1,140 research outputs found
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
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