the peak activity of L = 1.2\times10^{37} erg s^{-1} in 1989. On the other hand, the absorption column density showed a drastic increase over the Ginga observation series. It reached a maximum of 1.4\times10^{24} cm^{-2} in 1991, indicating a rapid accumulation of matter in the vicinity of the source. The ASCA observation was performed on 1994/09/15, a month before the transition into a spinup phase. The source brightened again to L = 10^{37} erg s^{-1}. The absorption column density was observed to decrease for the first time to 2\times10^{23} cm^{-2}. The ionization degree of iron in the absorbing matter was determined to be Fe I--Fe IV using the ratio of the line-center energy to the absorption-edge energy. The low ionization degree is consistent with an absorbing matter distribution extending 10^{12} cm from the source. We compared the results with optical observations, and found the optical data also supports the picture. Based on the geometrical model, possible causes of the bimodal behavior of the source are discussed.

During a systematic search for periodic signals in a sample of ROSAT PSPC (0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the galactic plane. The energy spectrum is well fit by a power law with a photon index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1. The source is probably a long period, low luminosity X-ray pulsar, similar to X Per, or an intermediate polar.

During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~8.9 s X-ray pulsations in 2E 0050.1-7247, a variable X-ray source in the Small Magellanic Cloud. The source was detected several times between 1979 and 1993 at luminosity levels ranging from ~5 × 10³⁴ ergs s^-1 up to 1.4 × 10³⁶ ergs s^-1 with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power-law spectrum that steepens as the source luminosity decreases. We revealed a pronounced Hα activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E 0050.1-7247 is in a Be-type massive binary.

During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~ 8.9s X-ray pulsations in 2E0050.1-7247, a variable X-ray source in the Small Magellanic Cloud. The source was detected several times between 1979 and 1993 at luminosity levels ranging from about 5 times 10^{34} erg/s up to 1.4 times 10^{36} erg/s with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power law spectrum which steepens as the source luminosity decreases. We revealed a pronounced Halpha activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E0050.1-7247 is in a Be-type massive binary.