We report on the results of the first XMM-Newton satellite observation of the luminous and helium-rich O-type subdwarf BD +37{deg} 1977 carried out in April 2014. X-ray emission is detected with a flux of about 4*10^(-14) erg/cm2/s (0.2-1.5 keV), cor
responding to a f_X/f_bol ratio about 10^(-7); the source spectrum is very soft, and is well fit by the sum of two plasma components at different temperatures. Both characteristics are in agreement with what is observed in the main-sequence early-type stars, where the observed X-ray emission is due to turbulence and shocks in the stellar wind. A smaller but still significant stellar wind has been observed also in BD +37{deg} 1977; therefore, we suggest that also in this case the detected X-ray flux has the same origin.
We report on an X-ray observation of the Be X-ray Binary Pulsar RX J0059.2-7138, performed by XMM-Newton in March 2014. The 19 ks long observation was carried out about three months after the discovery of the latest outburst from this Small Magellani
c Cloud transient, when the source luminosity was Lx ~ 10$^{38}$ erg/s. A spin period of P=2.762383(5) s was derived, corresponding to an average spin-up of $dot{P}_{mathrm{spin}} = -(1.27pm0.01)times10^{-12}$ s $s^{-1}$ from the only previous period measurement, obtained more than 20 years earlier. The time-averaged continuum spectrum (0.2-12 keV) consisted of a hard power-law (photon index ~0.44) with an exponential cut-off at a phase-dependent energy (20-50 keV) plus a significant soft excess below about 0.5 keV. In addition, several features were observed in the spectrum: an emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9-1 keV likely due to a blend of Fe L-shell lines, and narrow emission and absorption lines consistent with transitions in highly ionized oxygen, nitrogen and iron visible in the high resolution RGS data (0.4-2.1 keV). Given the different ionization stages of the narrow line components, indicative of photoionization from the luminous X-ray pulsar, we argue that the soft excess in RX J0059.2-7138 is produced by reprocessing of the pulsar emission in the inner regions of the accretion disc.
The ASTRI project aims to develop, in the framework of the Cherenkov Telescope Array, an end-to-end prototype of the small-size telescope, devoted to the investigation of the energy range ~ 1-100 TeV. The proposed design is characterized by two chall
enging but innovative technological solutions which will be adopted for the first time on a Cherenkov telescope: a dual-mirror Schwarzschild-Couder configuration and a modular, light and compact camera based on Silicon photo-multipliers. Here we describe the prototype design, the expected performance and the possibility to realize a mini array composed by a few such telescopes, which shall be placed at the final CTA Southern Site.
X-ray observations of sdO stars are a useful tool to investigate their properties, but so far only two sdO stars were detected at X-rays. We observed a complete flux-limited sample of 19 sdO stars with the Chandra HRC-I camera to measure the count ra
te of the detected sources or to set a tight upper limit on it for the undetected sources. We obtained a robust detection of BD+37 1977 and Feige 34 and a marginal detection of BD+28 4211. The estimated luminosity of BD+37 1977 is above 10^31 erg/s, which is high enough to suggest the possible presence of an accreting compact companion. This possibility is unlikely for all the other targets (both detected and undetected), since in their case L_X < 10^30 erg/s. On the other hand, for all 19 targets the estimated value of L_X (or its upper limit) implies an X-ray/bolometric flux ratio that agrees with log(L_X/L_bol) = -6.7 +/- 0.5, which is the range of values typical of main-sequence and giant O stars. Therefore, for Feige 34 and BD+28 4211 the observed X-ray flux is most probably due to intrinsic emission. The same is possibile for the 16 undetected stars.
We report on the main results obtained thanks to an observation campaign with XMM-Newton of four persistent, low-luminosity (Lx ~ 10^34 erg/s) and long-period (P > 200 s) Be accreting pulsars. We found that all sources considered here are characteriz
ed by a spectral excess that can be described with a blackbody component of high temperature (kTbb > 1 keV) and small area (Rbb < 0.5 km). We show that: 1) this feature is a common property of several low-luminosity X-ray binaries; 2) for most sources the blackbody parameters (radius and temperature) are within a narrow range of values; 3) it can be interpreted as emission from the NS polar caps.
We report on the results we obtained with XMM-Newton observations of HD49798 and BD+37 442, the only two sdO stars for which X-ray emission has been observed so far. HD is a single-lined spectroscopic binary with orbital period of 1.5 days. We could
establish that its companion is a massive white dwarf with M = 1.28 Msun, which makes it a candidate type Ia supernova progenitor; we also detected a significant X-ray emission during the white-dwarf eclipse, which could be X-ray emission of the sdO star itself. In the case of BD+37 442, a luminous He-rich sdO that up to now was believed to be a single star, we discovered soft X-ray emission with a periodicity of 19.2 s. This indicates that also this hot subdwarf has a compact binary companion, either a white dwarf or a neutron star, most likely powered by accretion from the wind of the sdO star.
ASTRI is a flagship project of the Italian Ministry of Education, University and Research, which aims to develop an end-to-end prototype of the CTA small-size telescope. The proposed design is characterized by a dual-mirror Schwarzschild-Couder confi
guration and a camera based on Silicon photo-multipliers, two challenging but innovative technological solutions which will be adopted for the first time on a Cherenkov telescope. Here we describe the current status of the project, the expected performance and the possibility to realize a mini-array composed by a few small-size telescopes, which shall be placed at the final CTA Southern Site.
The X-ray source RX J0648.0-4418 is the only confirmed binary system in which a compact object, most likely a massive white dwarf, accretes from a hot subdwarf companion, the bright sdO star HD 49798. The X-ray emission from this system is characteri
zed by two periodic modulations caused by an eclipse, at the orbital period of 1.55 d, and by the rotation of the compact object with a spin period of 13.2 s. In 2011 we obtained six short XMM-Newton observations centered at orbital phase 0.75, in order to study the system during the eclipse, and spaced at increasingly long time intervals in order to obtain an accurate measure of the spin-period evolution through phase-connected timing. The duration of the eclipse ingress and egress, 500 s, indicates the presence of an X-ray emitting region with dimensions of the order of a few 10^4 km, surrounding the pulsar and probably due to scattering in the companions wind. We derived an upper limit on the spin-period derivative |Pdot|<6x10^-15 s/s, more than two orders of magnitude smaller than the previously available value. Significant X-ray emission is detected also during the 1.2 hours-long eclipse, with a luminosity of about 3x10^30 erg/s. The eclipse spectrum shows prominent emission lines of H- and He-like nitrogen, an overabundant element in HD 49798. These findings support the suggestion that the X-ray emission observed during the eclipse originates in HD 49798 and that the processes responsible for X-ray emission in the stellar winds of massive O stars are also at work in the much weaker winds of hot subdwarfs.
We report on the main results obtained thanks to an observation campaign, performed with XMM-Newton, of four persistent, low-luminosity (Lx ~ 10^34 erg/s) and long-period (P > 200 s) Be accreting pulsars. We found that all sources considered here are
characterized by a spectral excess that can be described with a blackbody component of high temperature (kT > 1 keV) and small area (R < 0.5 km). We show that: 1) this feature is a common property of several low-luminosity X-ray binaries; 2) for most sources the blackbody parameters (radius and temperature) are within a narrow range of values; 3) it can be interpreted as emission from the NS polar caps.
We report the results of the first X-ray observation of the luminous and helium-rich O-type subdwarf BD+37 442, carried out with the XMM-Newton satellite in August 2011. X-ray emission is detected with a flux of about 3x10^(-14) erg/cm2/s (0.2-1 keV)
and a very soft spectrum, well fit by the sum of a blackbody with temperature kT_BB = 45^(+11)_(-9) eV and a power law with a poorly constrained photon index. Significant pulsations with a period of 19.2 s are detected, indicating that the X-ray emission originates in a white dwarf or neutron star companion, most likely powered by accretion from the wind of BD+37 442.
