584 research outputs found
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
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
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
Far Infrared Variability of Sagittarius A*: 25.5 Hours of Monitoring with
Variable emission from Sgr~A*, the luminous counterpart to the super-massive
black hole at the center of our Galaxy, arises from the innermost portions of
the accretion flow. Better characterization of the variability is important for
constraining models of the low-luminosity accretion mode powering Sgr~A*, and
could further our ability to use variable emission as a probe of the strong
gravitational potential in the vicinity of the
black hole. We use the \textit{Herschel}
Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr~A* at
wavelengths that are difficult or impossible to observe from the ground. We
find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal
structure that is highly correlated across these wavelengths. While the
variations correspond to 1% changes in the total intensity in the
\textit{Herschel} beam containing Sgr~A*, comparison to independent,
simultaneous observations at 0.85 mm strongly supports the reality of the
variations. The lowest point in the light curves, 0.5 Jy below the
time-averaged flux density, places a lower bound on the emission of Sgr~A* at
0.25 mm, the first such constraint on the THz portion of the SED. The
variability on few hour timescales in the SPIRE light curves is similar to that
seen in historical 1.3 mm data, where the longest time series is available, but
the distribution of variations in the sub-mm do not show a tail of
large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from
XMM-Newton shows no significant variation within our observing period, which
may explain the lack of very large variations if X-ray and submillimeter flares
are correlated.Comment: Accepted for publication in Ap
Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma
We have carried out Very Large Array (VLA) continuum observations to study
the variability of Sgr A* at 43 GHz (=7mm) and 22 GHz
(=13mm). A low level of flare activity has been detected with a
duration of 2 hours at these frequencies, showing the peak flare
emission at 43 GHz leading the 22 GHz peak flare by to 40 minutes. The
overall characteristics of the flare emission are interpreted in terms of the
plasmon model of Van der Laan (1966) by considering the ejection and
adiabatically expansion of a uniform, spherical plasma blob due to flare
activity. The observed peak of the flare emission with a spectral index
of =1.6 is consistent with the prediction that the peak
emission shifts toward lower frequencies in an adiabatically-expanding
self-absorbed source. We present the expected synchrotron light curves for an
expanding blob as well as the peak frequency emission as a function of the
energy spectral index constrained by the available flaring measurements in
near-IR, sub-millimeter, millimeter and radio wavelengths. We note that the
blob model is consistent with the available measurements, however, we can not
rule out the jet of Sgr A*. If expanding material leaves the gravitational
potential of Sgr A*, the total mass-loss rate of nonthermal and thermal
particles is estimated to be M yr. We
discuss the implication of the mass-loss rate since this value matches closely
with the estimated accretion rate based on polarization measurements.Comment: Revised with new Figures 1 and 2, 17 pages, 4 figures, ApJ (in press
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
Radio Continuum Emission from the Magnetar SGR J1745-2900: Interaction with Gas Orbiting Sgr A*
We present radio continuum light curves of the magnetar SGR J17452900 and
Sgr A* obtained with multi-frequency, multi-epoch Very Large Array observations
between 2012 and 2014. During this period, a powerful X-ray outburst from SGR
J17452900 occurred on 2013-04-24. Enhanced radio emission is delayed with
respect to the X-ray peak by about seven months. In addition, the flux density
of the emission from the magnetar fluctuates by a factor of 2 to 4 at
frequencies between 21 and 41 GHz and its spectral index varies erratically.
Here we argue that the excess fluctuating emission from the magnetar arises
from the interaction of a shock generated from the X-ray outburst with the
orbiting ionized gas at the Galactic center. In this picture, variable
synchrotron emission is produced by ram pressure variations due to
inhomogeneities in the dense ionized medium of the Sgr A West bar. The pulsar
with its high transverse velocity is moving through a highly blue-shifted
ionized medium. This implies that the magnetar is at a projected distance of
pc from Sgr A* and that the orbiting ionized gas is partially or
largely responsible for a large rotation measure detected toward the magnetar.
Despite the variability of Sgr A* expected to be induced by the passage of the
G2 cloud, monitoring data shows a constant flux density and spectral index
during this periodComment: 12 pages, 3 figures, ApJL (in press
Flaring Activity of Sgr A*: Expanding Hot Blobs
Sgr A* is considered to be a massive black hole at the Galactic center and is
known to be variable in radio, millimeter, near-IR and X-rays. Recent
multi-wavelength observing campaigns show a simultaneous X-ray and near-IR
flare, as well as sub-millimeter and near-IR flares from Sgr A*. The flare
activity is thought to be arising from the innermost region of Sgr A*. We have
recently argued that the duration of flares in near-IR and submillimeter
wavelengths implies that the burst of emission expands and cools on a dynamical
time scale before the flares leave Sgr A*. The detection of radio flares with a
time delay in the range of 20 and 40 minutes between 7 and 12mm peak emission
implies adiabatic expansion of a uniform, spherical hot blob due to flare
activity. We suspect that this simple outflow picture shows some of the
characteristics that are known to take place in microquasars, thus we may learn
much from comparative study of Sgr A* and its environment vs. microquasars.Comment: 10 pages, 5 figures, to be published in IV Microquasar Workshop:
Microquasars and Beyond, September 18-22 2006, Como, Ital
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