219 research outputs found
XMM observation of 1RXS J180431.1-273932: a new M-type X-ray binary with a 494 s-pulse period neutron star?
Low-mass X-ray binaries are binary systems composed of a compact object and a
low-mass star. Recently, a new class of these systems, known as symbiotic
-ray binaries (with a neutron star with a M-type giant companion), has been
discovered. Here, we present long-duration observations of the
source 1RXS J180431.1-273932. Temporal and spectral analysis of the source was
performed along with a search for an optical counterpart. We used a
Lomb-Scargle periodogram analysis for the period search and evaluated the
confidence level using Monte-Carlo simulations. The source is characterized by
regular pulses so that it is most likely a neutron star. A modulation of
s (3 error) was found with a confidence level of 99%.
Evidence of variability is also present, since the data show a rate of change
in the signal of counts s hr. A longer
observation will be necessary in order to determine if the source shows any
periodic behavior. The spectrum can be described by a power law with photon
index and a Gaussian line at 6.6 keV. The X-ray flux in the
0.2--10 keV energy band is erg s cm. The
identification of an optical counterpart (possibly an M6III red-giant star with
an apparent visual magnitude of ) allows a conservative distance
of kpc to be estimated. Other possibilities are also discussed. Once
the distance was estimated, we got an -ray luminosity of L_X\ut<6\times
10^{34} erg s, which is consistent with the typical -ray luminosity
of a symbiotic LMXB system.Comment: in press on A&
Detecting planetary transits in the presence of stellar variability. Optimal filtering and the use of colour information
Copyright © The European Southern Observatory (ESO)Space-based photometric transit searches, such as the ESA mission Eddington (planned for launch in 2007), are expected to detect large numbers of terrestrial planets outside the solar system, including some Earth-like planets. Combining simplicity of concept with efficiency, the transit method consists in detecting the periodic luminosity drop in stellar light curves caused by the transit of a planet in front of its parent star. In a previous paper (Aigrain & Favata 2002, hereafter Paper I), we developed a Bayesian transit detection algorithm and evaluated its performance on simulated light curves dominated by photon noise. In this paper, we examine the impact of intrinsic stellar variability. Running the algorithm on light curves with added stellar variability (constructed using data from the VIRGO/PMO6 instrument on board SoHO) demonstrated the need for pre-processing to remove the stellar noise. We have developed an effective variability filter, based on an ad-hoc optimal approach, and run extensive simulations to test the filter and detection algorithm combination for a range of stellar magnitudes and activity levels. These show that activity levels up to solar maximum are not an obstacle to habitable planet detection. We also evaluated the benefits of using colour information in the detection process, and concluded that in the case of Eddington they are outweighed by the implied loss of photometric accuracy
On the optical counterpart of NGC300 X-1 and the global Wolf-Rayet content of NGC300
(Conext:) Surveys of Wolf-Rayet (WR) populations in nearby galaxies provide
tests of evolutionary models plus Type Ib/c supernova progenitors. This
spectroscopic study complements the recent imaging survey of the spiral galaxy
NGC 300 by Schild et al. (Aims): Revisions to the known WR content of NGC 300
are presented. We investigate the WR nature of candidate #41 from Schild et al.
which is spatially coincident with the bright X-ray point source NGC 300 X-1;
(Methods:) VLT/FORS2 multi-object spectroscopy of WR candidates in NGC 300 is
obtained; (Results:) We establish an early-type WN nature of #41, i.e. similar
to the optical counterpart of IC 10 X-1, which closely resembles NGC 300 X-1.
We confirm 9 new WR stars, bringing the current WR census of the inner disk to
31, with N(WC)/N(WN)~0.9. (Conclusions:) If #41 is the optical counterpart for
NGC 300 X-1, we estimate a WR mass of 38 Msun based upon ground-based
photometry, from which a black hole mass of > 10 Msun results from the 32.8 hr
period of the system and WR wind velocity of 1250 km/s. We estimate an 95%
completeness among WC stars and 70% among WN stars, such that the total WR
content is ~40, with N(WC)/N(WN)~0.7. From the Halpha-derived star formation
rate of the inner galaxy, we infer N(WR)/N(O)~0.04Comment: 5 pages, 5 figures, accepted for A&A Letter
The Frequency Content of the VIRGO/SoHO Lightcurves: Implications for Planetary Transit Detection from Space
Stellar micro-variability poses a serious threat to the capacities of
space-based planet-finding missions such as Kepler or Eddington. The solar
lightcurves obtained by the VIRGO PMO6 and SPM instruments on board SoHO from
1996 to 2001 have been studied in order to follow variability changes through
the activity cycle. In all datasets, active regions-induced variability, below
2 microHz, is closely correlated to the BBSO Ca II K-line index. The PMO6
(total irradiance) data shows evidence for a meso-granulation component around
tau = 8x10^3 s, while all narrow-band SPM datasets (red, green and blue) show
super-granulation (tau = 5x10^4 s) but no meso-granulation. Both actvity and
granulation related components have significantly smaller amplitudes in the red
than in the blue channel. These results, coupled with available stellar data,
allow us to generate simulated lightcurves with enhanced variability as a
testbed for pre-processing and detection methods, and influence the case for
using colour information in this kind of mission.Comment: 4 pages, 1 figure, to appear in Conference Proceedings "Scientific
Frontiers in Research on Extrasolar Planets", Drake Dreming (ed
Phase connected X-ray light curve and He II RV measurements of NGC 300 X-1
NGC300 X-1 and IC10 X-1 are currently the only two robust extragalactic
candidates for being Wolf-Rayet/black hole X-ray binaries, the Galactic
analogue being Cyg X-3. These systems are believed to be a late product of
high-mass X-ray binary evolution and direct progenitors of black hole mergers.
From the analysis of Swift data, the orbital period of NGC 300 X-1 was found to
be 32.8h. We here merge the full set of existing data of NGC300 X-1, using
XMM-Newton, Chandra and Swift observations to derive a more precise value of
the orbital period of 32.7932+-0.0029h above a confidence level of 99.99%. This
allows us to phase connect the X-ray light curve of the source with radial
velocity measurements of He II lines performed in 2010. We show that, as for
IC10 X-1 and Cyg X-3, the X-ray eclipse corresponds to maximum of the blueshift
of the He II lines, instead of the expected zero velocity. This indicates that
for NGC300 X-1 as well, the wind of the WR star is completely ionised by the
black hole radiation and that the emission lines come from the region of the WR
star that is in the shadow. We also present for the first time the light curve
of two recent very long XMM-Newton observations of the source, performed on the
16th to 20th of December 2016.Comment: 7 pages, 4 figures, from IAU Symposium 346: High-mass X-ray binaries:
illuminating the passage from massive binaries to merging compact object
NGC 300 X-1 is a Wolf-Rayet/Black-Hole binary
We present VLT/FORS2 time-series spectroscopy of the Wolf-Rayet star #41 in
the Sculptor group galaxy NGC 300. We confirm a physical association with NGC
300 X-1, since radial velocity variations of the HeII 4686 line indicate an
orbital period of 32.3 +/- 0.2 hr which agrees at the 2 sigma level with the
X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of
267 +/- 8 km/s, from which a mass function of 2.6 +/- 0.3 Msun is obtained. A
revised spectroscopic mass for the WN-type companion of 26+7-5 Msun yields a
black hole mass of 20 +/- 4 Msun for a preferred inclination of 60-75 deg. If
the WR star provides half of the measured visual continuum flux, a reduced WR
(black hole) mass of 15 +4 -2.5 Msun (14.5 +3 -2.5 Msun) would be inferred. As
such, #41/NGC 300 X-1 represents only the second extragalactic Wolf-Rayet plus
black-hole binary system, after IC 10 X-1. In addition, the compact object
responsible for NGC 300 X-1 is the second highest stellar-mass black hole known
to date, exceeded only by IC 10 X-1.Comment: 5 pages, 6 figures, MNRAS Letters in press. Images and animations
available from http://www.eso.org/public/news/eso1004/ at 11am GMT on 27 Jan
1
An ultraluminous supersoft source with a 4 hour modulation in NGC 4631
Context. Supersoft X-ray sources (SSSs) are characterised by very low
temperatures (< 100 eV). Classical SSSs have bolometric luminosities in the
range of 10^36-10^38 erg/s and are modelled with steady nuclear burning of
hydrogen on the surfaces of white dwarfs. However, several SSSs have been
discovered with much higher luminosities. Their nature is still unclear. Aims.
We report the discovery of a 4h modulation for an ultraluminous SSS in the
nearby edge-on spiral galaxy NGC 4631, observed with XMM-Newton in 2002 June.
Temporal and spectral analysis of the source is performed. Methods. We use a
Lomb-Scargle periodogram analysis for the period search and evaluate the
confidence level using Monte-Carlo simulations. We measure the source
temperature, flux and luminosity through spectral fitting. Results. A
modulation of 4.2+-0.4 h (3 sigma error) was found for the SSS with a
confidence level >99%. Besides dips observed in the light curve, the flux
decreased by a factor of 3 within ~10h. The spectrum can be described with an
absorbed blackbody model with kT~67eV. The absorbed luminosity in the 0.2-2 kev
energy band was 2.7x10^38 erg/sec while the bolometric luminosity was a hundred
time higher (3.2x10^40 erg/s), making the source one of the most luminous of
its class, assuming the best fit model is correct. Conclusions. This source is
another very luminous SSS for which the standard white dwarf interpretation
cannot be applied, unless a strong beaming factor is considered. A stellar-mass
black hole accreting at a super Eddington rate is a more likely interpretation,
where the excess of accreted matter is ejected through a strong optically-thick
outflow. The 4 h modulation could either be an eclipse from the companion star
or the consequence of a warped accretion disk.Comment: 4 pages, 4 figures, accepted as a Letter in Astronomy & Astrophysic
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