arXiv subjects

A. La Barbera

Publications and source records attributed to A. La Barbera.

93 records · Page 6Linked to original sources

arXiv2004

GX 301-2 as seen by Integral

We present observations of the High Mass X-ray Binary GX 301-2 taken during the Galactic Plane Scan (GPS) with Integral following our RXTE observations (Kreykenbohm et al. 2004). The optical companion of GX 301-2 is the B1Ia+ hypergiant Wray 977 with a luminosity of 1.3 x 10^6 Lsun and a mass of ~48 Msun making the system one of the most massive X-ray binaries known. The system was observed 24 times during the GPS thus covering many orbital phases including the pre-periastron flare. The source is clearly detected in all Integral instruments in most pointings. The derived X-ray light curves show strong variability on all timescales. Depending on the orbital phase of the pointings, the luminosity changes by factor of more than 10 during the pre-periastron flare compared to other pointings. Furthermore, broad band spectra using all instruments available on Integral are compared with data taken by the RXTE instruments PCA and HEXTE (using the same data as in Kreykenbohm et al. 2004).

astro-ph↗

arXiv2001

The 0.1-100 keV Spectrum of LMC X-4 in the High State: Evidence for a High Energy Cyclotron Absorption Line

We report on the spectral analysis of the X-ray pulsar LMC X-4 in its high state out of eclipse observed by BeppoSAX. During this observation no coherent pulsations are detected. The primary continuum is well described by a power law with a high energy cutoff (E_cutoff ~ E_fold ~ 18 keV). The addition of a cyclotron absorption line at ~100 keV improves the fit significantly. The inferred magnetic moment is 1.1 10^{31} Gauss cm^3, in agreement with the value estimated assuming that the neutron star is at the spin equilibrium, as it has been proposed for this source. The remaining excess at low energies can be fitted by a Comptonization of soft photons by moderately hot electrons (kT ~0.9 keV), with an optical depth τ~ 16. The seed photons for this Comptonization are consistent with black body emission from the accretion disk at the magnetospheric radius. Another possibility is to fit the soft excess with black body and thermal bremsstrahlung. In this case the black body would originate from cold plasma at the magnetosphere while the bremsstrahlung component may be produced by the strong stellar wind from the companion star, ionized by the X-ray emission from the pulsar.

astro-ph↗

arXiv2000

The Composite Broad Band Spectrum of Cir X-1 at the Periastron: a Peculiar Z-source

We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A flare lasting $\sim 6 \times 10^3$ s is present at the beginning of the observation. The luminosity during the persistent emission is $1 \times 10^{38}$ erg s$^{-1}$, while during the flare is $2 \times 10^{38}$ erg s$^{-1}$. We produced broad band (0.1-100 keV) energy spectra during the flare and the persistent emission. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of $\sim 0.4$ keV, by electrons at a temperature of $\sim 1$ keV. Out of the flare a power-law component, with photon index $\sim 3$, is dominant at energies higher than 10 keV. This component contributes to $\sim 4%$ of the total luminosity. During the flare its addition is not statistically significant. An absorption edge at $\sim 8.4$ keV, with optical depth $\sim 1$, corresponding to the K-edge of Fe XXIII-XXV, and an iron emission line at 6.7 keV are also present. The iron line energy is in agreement with the ionization level of the absorption edge. The hydrogen column deduced from the absorption edge is $\sim 10^{24}$ cm$^{-2}$, two order of magnitude larger than the absorption measured in this source. We calculated the radius of the region originating the comptonized seed photons, $R_W \sim 150$ km. We propose a scenario where $R_W$ is the inner disk radius, a highly ionized torus surrounds the accretion disk and a magnetosphere is large up to $R_W$. The absorption edge and the emission line could originate in the photoionized torus, while the comptonized component originates in an inner region of the disk.

astro-ph↗