1,234 research outputs found
XMM-Newton observations of Nova Sgr 1998
We report on X-ray observations of Nova Sagittarius 1998 (V4633 Sgr),
performed with XMM-Newton at three different epochs, 934, 1083 and 1265 days
after discovery. The nova was detected with the EPIC cameras at all three
epochs, with emission spanning the whole energy range from 0.2 to 10 keV.
The X-ray spectra do not change significantly at the different epochs, and
are well fitted for the first and third observations with a multi-temperature
optically thin thermal plasma, while lower statistics in the second
observations lead to a poorer fit. The thermal plasma emission is most probably
originated in the shock heated ejecta, with chemical composition similar to
that of a CO nova. However, we can not completely rule out reestablished
accretion as the origin of the emission. We also obtain upper limits for the
temperature and luminosity of a potential white dwarf atmospheric component,
and conclude that hydrogen burning had already turned-off by the time of our
observations.Comment: 18 pages, 3 figures. Accepted in Astrophysical Journa
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
Constraining the Accretion Geometry of the Intermediate Polar EX Hya Using NuSTAR, Swift, and Chandra Observations
In magnetically accreting white dwarfs, the height above the white dwarf surface where the standing shock is formed is intimately related with the accretion rate and the white dwarf mass. However, it is difficult to measure. We obtained new data with NuSTAR and Swift that, together with archival Chandra data, allow us to constrain the height of the shock in the intermediate polar EX Hya. We conclude that the shock has to form at least at a distance of about one white dwarf radius from the surface in order to explain the weak Fe Kα 6.4 keV line, the absence of a reflection hump in the high-energy continuum, and the energy dependence of the white dwarf spin pulsed fraction. Additionally, the NuSTAR data allowed us to measure the true, uncontaminated hard X-ray (12-40 keV) flux, whose measurement was contaminated by the nearby galaxy cluster Abell 3528 in non-imaging X-ray instruments.Fil: Luna, Gerardo Juan Manuel. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Mukai, K.. National Aeronautics and Space Administration; Estados UnidosFil: Orio, M.. Università di Padova; ItaliaFil: Zemko, P.. Università di Padova; Itali
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 both during the low and the high optical states. They are BZ Cam, MV Lyr, TT Ari, and V794 Aql. Using archival ROSAT, Swift and SUZAKU observations we found that the X-ray flux for BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) that would indicate the thermonuclear burning predicted in a previous article. The X-ray flux is lower by a factor 2–10 in the low than the high state in other systems, and does not reflect the drop in ˙M inferred from optical and UV data. The best fit model for the X-ray spectra is a collisionally ionized plasma model. The X-ray flux may originate in a shocked wind or in accretion onto polar caps in intermediate polar systems that continues even during the low state
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
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