338 research outputs found
LISA Sources in Globular Clusters
Globular clusters house a population of compact binaries that will be
interesting gravitational wave sources for LISA. We provide estimates for the
numbers of sources of several categories and discuss the sensitivity of LISA to
detecting these sources. The estimated total number of detectable sources
ranges from about 10 to about 1000 with gravitational wave frequencies above 1
mHz. These sources are typically undetectable by any other means and thus offer
an opportunity for doing true gravitational-wave astronomy. The detection of
these sources would provide information about both binary star evolution and
the dynamics of globular clusters.Comment: Contribution to Proceedings of 3rd LISA Symposium 7 pages, added
reference
HST - WFPC2 photometry of the globular cluster ngc 288: binary systems, blue stragglers and very blue stars
We report on new WFPC2 observations of the globular cluster NGC 288, focusing
our attention on peculiar stars. A very pronounced binary sequence, paralleling
the ordinary Main Sequence (MS) is clearly observed in the Color Magnitude
Diagram (CMD) and a huge relative fraction of Blue Straggler Stars is measured.
The dataset offers the opportunity of studying the evolution of a large
population of binaries (and binary evolution by-products) in a low density
environment, where the evolution of such systems is not dominated by collisions
and encounters. Three (very) Extreme Horizontal Branch Stars have been found,
all lying outside of the cluster core.Comment: 6 pages, 3 figures, in press in the chemical evolution of the Milky
Way: stars versus clusters, F. Matteucci and F. Giovannelli eds, Kluwe
Imaging X-ray, Optical, and Infrared Observations of the Transient Anomalous X-ray Pulsar XTE J1810-197
We report X-ray imaging, timing, and spectral studies of XTE J1810-197, a
5.54s pulsar discovered by Ibrahim et al. (2003) in recent RXTE observations.
In a set of short exposures with the Chandra HRC camera we detect a strongly
modulated signal (55+/-4% pulsed fraction) with the expected period located at
(J2000) 18:09:51.08, -19:43:51.7, with a uncertainty radius of 0.6 arcsec (90%
C.L.). Spectra obtained with XMM-Newton are well fitted by a two-component
model that typically describes anomalous X-ray pulsars (AXPs), an absorbed
blackbody plus power law with parameters kT = 0.67+/-0.01 keV, Gamma=3.7+/-0.2,
N_H=(1.05+/-0.05)E22 cm^-2, and Fx(0.5-10 keV) = 3.98E-11 ergs/cm2/s.
Alternatively, a 2T blackbody fit is just as acceptable. The location of CXOU
J180951.1-194351 is consistent with a point source seen in archival Einstein,
Rosat, & ASCA images, when its flux was nearly two orders-of-magnitude fainter,
and from which no pulsations are found. The spectrum changed dramatically
between the "quiescent" and "active" states, the former can be modeled as a
softer blackbody. Using XMM timing data, we place an upper limit of 0.03 lt-s
on any orbital motion in the period range 10m-8hr. Optical and infrared images
obtained on the SMARTS 1.3m telescope at CTIO show no object in the Chandra
error circle to limits V=22.5, I=21.3, J=18.9, & K=17.5. Together, these
results argue that CXOU J180951.1-194351 is an isolated neutron star, one most
similar to the transient AXP AX J1844.8-0256. Continuing study of XTE J1810-197
in various states of luminosity is important for understanding and possibly
unifying a growing class of isolated, young neutron stars that are not powered
by rotation.Comment: 12 pages, 7 figures, AAS LaTex, uses emulateapj5.sty. Updated to
include additional archival data and a new HRC observation. To appear in The
Astrophysical Journa
Polarised infrared emission from X-ray binary jets
Near-infrared (NIR) and optical polarimetric observations of a selection of
X-ray binaries are presented. The targets were observed using the Very Large
Telescope and the United Kingdom Infrared Telescope. We detect a significant
level (3 sigma) of linear polarisation in four sources. The polarisation is
found to be intrinsic (at the > 3 sigma level) in two sources; GRO J1655-40 (~
4-7% in H and Ks-bands during an outburst) and Sco X-1 (~ 0.1-0.9% in H and K),
which is stronger at lower frequencies. This is likely to be the signature of
optically thin synchrotron emission from the collimated jets in these systems,
whose presence indicates a partially-ordered magnetic field is present at the
inner regions of the jets. In Sco X-1 the intrinsic polarisation is variable
(and sometimes absent) in the H and K-bands. In the J-band (i.e. at higher
frequencies) the polarisation is not significantly variable and is consistent
with an interstellar origin. The optical light from GX 339-4 is also polarised,
but at a level and position angle consistent with scattering by interstellar
dust. The other polarised source is SS 433, which has a low level (0.5-0.8%) of
J-band polarisation, likely due to local scattering. The NIR counterparts of
GRO J0422+32, XTE J1118+480, 4U 0614+09 and Aql X-1 (which were all in or near
quiescence) have a linear polarisation level of < 16% (3 sigma upper limit,
some are < 6%). We discuss how such observations may be used to constrain the
ordering of the magnetic field close to the base of the jet in such systems.Comment: Accepted to be published in MNRAS; 13 pages, 6 figure
Optical and Infrared Photometry of the Micro-Quasar GRO J1655-40 in Quiescence
We present BVIJK photometry of the black-hole candidate GRO J1655-40 in full
quiescence. We report a refined orbital period of 2.62191 +/- 0.00020 days. The
light curves are dominated by ellipsoidal variations from the secondary star.
We model the light curves with an upgraded code which includes a more accurate
treatment of limb darkening. Previous models containing a large cool disk are
ruled out, and indeed our data can be fit with a pure ellipsoidal light curve
without any disk contribution. In general agreement with previous results, we
derive a confidence region of the correlated quantities of inclination and mass
ratio, centered on an inclination of 70.2 +/ 1.9 degrees, and mass ratio 2.6
+/- 0.3, resulting in a primary mass M = 6.3 +/- 0.5 Mo (all 95% confidence).
The complex limits and errors on these values, and on the possible disk
contribution to the light curve, are discussed.Comment: 14 pages, 5 figures. To be published in The Astrophysical Journa
X-ray states and radio emission in the black hole candidate XTE J1550-564
We report on radio and X-ray observations of the black hole candidate (BHC)
XTE J1550-564 performed during its 2000 X-ray outburst. Observations have been
conducted with the Australia Telescope Compact Array (ATCA) and have allowed us
to sample the radio behavior of XTE J1550-564 in the X-ray Low Hard and
Intermediate/Very High states. We observed optically thin radio emission from
XTE J1550-564 five days after a transition to an Intermediate/Very High state,
but we observed no radio emission six days later, while XTE J1550-564 was still
in the Intermediate/Very High state. In the Low Hard state, XTE J1550-564 is
detected with an inverted radio spectrum. The radio emission in the Low Hard
state most likely originates from a compact jet; optical observations suggest
that the synchrotron emission from this jet may extend up to the optical range.
The total power of the compact jet might therefore be a significant fraction of
the total luminosity of the system. We suggest that the optically thin
synchrotron radio emission detected five days after the transition to the
Intermediate/Very High state is due to a discrete ejection of relativistic
plasma during the state transition. Subsequent to the decay of the optically
thin radio emission associated with the state transition, it seems that in the
Intermediate/Very High state the radio emission is quenched by a factor greater
than 50, implying a suppression of the outflow. We discuss the properties of
radio emission in the X-ray states of BHCs.Comment: 15 pages, including 3 figures. Accepted for publication in ApJ,
scheduled for the vol. 553 Jun 1, 2001 issu
New evidence on the origin of the microquasar GRO J1655-40
Aims. Motivated by the new determination of the distance to the microquasar
GRO J1655-40 by Foellmi et al. (2006), we conduct a detailed study of the
distribution of the atomic and molecular gas, and dust around the open cluster
NGC 6242, the possible birth place of the microquasar. The proximity and
relative height of the cluster on the galactic disk provides a unique
opportunity to study SNR evolution and its possible physical link with
microquasar formation. Methods. We search in the interstellar atomic and
molecular gas around NGC 6242 for traces that may have been left from a
supernova explosion associated to the formation of the black hole in GRO
J1655-40. Furthermore, the 60/100 mu IR color is used as a tracer of
shocked-heated dust. Results. At the kinematical distance of the cluster the
observations have revealed the existence of a HI hole of 1.5*1.5 degrees in
diameter and compressed CO material acumulated along the south-eastern internal
border of the HI cavity. In this same area, we found extended infrared emission
with characteristics of shocked-heated dust. Based on the HI, CO and FIR
emissions, we suggest that the cavity in the ISM was produced by a supernova
explosion occured within NGC 6242. The lower limit to the kinematic energy
transferred by the supernova shock to the surrounding interstellar medium is ~
10^{49} erg and the atomic and molecular mass displaced to form the cavity of ~
16.500 solar masses. The lower limit to the time elapsed since the SN explosion
is ~ 2.2*10^{5} yr, which is consistent with the time required by GRO J1655-40
to move from the cluster up to its present position. The observations suggest
that GRO J1655-40 could have been born inside NGC 6242, being one of the
nearest microquasars known so far.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Microscopic Theory of Magnon-Drag Thermoelectric Transport in Ferromagnetic Metals
A theoretical study of the magnon-drag Peltier and Seebeck effects in
ferromagnetic metals is presented. A magnon heat current is described
perturbatively from the microscopic viewpoint with respect to electron--magnon
interactions and the electric field. Then, the magnon-drag Peltier coefficient
\Pi_\MAG is obtained as the ratio between the magnon heat current and the
electric charge current. We show that \Pi_\MAG=C_\MAG T^{5/2} at a low
temperature ; that the coefficient C_\MAG is proportional to the spin
polarization of the electric conductivity; and that for C_\MAG<0,
but . From experimental results for magnon-drag Peltier
effects, we estimate that the strength of the electron--magnon interaction is
about 0.3 eV for permalloy.Comment: 3 pages, 2 figures, accepted for publication in Journal of the
Physical Society of Japa
Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J1655--40)
We have obtained intermediate resolution optical spectra of the black-hole
candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical
active state (R~15.05). We measure the radial velocity curve of the secondary
star and obtain a semi-amplitude of 279+/-10 km/s; a value which is 30 per cent
larger than the value obtained when the source is in quiescence. Our large
value for K_2 is consistent with 60 +9,-7 per cent of the secondary star's
surface being heated; compared to 35 per cent, which is what one would expect
if only the inner face of the secondary star were irradiated. Effects such as
irradiation-induced flows on the secondary star may be important in explaining
the observed large value for K_2.Comment: 5 pages, 2 figures, accepted by MNRA
Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550-564
We report on the X-ray spectral behavior of the exceptionally bright X-ray
nova XTE J1550-564 during its 1998-99 outburst. Our study is based on 209
pointed observations using the PCA and HEXTE instruments onboard the Rossi
X-ray Timing Explorer spanning 250 days and covering the entire double-peaked
eruption that occurred from 1998 September until 1999 May. The spectra are fit
to a model including multicolor blackbody disk and power-law components. The
source is observed in the very high and high/soft outburst states of black hole
X-ray novae. During the very high state, when the power-law component dominated
the spectrum, the inner disk radius is observed to vary by more than an order
of magnitude; the radius decreased by a factor of 16 in one day during a 6.8
Crab flare. If the larger of these observed radii is taken to be the last
stable orbit, then the smaller observed radius would imply that the inner edge
of the disk is inside the event horizon! However, we conclude that the apparent
variations of the inner disk radius observed during periods of increased
power-law emission are probably caused by the failure of the multicolor
disk/power-law model; the actual physical radius of the inner disk may remain
fairly constant. This interpretation is supported by the fact that the observed
inner disk radius remains approximately constant over 120 days in the high
state, when the power-law component is weak, even though the disk flux and
total flux vary by an order of magnitude. The mass of the black hole inferred
by equating the approximately constant inner disk radius observed in the
high/soft state with the last stable orbit for a Schwarzschild black hole is
M_BH = 7.4 M_sun (D/6 kpc) (cos i)^{-1/2}.Comment: Submitted to ApJ, 20 pages including 6 figures + 4 large table
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