8 research outputs found
Optical and X-ray Properties of CAL 83: I. Quasi-periodic Optical and Supersoft Variability
We have studied the long-term (~ years) temporal variability of the prototype
supersoft X-ray source (SSS) CAL 83 in the LMC, using data from the MACHO and
OGLE projects. The CAL 83 light curve exhibits dramatic brightness changes of
~1 mag on timescales of ~450 days, and spends typically ~200 days in the
optical low state. Combined with archival XMM-Newton X-ray observations these
represent the most extensive X-ray/optical study to date of this system, and
reveal in much greater detail that the X-ray light curve is anti-correlated
with the optical behaviour. This is remarkably similar to the behaviour of the
"transient" SSS, RX J0513.9-6951, where the SSS outbursts recur on a timescale
of ~168 days, and also anti-correlate with the optical flux. We performed
simple blackbody fits to both high and low state X-ray spectra, and find that
the blackbody temperature and luminosity decrease when the optical counterpart
brightens. We interpret these long-term variations in terms of the limit-cycle
model of Hachisu & Kato (2003a), which provides further support for these
systems containing massive (~1.3 Msun) white dwarfs. In addition, we have
refined their orbital periods in the MACHO and OGLE-III light curves to values
of 1.047529(1) days and 0.762956(5) days for CAL 83 and RX J0513.9-6951,
respectively.Comment: 9 pages, 8 figures, 3 tables, accepted for publication in MNRA
NGC 3105: a young open cluster with low metallicity
NGC 3105 is a young open cluster hosting blue, yellow and red supergiants.
This rare combination makes it an excellent laboratory to constrain
evolutionary models of high-mass stars. It is poorly studied and fundamental
parameters such as its age or distance are not well defined. We intend to
characterize in an accurate way the cluster as well as its evolved stars, for
which we derive for the first time atmospheric parameters and chemical
abundances. We identify 126 B-type likely members within a radius of
2.70.6 arcmin, which implies an initial mass, 4100
M. We find a distance of 7.20.7 kpc for NGC 3105, placing it at
=10.01.2 kpc. Isochrone fitting supports an age of 286 Ma,
implying masses around 9.5 M for the supergiants. A high fraction of
Be stars (25 %) is found at the top of the main sequence down to
spectral type b3. From the spectral analysis we estimate for the cluster a
=+46.90.9 km s and a low metallicity,
[Fe/H]=-0.290.22. We also have determined, for the first time, chemical
abundances for Li, O, Na, Mg, Si, Ca, Ti, Ni, Rb, Y, and Ba for the evolved
stars. The chemical composition of the cluster is consistent with that of the
Galactic thin disc. An overabundance of Ba is found, supporting the enhanced
-process. NGC 3105 has a low metallicity for its Galactocentric distance,
comparable to typical LMC stars. It is a valuable spiral tracer in a very
distant region of the Carina-Sagittarius spiral arm, a poorly known part of the
Galaxy. As one of the few Galactic clusters containing blue, yellow and red
supergiants, it is massive enough to serve as a testbed for theoretical
evolutionary models close to the boundary between intermediate and high-mass
stars.Comment: 18 pages, 13 figures. Accepted for publication in A&
Highly absorbed X-ray binaries in the Small Magellanic Cloud
Many of the high mass X-ray binaries (HMXRBs) discovered in recent years in
our Galaxy are characterized by a high absorption, most likely intrinsic to the
system, which hampers their detection at the softest X-ray energies. We have
undertaken a search for highly-absorbed X-ray sources in the Small Magellanic
Cloud (SMC) with a systematic analysis of 62 XMM-Newton SMC observations. We
obtained a sample of 30 sources showing evidence for an equivalent hydrogen
column density larger than 3x10^23 cm^-2. Five of these sources are clearly
identified as HMXRBs: four were already known (including three X-ray pulsars)
and one, XMM J005605.8-720012, reported here for the first time. For the
latter, we present optical spectroscopy confirming the association with a Be
star in the SMC. The other sources in our sample have optical counterparts
fainter than magnitude ~16 in the V band, and many of them have possible NIR
counterparts consistent with highly reddened early type stars in the SMC. While
their number is broadly consistent with the expected population of background
highly-absorbed active galactic nuclei, a few of them could be HMXRBs in which
an early type companion is severely reddened by local material.Comment: 10 pages, 4 figures, 4 tables. Accepted for publication by Astronomy
& Astrophysic
The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary systems
In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC),
two new bright X-ray sources were discovered exhibiting the spectral
characteris- tics of High Mass X-ray Binaries - but revealing only weak
evidence for pulsations in just one of the objects(at 153s in
XMMUJ010743.1-715953). The accurate X- ray source locations permit the
identification of these X-ray source with Be stars, thereby strongly suggesting
these systems are new Be/X-ray binaries. From blue spectra the proposed
classification for XMMUJ010633.1-731543 is B0.5-1Ve and for
XMMUJ010743.1-715953 it is B2IV-Ve.Comment: MNRAS (accepted), 12 pages, 17 figures, 4 table
Supersoft X-Ray Source CAL 83: A Possible AE Aqr-like System
CAL83 is a close binary supersoft X-ray source in the Large Magellanic Cloud. A ~67 s periodicity detected in supersoft X-rays is most probably associated with the spin period of a highly spun-up white dwarf (WD). The variability in the period is ascribed to the obscuration of the WD by the hydrogen burning envelope surrounding it, rotating with a period that is close to, but not quite synchronized with, the WD rotation period. Optical spectra obtained with SALT exhibit accretion disc emission lines with broad wing structures and P Cyg profiles, indicating mass outflows. Timing analysis of photometrical observations performed at the South African Astronomical Observatory (SAAO) revealed variable signals at ≤1 mHz which are thought to be associated with quasi-periodic oscillations from an accretion disc. The short spin period inferred for CAL83 can be the result of spin-up by accretion disc torques during a long mass transfer history, placing this source on a similar evolutionary track as the cataclysmic variable AE Aqr
On the impossible NGC 4372 v1 and v2: An extended AGB to the [Fe/H]=-2.2 cluster
The asymptotic giant branch (AGB) of the globular cluster NGC 4372 appears to
extend to unexpectedly high luminosities. We show, on the basis of proper
motions and spatial distribution, that the extended AGB is indeed a likely part
of the cluster. We also present the first spectra of the very cool (2600 K),
very luminous (8000 Lsun), very dusty, oxygen-rich, purported long-period
variable stars V1 and V2 that define the AGB tip. In particular, on the basis
of their radial velocities, we conclude that V1 and V2 are probably members. We
find that V1 and V2 are likely undergoing the superwind phase that terminates
their nuclear-burning evolution. We hypothesise that the mass-loss processes
that terminate the AGB are inhibited in NGC 4372 due to a lack of atmospheric
pulsation and the high gas-to-dust ratio in the ejecta, leading to a delay in
the associated enhanced mass loss and dust production. Previously predicted,
but never observed, this explains the high mass of the white dwarf in Pease 1
in M15 without the need to invoke a stellar merger. If commonplace, this
phenomenon has implications for the mass return from stars, the production of
carbon stars and supernovae through the Universe's history, and the AGB
contribution to light from unresolved metal-poor populations.Comment: 5 pages, 5 figures, accepted MNRAS Letter