XMM-Newton observation of the Galactic supernova remnant W51C (G49.1-0.1)

The supernova remnant (SNR) W51C is a Galactic object located in a strongly inhomogeneous interstellar medium with signs of an interaction of the SNR blast wave with dense molecular gas. Diffuse X-ray emission from the interior of the SNR can reveal element abundances in the different emission regions and shed light on the type of supernova (SN) explosion and its progenitor. The hard X-ray emission helps to identify possible candidates for a pulsar formed in the SN explosion and for its pulsar wind nebula (PWN). We have analysed X-ray data obtained with XMM-Newton. Spectral analyses in selected regions were performed. Ejecta emission in the bright western part of the SNR, located next to a complex of dense molecular gas, was confirmed. The Ne and Mg abundances suggest a massive progenitor with a mass of > 20 M_sun. Two extended regions emitting hard X-rays were identified (corresponding to the known sources [KLS2002] HX3 west and CXO J192318.5+140305 discovered with ASCA and Chandra, respectively), each of which has an additional point source inside and shows a power-law spectrum with Gamma ~ 1.8. Based on their X-ray emission, both sources can be classified as PWN candidates.Comment: 6 pages, 4 figures, accepted for publication in Astronomy and Astrophysic

Comparison of techniques to reconstruct VHE gamma-ray showers from multiple stereoscopic Cherenkov images

For air showers observed simultaneously by more than two imaging atmospheric Cherenkov telescopes, the shower geometry is overconstrained by the images and image information should be combined taking into account the quality of the images. Different algorithms are discussed and tested experimentally using data obtained from observations of Mkn 501 with the HEGRA IACT system. Most of these algorithms provide an estimate of the accuracy of the reconstruction of shower geometry on an event-by-event basis, allowing, e.g., to select higher-quality subsamples for precision measurements.Comment: 14 Pages, 6 figures, Late

A non-pulsating neutron star in the supernova remnant HESS J1731-347 / G353.6-0.7 with a carbon atmosphere

Context: The CCO candidate in the center of the supernova remnant shell HESS J1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-ray spectrum. If the absence of pulsations is interpreted as evidence for the emitting surface area being the entire neutron star surface, the assumption of the measured flux being due to a blackbody emission translates into a source distance that is inconsistent with current estimates of the remnant's distance. Aims: With the best available observational data, we extended the pulse period search down to a sub-millisecond time scale and used a carbon atmosphere model to describe the X-ray spectrum of the CCO and to estimate geometrical parameters of the neutron star. Methods: To search for pulsations we used data of an observation of the source with XMM-Newton performed in timing mode. For the spectral analysis, we used earlier XMM-Newton observations performed in imaging mode, which permits a more accurate treatment of the background. The carbon atmosphere models used to fit the CCO spectrum are computed assuming hydrostatic and radiative equilibria and take into account pressure ionization and the presence of spectral lines. Results: Our timing analysis did not reveal any pulsations with a pulsed fraction above ~8% down to 0.2 ms. This finding further supports the hypothesis that the emitting surface area is the entire neutron star surface. The carbon atmosphere model provides a good fit to the CCO spectrum and leads to a normalization consistent with the available distance estimates of the remnant. The derived constraints on the mass and radius of the source are consistent with reasonable values of the neutron star mass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7 is the second object of this class for which a carbon atmosphere model provides a consistent description of X-ray emission.Comment: 6 pages, 5 figures, accepted for publication in Astronomy&Astrophysic

Suzaku observation of the unidentified VHE gamma-ray source HESS J1702-420

A deep X-ray observation of the unidentified very high energy (VHE) gamma-ray source HESS J1702-420, for the first time, was carried out by Suzaku. No bright sources were detected in the XIS field of view (FOV) except for two faint point-like sources. The two sources, however, are considered not to be related to HESS J1702-420, because their fluxes in the 2-10 keV band (~ 10^-14 erg s^-1 cm^-2) are ~ 3 orders of magnitude smaller than the VHE gamma-ray flux in the 1-10 TeV band (F_{TeV} = 3.1 x 10^-11 erg s^-1 cm^-2). We compared the energy spectrum of diffuse emission, extracted from the entire XIS FOV with those from nearby observations. If we consider the systematic error of background subtraction, no significant diffuse emission was detected with an upper limit of F_X <2.7 x 10^-12 erg s^-1 cm^-2 in the 2-10 keV band for an assumed power-law spectrum of \Gamma=2.1 and a source size same as that in the VHE band. The upper limit of the X-ray flux is twelve times as small as the VHE gamma-ray flux. The large flux ratio (F_{TeV}/F_X) indicates that HESS J1702-420 is another example of a "dark" particle accelerator. If we use a simple one-zone leptonic model, in which VHE gamma-rays are produced through inverse Compton scattering of the cosmic microwave background and interstellar far-infrared emission, and the X-rays via the synchrotron mechanism, an upper limit of the magnetic field (1.7 \mu G) is obtained from the flux ratio. Because the magnetic field is weaker than the typical value in the Galactic plane (3-10 \mu G), the simple one-zone model may not work for HESS J1702-420 and a significant fraction of the VHE gamma-rays may originate from protons.Comment: 7 pages, accepted for publication in PASJ (Suzaku and MAXI special issue