949 research outputs found
The nature of very faint X-ray binaries; hints from light curves
Very faint X-ray binaries (VFXBs), defined as having peak luminosities Lx of
10^34-10^36 erg/s, have been uncovered in significant numbers, but remain
poorly understood. We analyse three published outburst light curves of two
transient VFXBs using the exponential and linear decay formalism of King and
Ritter (1998). The decay timescales and brink luminosities suggest orbital
periods of order 1 hour. We review various estimates of VFXB properties, and
compare these with suggested explanations of the nature of VFXBs. We suggest
that: 1) VFXB outbursts showing linear decays might be explained as partial
drainings of the disc of "normal" X-ray transients, and many VFXB outbursts may
belong to this category; 2) VFXB outbursts showing exponential decays are best
explained by old, short-period systems involving mass transfer from a low-mass
white dwarf or brown dwarf; 3) persistent (or quasi-persistent) VFXBs, which
maintain an Lx of 10^34-10^35 erg/s for years, may be explained by
magnetospheric choking of the accretion flow in a propeller effect, permitting
a small portion of the flow to accrete onto the neutron star's surface. We thus
predict that (quasi-)persistent VFXBs may also be transitional millisecond
pulsars, turning on as millisecond radio pulsars when their Lx drops below
10^32 erg/s.Comment: 12 pages, 4 figures. MNRAS, in pres
The soft quiescent spectrum of the transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5
We report on the quiescent X-ray properties of the recently discovered
transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5.
Using two archival Chandra observations, we demonstrate that the quiescent
spectrum of this neutron star low-mass X-ray binary is soft and can be fit to a
neutron star atmosphere model with a temperature of kT^inf~73 eV. A powerlaw
spectral component is not required by the data and contributes at most ~20% to
the total unabsorbed 0.5-10 keV flux of ~9E-14 erg/cm2/s. Such a soft quiescent
spectrum is unusual for neutron stars with relatively high inferred magnetic
fields and casts a different light on the interpretation of the hard spectral
component, which is often attributed to magnetic field effects. For a distance
of 5.5 kpc, the estimated quiescent thermal bolometric luminosity is ~6E32
erg/s. If the thermal emission is interpreted as cooling of the neutron star,
the observed luminosity requires that the system is quiescent for at least ~100
years. Alternatively, enhanced neutrino emissions can cool the neutron star to
the observed quiescent luminosity.Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letter
Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440
We have identified a new transient luminous low-mass X-ray binary, NGC 6440
X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular
cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at
L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other
outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have
been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et
al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440
X-2), with similar peak luminosities and decay times. Optical and infrared
imaging did not detect a clear counterpart, with best limits of V>21, B>22 in
quiescence from archival HST imaging, g'>22 during the August outburst from
Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the July outburst from
CTIO 4-m ISPI imaging.
Archival Chandra X-ray images of the core do not detect the quiescent
counterpart, and place a bolometric luminosity limit of L_{NS}< 6*10^31 ergs/s
(one of the lowest measured) for a hydrogen atmosphere neutron star. A short
Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's
position, suggesting enhanced quiescent emission at L_X~6*10^31 ergs/s .
NGC 6440 X-2 currently shows the shortest recurrence time (~31 days) of any
known X-ray transient, although regular outbursts were not visible in the bulge
scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may
be easily missed by current X-ray monitoring.Comment: 13 pages (emulateapj), 8 (color) figures, ApJ in press. Revised
version adds 5th outburst (Oct./Nov. 2009), additional discussion of possible
causes of short outburst recurrence time
Near-infrared counterparts of three transient very faint neutron star X-ray binaries
We present near-infrared (NIR) imaging observations of three transient
neutron star X-ray binaries, SAX J1753.5-2349, SAX J1806.5-2215 and AX
J1754.2-2754. All three sources are members of the class of `very faint' X-ray
transients which exhibit X-ray luminosities erg s.
The nature of this class of sources is still poorly understood. We detect NIR
counterparts for all three systems and perform multi-band photometry for both
SAX J1753.5-2349 and SAX J1806.5-2215, including narrow-band Br
photometry for SAX J1806.5-2215. We find that SAX J1753.5-2349 is significantly
redder than the field population, indicating that there may be absorption
intrinsic to the system, or perhaps a jet is contributing to the infrared
emission. SAX J1806.5-2215 appears to exhibit absorption in Br,
providing evidence for hydrogen in the system. Our observations of AX
J1754.2--2754 represent the first detection of a NIR counterpart for this
system. We find that none of the measured magnitudes are consistent with the
expected quiescent magnitudes of these systems. Assuming that the infrared
radiation is dominated by either the disc or the companion star, the observed
magnitudes argue against an ultracompact nature for all three systems.Comment: 10 pages, 10 figures, accepted for publication in MNRA
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
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