We have investigated the X-ray spectral properties of a sample of 138 X-ray sources detected serendipitously in $XMM-Newton$ observations of the Galactic plane, at an intermediate to faint flux level. We divide our sample into 5 subgroups according t
o the spectral hardness of the sources, and stack (i.e. co-add) the individual source spectra within each subgroup. As expected these stacked spectra show a softening trend from the hardest to the softest subgroups, which is reflected in the inferred line-of-sight column density. The spectra of the three hardest subgroups are characterized by a hard continuum plus superimpose Fe-line emission in the 6--7 keV bandpass. The average equivalent width (EW) of the 6.7-keV He-like Fe-K$alpha$ line is 170$^{+35}_{-32}$ eV, whereas the 6.4-keV Fe-K fluorescence line from neutral iron and the 6.9-keV H-like Fe-Ly$alpha$ line have EWs of 89$^{+26}_{-25}$ eV and 81$^{+30}_{-29}$ eV respectively, i.e. roughly half that of the 6.7-keV line. The remaining subgroups exhibit soft thermal spectra. Virtually all of the spectrally-soft X-ray sources can be associated with relatively nearby coronally-active late-type stars, which are evident as bright near-infrared (NIR) objects within the X-ray error circles. On a similar basis only a minority of the spectrally-hard X-ray sources have likely NIR identifications. The average continuum and Fe-line properties of the spectrally-hard sources are consistent with those of magnetic cataclysmic variables but the direct identification of large numbers of such systems in Galactic X-ray surveys, probing intermediate to faint flux levels, remains challenging.
We present a systematic search for radio counterparts to Ultra Luminous X-ray (ULX) source candidates based on a cross-correlation of the Swartz et al. (2004) ULX catalogue based on Chandra data and the FIRST radio survey. We find seven cases of cons
piscuous peaks of radio emission that could be associated to ULX sources. Among these seven ULX radio candidates, three X-ray sources are located within 5 of the FIRST radio peaks. These three cases are shown and discussed individually.
Aims: We present multiwavelength observations of one of the most distant gamma-ray bursts detected so far, GRB 080913. Based on these observations, we consider whether it could be classified as a short-duration GRB and discuss the implications for th
e progenitor nature and energy extraction mechanisms. Methods: Multiwavelength X-ray, near IR and millimetre observations were made between 20.7 hours and 16.8 days after the event. Results: Whereas a very faint afterglow was seen at the 3.5m CAHA telescope in the nIR, the X-ray afterglow was clearly detected in both Swift and XMM-Newton observations. An upper limit is reported in the mm range. We have modeled the data assuming a collimated $theta_0$ $gtrsim$ 3$^circ$ blast wave with an energy injection at 0.5 days carrying $5sim 10^{52}$ erg or approximately 12 times the initial energy of the blast wave. We find that GRB 080913 shares many of the gamma-ray diagnostics with the more recent burst GRB 090423 for being classified as short had they ocurred at low redshift. If the progenitor were a compact binary merger, it is likely composed by a NS and BH. The Blandford-Znajek (BZ) mechanism is the preferred one to extract energy from the central, maximally-rotating BH. Both the magnetic field close to the event horizon (B) and the BH mass ($M_{bh}$) are restricted within a relatively narrow range, such that $(B / 3times 10^{16} rm{G}) (M_{bh} / 7 M_odot) sim 1$. Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand.
