281 research outputs found
New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate
We present detailed analysis of three globular cluster X-ray sources in the
XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular
cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its
combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%,
and its 0.3--7.0 keV emission spectrum was well described by an absorbed power
law with photon index 1.440.12. Its variability and emission is
characteristic of low mass X-ray binaries (LMXBs) in the low-hard state,
whether the accretor is a neutron star or black hole. Such behaviour is
typically observed at luminosities \la10% Eddington. However, XBo 45
exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of
2.5 erg s, or{~140%} Eddington for a 1.4
neutron star accreting hydrogen. Hence, we identify XBo 45 as a new
candidate black hole LMXB. XBo 45 appears to have been consistently bright for
~30 years, consistent with theoretical prediction for a globular cluster black
hole binary formed via tidal capture. Bo 375 was observed in the 2007, January
2 XMM-Newton observation, and has a two-component spectrum that is typical for
a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and
could contain either a black hole or neutron star.Comment: Accepted by ApJ, 16 pages, 5 figures. This version includes the final
changes made at the request of the refere
XMM-Newton detection of Nova Muscae 1991 in Quiescence
The soft X-ray transient GU Mus has been detected by XMM-Newton in the
quiescent state. The source is very faint, with a 0.5-10.0 keV unabsorbed flux
of ergs cm s. The spectra is well
fit by an absorbed powerlaw with a photon index of ,
close to the value seen when the source was in the low/hard state in Aug. 1991.
From our observed luminosity, it seems unlikely that the quiescent state
emission is dominated by coronal X-rays from the secondary. The flux also
appears to be in agreement with the ADAF model of BH-transients in quiescence.Comment: 6 pages including 3 figures. Accepted for publication, Astronomy and
Astrophysic
Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic
We have computed, based on 17 infrared radial velocities, the first set of
orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The
giant's companion is a neutron star, the bright X-ray source GX 1+4. We find an
orbital period of 1161 days by far the longest of any known X-ray binary. The
orbit has a modest eccentricity of 0.10 with an orbital circularization time of
less than 10^6 years. The large mass function of the orbit significantly
restricts the mass of the M giant. Adopting a neutron-star mass of 1.35M(Sun),
the maximum mass of the M giant is 1.22M(Sun), making it the less massive star.
Derived abundances indicate a slightly subsolar metallicity. Carbon and
nitrogen are in the expected ratio resulting from the red-giant first dredge-up
phase. The lack of O-17 suggests that the M-giant has a mass less than
1.3M(Sun), consistent with our maximum mass. The red giant radius is 103R(Sun),
much smaller than the estimated Roche lobe radius. Thus, the mass loss of the
red giant is via a stellar wind. Although the M giant companion to the neutron
star has a mass similar to the late-type star in low-mass X-ray binaries, its
near-solar abundances and apparent runaway velocity are not fully consistent
with the properties of this class of stars.Comment: In press to The Astrophysical Journal (10 April 2006 issue). 23 page
MAXI J1659-152: The shortest orbital period black-hole transient in outburst
MAXI J1659-152 is a bright X-ray transient black-hole candidate binary system
discovered in September 2010. We report here on MAXI, RXTE, Swift, and
XMM-Newton observations during its 2010/2011 outburst. We find that during the
first one and a half week of the outburst the X-ray light curves display drops
in intensity at regular intervals, which we interpret as absorption dips. About
three weeks into the outbursts, again drops in intensity are seen. These dips
have, however, a spectral behaviour opposite to that of the absorption dips,
and are related to fast spectral state changes (hence referred to as transition
dips). The absorption dips recur with a period of 2.414+/-0.005 hrs, which we
interpret as the orbital period of the system. This implies that MAXI J1659-152
is the shortest period black-hole candidate binary known to date. The
inclination of the accretion disk with respect to the line of sight is
estimated to be 65-80 degrees. We propose the companion to the black-hole
candidate to be close to an M5 dwarf star, with a mass and radius of about
0.15-0.25 M_sun and 0.2-0.25 R_sun, respectively. We derive that the companion
had an initial mass of about 1.5 M_sun, which evolved to its current mass in
about 5-6 billion years. The system is rather compact (orbital separation of
larger than ~1.33 R_sun), and is located at a distance of 8.6+/-3.7 kpc, with a
height above the Galactic plane of 2.4+/-1.0 kpc. The characteristics of short
orbital period and high Galactic scale height are shared with two other
transient black-hole candidate X-ray binaries, i.e., XTE J1118+480 and Swift
J1735.5-0127. We suggest that all three are kicked out of the Galactic plane
into the halo, rather than being formed in a globular cluster.Comment: 20 pages, 14 figures, accepted for publication in A&
The Symbiotic Neutron Star Binary GX 1+4/V2116 Ophiuchi
We present multiwavelength observations of this S-type symbiotic LMXB which
consists of a 2-min X-ray pulsar accreting from an M6 III giant. This is the
only symbiotic system definitely known to contain a neutron star. The steady
interstellar extinction toward the binary (Av=5) contrasts the variable N_H
inferred from X-ray measurements, most likely evidence for a stellar wind. The
mass donor is probably near the tip of the first-ascent red giant branch, in
which case the system is 3-6 kpc distant and has an X-ray luminosity of 10^37
erg/s. It is also possible, though less likely, that the donor star is just
beginning its ascent of the asymptotic giant branch, in which case the system
is 12-15 kpc distant and has an X-ray luminosity of 10^38 erg/s. However, our
measured Av argues against such a large distance. We show that the dense (10^9
cm^-3) emission-line nebula enshrouding the binary is powered by UV radiation
from an accretion disk. The emission-line spectrum constrains the temperature
and inner radius of the disk (and thus the pulsar's magnetic field strength),
and we discuss this in the context of the accretion torque reversals observed
in the pulsar. We also show that the binary period must be >100 d and is most
likely >260 d, making GX 1+4 the only known LMXB with Porb>10 d. If the mass
donor fills its Roche lobe, the mass transfer must be highly super-Eddington,
requiring much mass loss from the binary. We discuss the alternative that the
disk forms from the slow, dense stellar wind expected from the red giant.Comment: 46 pages including 7 PS figures. Accepted for publication in Ap
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