3,212 research outputs found
Constraints on physics of neutron stars from X-ray observations
I summarize some constraints on the physics of neutron stars arising from
X-ray observations of the surfaces of neutron stars, focusing on using models
of low-magnetic-field neutron star atmospheres to interpret their X-ray
spectra. I discuss observations of spectral lines, pulsation profiles, X-ray
bursts, radius measurements of transiently accreting neutron stars in
quiescence, crust and core cooling measurements of transiently accreting
neutron stars, and cooling of young neutron stars. These observations have
constrained the neutron star mass and radius (and thus the internal
composition, and dense matter equation of state), the superfluidity and
neutrino emissivity properties of the core, and the composition and superfluid
state of the crust.Comment: Refereed proceedings of XQCD 2012 conference (August 2012), to be
published online in the IOP Journal of Physics: Conference Series. 10 pages,
8 figure
X-ray Sources in Galactic Globular Clusters
I review recent work on X-ray sources in Galactic globular clusters,
identified with low-mass X-ray binaries (LMXBs), cataclysmic variables (CVs),
millisecond pulsars (MSPs) and coronally active binaries by Chandra. Faint
transient LMXBs have been identified in several clusters, challenging our
understanding of accretion disk instabilities. Spectral fitting of X-rays from
quiescent LMXBs offers the potential to constrain the interior structure of
neutron stars. The numbers of quiescent LMXBs scale with the dynamical
interaction rates of their host clusters, indicating their dynamical formation.
Large numbers of CVs have been discovered, including a very faint population in
NGC 6397 that may be at or beyond the CV period minimum. Most CVs in dense
clusters seem to be formed in dynamical interactions, but there is evidence
that some are primordial binaries. Radio millisecond pulsars show thermal
X-rays from their polar caps, and often nonthermal X-rays, either from
magnetospheric emission, or from a shock between the pulsar wind and material
still flowing from the companion. Chromospherically active binaries comprise
the largest number of X-ray sources in globular clusters, and their numbers
generally scale with cluster mass, but their numbers seem to be reduced in all
globular clusters compared to other old stellar populations.Comment: To appear in the proceedings of the conference "Binary Star
Evolution: Mass Loss, Accretion and Mergers" (ed. V. Kalogera and M. van der
Sluys, AIP Conf. Ser.), held in Mykonos, Greece, held in June 22-25, 2010. 9
pages, 4 color figures. Slightly altered (from conf. version) to mention the
new transient LMXB in Terzan
A Chandra X-ray Observatory Study of PSR J1740--5340 and Candidate Millisecond Pulsars in the Globular Cluster NGC 6397
We present a deep Chandra X-ray Observatory study of the peculiar binary
radio millisecond pulsar PSR J1740--5340 and candidate millisecond pulsars
(MSPs) in the globular cluster NGC 6397. The X-rays from PSR J1740--5340 appear
to be non-thermal and exhibit variability at the binary period. These
properties suggest the presence of a relativistic intrabinary shock formed due
to interaction of a relativistic rotation-powered pulsar wind and outflow from
the unusual "red-straggler/sub-subgiant" companion. We find the X-ray source
U18 to show similar X-ray and optical properties to those of PSR J1740--5340,
making it a strong MSP candidate. It exhibits variability on timescales from
hours to years, also consistent with an intrabinary shock origin of its X-ray
emission. The unprecedented depth of the X-ray data allows us to conduct a
complete census of MSPs in NGC 6397. Based on the properties of the present
sample of X-ray--detected MSPs in the Galaxy we find that NGC 6397 probably
hosts no more than 6 MSPs.Comment: 10 pages, 6 figures, 3 tables; accepted for publication in The
Astrophysical Journa
XMM-Newton observations of IGR J00291+5934: signs of a thermal spectral component during quiescence
We present X-ray observations of the transient accretion-powered millisecond
pulsar IGR J00291+5934 during quiescence. IGR J00291+5934 is the first source
among accretion powered millisecond pulsars to show signs of a thermal
component in its quiescent spectrum. Fitting this component with a neutron star
atmosphere or a black body model we obtain soft temperatures (~64 eV and ~110
eV, respectively). As in other sources of this class a hard spectral component
is also present, comprising more than 60% of the unabsorbed 0.5-10 keV flux.
Interpreting the soft component as cooling emission from the neutron star, we
can conclude that the compact object can be spun up to milliscond periods by
accreting only <0.2 solar masses.Comment: 5 pages - 2 figures. Accepted for publication on ApJ
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
Discovery of a candidate quiescent low-mass X-ray binary in the globular cluster NGC 6553
This paper reports the search for quiescent low-mass X-ray binaries (qLMXBs)
in the globular cluster (GC) NGC 6553 using an XMM-Newton observation designed
specifically for that purpose. We spectrally identify one candidate qLMXB in
the core of the cluster, based on the consistency of the spectrum with a
neutron star H-atmosphere model at the distance of NGC 6553. Specifically, the
best-fit radius found using the three XMM European Photon Imaging Camera
spectra is R_NS=6.3(+2.3)(-0.8) km (for M_NS=1.4 Msun) and the best-fit
temperature is kTeff=136 (+21)(-34) eV. Both physical parameters are in
accordance with typical values of previously identified qLMXBs in GC and in the
field, i.e., R_NS~5-20 km and kTeff~50-150 eV. A power-law (PL) component with
a photon index Gamma=2.1(+0.5)(-0.8) is also required for the spectral fit and
contributes to ~33% of the total flux of the X-ray source. A detailed analysis
supports the hypothesis that the PL component originates from nearby sources in
the core, unresolved with XMM. The analysis of an archived Chandra observation
provides marginal additional support to the stated hypothesis. Finally, a
catalog of all the sources detected within the XMM field of view is presented
here.Comment: 10 pages, 5 figures, 3 tables. Accepted to ApJ (to be published in
August 2011
Neutron Stars in Globular Clusters
Dynamical interactions that occur between objects in dense stellar systems
are particularly important for the question of formation of X-ray binaries. We
present results of numerical simulations of 70 globular clusters with different
dynamical properties and a total stellar mass of 2*10^7 Msun. We find that in
order to retain enough neutron stars to match observations we must assume that
NSs can be formed via electron-capture supernovae. Our simulations explain the
observed dependence of the number of LMXBs on ``collision number'' as well as
the large scatter observed between different globular clusters. For millisecond
pulsars, we obtain good agreement between our models and the numbers and
characteristics of observed pulsars in the clusters Terzan 5 and 47 TucComment: 5 pages, 3 figures, to appear in "Dynamical Evolution of Dense
Stellar Systems", IAUS 246, ed. E. Vesperin
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