Chandra and XMM-Newton observations of the exceptional pulsar PSR B0628-28

PSR B0628-28 is a radio pulsar which was first detected in the X-ray band by ROSAT and then later observed with Chandra and XMM-Newton. The Chandra observation yielded an X-ray luminosity two orders of magnitude higher than what is expected for spin-powered pulsars, also there were no pulsations detected. The XMM-Newton observation, however, reveals pulsations at the expected radio period, P=1.244 s. The simultaneously analyzed spectra also gives a luminosity (in cgs) Log Lx=30.34, which is ~350 times greater than what would be expected from the correlation between Lx-Edot.Comment: 13 pages, 4 figures, to be published in ApJ

Variability and multi-periodic oscillations in the X-ray light curve of the classical nova V4743 Sgr

The classical nova V4743 Sgr was observed with XMM-Newton for about 10 hours on April 4 2003, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X-ray source ever observed. We present the results of a time series analysis performed on the X-ray light curve obtained in this observation, and in a previous shorter observation done with Chandra 16 days earlier. Intense variability, with amplitude as large as 40% of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 day time interval between the two observations. We suggest that a periods in the power spectrum of both light curves at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf in the system, and that other observed frequencies are signatures of nonradial white dwarf pulsations. A possible signal with a 24000 sec period is also found in the XMM-Newton light curve: a cycle and a half are clearly identified. This period is consistent with the 24278 s periodicity discovered in the optical light curve of the source and thought to be the orbital period of the nova binary system.Comment: In press in Monthly Notices of the Royal Astronomical Societ

Two SMC Symbiotic stars undergoing steady hydrogen burning

Two symbiotic stars in the Small Magellanic Cloud (SMC), Lin 358 and SMC 3, have been supersoft X-ray sources (SSS) for more than 10 years. We fit atmospheric and nebular models to their X-ray, optical and UV spectra obtained at different epochs. The X-ray spectra are extremely soft, and appear to be emitted by the white dwarf atmosphere and not by the nebula like in some other symbiotics. The white dwarf of SMC 3, the hottest of the two sources, had a constant effective temperature ~500,000 K at various epochs during 12 years. No nova-like outbursts of these systems have been recorded in the last 50 years, despite continuous optical monitoring of the SMC, and there are no indications of cooling of the white dwarf, expected after a thermonuclear flash. The bolometric luminosity of this system in March of 2003 was more than an order of magnitude lower than three years later, however the time of the observation is consistent with a partial eclipse of the white dwarf, previously found in ROSAT and optical observations. The red giant wind is either very asymmetric or very clumpy. The conpact object of Lin 358 has been at T>~180,000 K since 1993, perhaps with a moderate increase. Atmospheric fits are obtained with log(g)>=9, appropriate only for WD mass >1.18 M(sol). The two systems are probably accreting and burning hydrogen steadily at the high rate required for type Ia supernova progenitors.Comment: In press on the Astrophysical Journa

Discovery of extended emission around the pulsar B0355+54

PSR B0355+54 is one of the handful of pulsars that have been observed with both Chandra and XMM-Newton. The analysis of the archival data has revealed the pulsar and a similar to 30" compact nebula surrounding it. XMM-Newton also has detected a trail that extends similar to 6' and, similar to the compact nebula, is also counteraligned with the proper motion of the pulsar. The spectra of both the pulsar and the extended emission are well described by an absorbed power-law model. The measured flux corresponds to an efficiency of converting the spin-down luminosity into X-rays in the 2-10 keV band of similar to 0.01% and similar to 1% for the pulsar and the extended emission, respectively. From the XMM-Newton data we have detected pulsations at the expected radio period. The energetics and the extent of the extended emission can be explained by a bow shock formed by the motion of the pulsar through the interstellar medium