Swift monitoring of the massive X-ray binary SAX J0635.2+0533

SAX J0635.2+0533 is a binary pulsar with a very short pulsation period ($P$ = 33.8 ms) and a high long-term spin down ($\dot P$ $>$ 3.8$\times10^{-13}$ s s$^{-1}$), which suggests a rotation-powered (instead of an accretion-powered) nature for this source. While it was discovered at a flux level around 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, between 2003 and 2004 this source was detected with XMM-Newton at an average flux of about 10$^{-13}$ erg cm$^{-2}$ s$^{-1}$; moreover, the flux varied of over one order of magnitude on time scales of a few days, sometimes decreasing below $3\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$. Since both the rotation-powered and the accretion-powered scenarios have difficulties to explain these properties, the nature of SAX J0635.2+0533 is still unclear. Here we report on our recent long-term monitoring campaign on SAX J0635.2+0533 carried out with Swift and on a systematic reanalysis of all the RXTE observations performed between 1999 and 2001. We found that during this time interval the source remained almost always active at a flux level above 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$.Comment: 8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy & Astrophysic

News on the X-ray emission from hot subdwarf stars

In latest years, the high sensitivity of the instruments on-board the XMM-Newton and Chandra satellites allowed us to explore the properties of the X-ray emission from hot subdwarf stars. The small but growing sample of X-ray detected hot subdwarfs includes binary systems, in which the X-ray emission is due to wind accretion onto a compact companion (white dwarf or neutron star), as well as isolated sdO stars, in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low-mass stars provide information which can be useful for our understanding of the weak winds of this type of stars and can lead to the discovery of particularly interesting binary systems. Here we report the most recent results we have recently obtained in this research area.Comment: 8 pages, 3 figures. To appear in the Proceedings of the 8th Meeting on Hot Subdwarf Stars and Related Objects, 9-15 July 2017, Cracow, Poland. Eds. A. Baran, A. E. Lynas-Gray, Open Astronomy, in pres

Three new X-ray emitting sdO stars discovered with Chandra

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 rate 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.Comment: 6 pages. Accepted for publication by Astronomy and Astrophysic

Follow-up observations of X-ray emitting hot subdwarf star: the He-rich sdO BD +37{\deg} 1977

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), corresponding 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.Comment: 6 pages. Accepted for publication by Astronomy and Astrophysic

Spectral properties of the soft excess pulsar RX J0059.2-7138 during its 2013 outburst

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 Magellanic 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 $\dot{P}_{\mathrm{spin}} = -(1.27\pm0.01)\times10^{-12}$ 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.Comment: Accepted for publication in Mon. Not. R. Astron. Soc. 9 pages, 5 figure

An ultra-massive fast-spinning white dwarf in a peculiar binary system

White dwarfs typically have masses in a narrow range centered at about 0.6 solar masses (Msun). Only a few ultra-massive white dwarfs (M>1.2 Msun) are known. Those in binary systems are of particular interest because a small amount of accreted mass could drive them above the Chandrasekhar limit, beyond which they become gravitationally unstable. Using data from the XMM-Newton satellite, we show that the X-ray pulsator RX J0648.0-4418 is a white dwarf with mass > 1.2 Msun, based only on dynamical measurements. This ultra-massive white dwarf in a post-common envelope binary with a hot subdwarf can reach the Chandrasekhar limit, and possibly explode as a Type Ia supernova, when its helium-rich companion will transfer mass at an increased rate through Roche lobe overflow.Comment: Science article and Supporting Online Material are available at http://www.sciencemag.org/ Submitted 13 May 2009; accepted 23 July 200

Spectral analysis of SXP59.0 during its 2017 outburst and properties of the soft excess in X-ray binary pulsars

We report the results provided by the XMM-Newton observation of the X-ray binary pulsar SXP59.0 during its most recent outburst in April 2017. The source was detected at $f_{\rm X}$(0.2-12 keV) = 8$\times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$, one of its highest flux levels reported to date. The measured pulse period was $P_{\rm spin}$ = 58.949(1) s, very similar to the periods measured in most of the previous observations. The pulsed emission was clearly detected over the whole energy range between 0.2 and 12 keV, but the pulse profile is energy dependent and the pulsed fraction increases as the energy increases. Although the time-averaged EPIC spectrum is dominated by a power-law component (with photon index $\Gamma = 0.76 \pm 0.01$), the data show an evident soft excess, which can be described with the sum of a black-body and a hot thermal plasma component (with temperatures $kT_{\rm BB} = 171^{+11}_{-14}$ eV and $kT_{\rm APEC} = 1.09^{+0.16}_{-0.09}$ keV, respectively). Moreover, the EPIC and RGS spectra show narrow emission lines due to N, O, Ne, Mg, and Fe. The phase-resolved spectral analysis of the EPIC data shows that the flux of the black-body component varies with the pulse phase, while the plasma component is almost constant. We show that the black-body component can be attributed to the reprocessing of the primary emission by the optically thick material at the inner edge of the accretion disc, while the hot plasma component is due to a diffuse gas far from the accretion region and the narrow emission lines of the RGS spectrum are most probably due to photoionized matter around the accreting source.Comment: 11 pages, 9 figures, 5 tables. Accepted for publication by Astronomy and Astrophysic