X-ray observations of VY Scl type nova-like binaries in the high and low state

Four VY Scl-type nova-like systems were observed in X-rays during both the low and the high optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari, and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multi-component model of plasma in collisional ionization equilibrium. The high state high resolution spectra of TT Ari taken with Chandra ACIS-S and the HETG gratings show a rich emission line spectrum, with prominent lines of in Mg, Si, Ne, and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disk emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.Comment: 13 pages, 8 figures, 7 tables. Accepted for publication by MNRA

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

V4743 Sgr, a magnetic nova?

Two XMM Newton observations of Nova V4743 Sgr (Nova Sgr 2002) were performed shortly after it returned to quiescence, 2 and 3.5 years after the explosion. The X-ray light curves revealed a modulation with a frequency of ~0.75 mHz, indicating that V4743 Sgr is most probably an intermediate polar (IP). The X-ray spectra have characteristics in common with known IPs, with a hard thermal plasma component that can be fitted only assuming a partially covering absorber. In 2004 the X-ray spectrum had also a supersoft blackbody-like component, whose temperature was close to that of the white dwarf (WD) in the supersoft X-ray phase following the outburst, but with flux by at least two orders of magnitude lower. In quiescent IPs, a soft X-ray flux component originates at times in the polar regions irradiated by an accretion column, but the supersoft component of V4743 Sgr disappeared in 2006, indicating a possible origin different from accretion. We suggest that it may have been due to an atmospheric temperature gradient on the WD surface, or to continuing localized thermonuclear burning at the bottom of the envelope, before complete turn-off. An optical spectrum obtained with SALT 11.5 years after the outburst showed a prominent He II 4686A line and the Bowen blend, which reveal a very hot region, but with peak temperature shifted to the ultraviolet (UV) range. V4743 Sgr is the third post-outburst nova and IP candidate showing a low-luminosity supersoft component in the X-ray flux a few years after the outburst.Comment: 9 pages, 5 figures, accepted to MNRA

The X-ray emission from Nova V382 Velorum: I. The hard component observed with BeppoSAX

We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), done in a broad X-ray band covering 0.1-300 keV only 15 days after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments in the energy range 1.8-10 keV and we attribute the emission to shocks in the ejecta. The plasma temperature was kT~6 keV and the unabsorbed flux was F(x)~4.3 x 10(-11) erg/cm**2/s. The nebular material was affected by high intrinsic absorption of the ejecta. 6 months after after the outburst, the intrinsic absorption did not play a role, the nova had turned into a bright supersoft source, and the hot nebular component previously detected had cooled to a plasma temperature kT<=1 keV. No emission was detected in either observation above 20 keV.Comment: 1 tex file, 2 figures as .ps, and 1 .sty file of MNRA

Measurements of the Cerenkov light emitted by a TeO2 crystal

Bolometers have proven to be good instruments to search for rare processes because of their excellent energy resolution and their extremely low intrinsic background. In this kind of detectors, the capability of discriminating alpha particles from electrons represents an important aspect for the background reduction. One possibility for obtaining such a discrimination is provided by the detection of the Cerenkov light which, at the low energies of the natural radioactivity, is only emitted by electrons. In this paper, the results of the analysis of the light emitted by a TeO2 crystal at room temperature when transversed by a cosmic ray are reported. Light is promptly emitted after the particle crossing and a clear evidence of its directionality is also found. These results represent a strong indication that Cerenkov light is the main, if not even the only, component of the light signal in a TeO2 crystal. They open the possibility to make large improvements in the performance of experiments based on this kind of material

The X-ray emission from Nova V382 Velorum: II. The super-soft component observed with BeppoSAX

Nova Velorum 1999 (V382 Vel) was observed by BeppoSAX 6 months after optical maximum and was detected as a bright X-ray supersoft source, with a count rate 3.454+-0.002 cts/s in the LECS. It was the softest and most luminous supersoft source observed with this instrument. The flux in the 0.1-0.7 keV range was not constant during the observation. It dropped by a factor of 2 in less than 1.5 hour and then was faint for at least 15 minutes, without significant spectral changes. The observed spectrum is not well fit with atmospheric models of a hot, hydrogen burning white dwarf. This is due mainly to a supersoft excess in the range 0.1-0.2 keV, but the fit can be significantly improved at higher energy if at least one emission feature is superimposed. We suggest that a ``pseudocontinuum'' was detected, consisting of emission lines in the supersoft X-ray range superimposed on the thermal continuum of a white dwarf atmosphere. As a result, an accurate determination of the effective temperature and gravity of the white dwarf at this post-outburst stage is not possible.Comment: To appear in MNRA