797 research outputs found
Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-ray Luminosity Functions
As part of our campaign to determine the nature of the various source
populations of the low-luminosity globular cluster X-ray sources, we have
obtained a Chandra X-ray Observatory ACIS-S3 image of the globular cluster NGC
6440. We detect 24 sources to a limiting luminosity of ~2 times 10^31 erg/s
(0.5-2.5keV) inside the cluster's half-mass radius, all of which lie within ~2
core radii of the cluster center. We also find excess emission in and around
the core which could be due to unresolved point sources. Based upon X-ray
luminosities and colors, we conclude that there are 4-5 likely quiescent
low-mass X-ray binaries and that most of the other sources are cataclysmic
variables. We compare these results to Chandra results from other globular
clusters and find the X-ray luminosity functions differ among the clusters.Comment: 9 pages, 4 figures, accepted by ApJ, minor changes, added table of
clusters' physical parameter
Identification of the LMXB and Faint X-ray Sources in NGC 6652
We have detected three new x-ray point sources, in addition to the known
low-mass x-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a
Chandra 1.6 ksec HRC-I exposure. Star 49 (M_{V}~4.7), suggested by Deutsch et
al.(1998) as the optical candidate for the LMXB, is identified (<0.3") not with
the LMXB, but with another, newly detected source (B). Using archival HST
images, we identify (<0.3") the LMXB (A) and one of the remaining new sources
(C) with blue variable optical counterparts at M_{V}~3.7 and 5.3 respectively.
The other new source (D) remains unidentified in the crowded cluster core. In
the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and
N_{H}=5.5*10^{20}, source A has intrinsic luminosity L_{X}~5.3*10^{35} ergs/s.
Assuming a 1 keV thermal bremsstrahlung spectrum, B has L_{X}~4.1*10^{33}
ergs/s, while C and D have L_{X}~8*10^{32}$ ergs/s. Source B is probably a
quiescent LMXB, while source C may be either a luminous CV or quiescent LMXB.Comment: 14 pages, 3 figures, accepted by Astrophysical Journa
Classical Novae as a Probe of the Cataclysmic Variable Population
Classical Novae (CNe) are the brightest manifestation of mass transfer onto a
white dwarf in a cataclysmic variable (CV). As such, they are probes of the
mass transfer rate, Mdot, and WD mass, Mwd, in these interacting binaries. Our
calculations of the dependence of the CN ignition mass, Mign, on Mdot and Mwd
yields the recurrence times of these explosions. We show that the observed CNe
orbital period distribution is consistent with the interrupted magnetic braking
evolutionary scenario, where at orbital periods Porb > 3 hr mass transfer is
driven by angular momentum loss via a wind from the companion star and at Porb
< 3 hr by gravitational radiation. About 50% of CNe occur in binaries accreting
at Mdot ~= 10^{-9} Msun/yr with Porb = 3-4 hr, with the remaining 50% split
evenly between Porb longer (higher Mdot) and shorter (lower Mdot) than this.
This resolution of the relative contribution to the CN rate from different CVs
tells us that 3(9)x10^5 CVs with WD mass 1.0(0.6)Msun are needed to produce one
CN per year. Using the K-band specific CN rate measured in external galaxies,
we find a CV birthrate of 2(4)x10^{-4}/yr per 10^{10}Lsun,K, very similar to
the luminosity specific Type Ia supernova rate in elliptical galaxies.
Likewise, we predict that there should be 60-180 CVs for every 10^6Lsun,K in an
old stellar population, similar to the number of X-ray identified CVs in the
globular cluster 47 Tuc, showing no overabundance relative to the field. Using
a two-component steady state model of CV evolution we show that the fraction of
CVs which are magnetic (22%) implies a birthrate of 8% relative to non-magnetic
CVs, similar to the fraction of strongly magnetic field WDs. (abridged)Comment: 6 pages, 2 figures, Accepted to the Astrophysical Journa
New BeppoSAX-WFC results on superbursts
Presently seven superbursters have been identified representing 10% of the
total Galactic X-ray burster population. Four superbursters were discovered
with the Wide Field Cameras (WFCs) on BeppoSAX and three with the All-Sky
Monitor and Proportional Counter Array on RXTE. We discuss the properties of
superbursters as derived from WFC observations. There are two interesting
conclusions. First, the average recurrence time of superbursts among X-ray
bursters that are more luminous than 10% of the Eddington limit is 1.5 yr per
object. Second, superbursters systematically have higher alpha values and
shorter ordinary bursts than most bursters that have not exhibited superbursts,
indicating a higher level of stable thermonuclear helium burning. Theory
predicts hitherto undetected superbursts from the most luminous neutron stars.
We investigate the prospects for finding these in GX~17+2.Comment: Submitted in January 2004 for the Proceedings of the meeting 'X-Ray
Timing 2003: Rossi and Beyond', eds. P. Kaaret, F. K. Lamb, & J. H. Swank
(Melville, NY: American Institute of Physics
Effects of Magnetic Braking and Tidal Friction on Hot Jupiters
Tidal friction is thought to be important in determining the long-term
spin-orbit evolution of short-period extrasolar planetary systems. Using a
simple model of the orbit-averaged effects of tidal friction Eggleton, Kiseleva
& Hut (1998), we analyse the effects of the inclusion of stellar magnetic
braking on the evolution of such systems. A phase-plane analysis of a
simplified system of equations, including only the stellar tide together with a
model of the braking torque proposed by Verbunt & Zwaan (1981), is presented.
The inclusion of stellar magnetic braking is found to be extremely important in
determining the secular evolution of such systems, and its neglect results in a
very different orbital history. We then show the results of numerical
integrations of the full tidal evolution equations, using the misaligned spin
and orbit of the XO-3 system as an example, to study the accuracy of simple
timescale estimates of tidal evolution. We find that it is essential to
consider coupled evolution of the orbit and the stellar spin in order to model
the behaviour accurately. In addition, we find that for typical Hot Jupiters
the stellar spin-orbit alignment timescale is of the same order as the inspiral
time, which tells us that if a planet is observed to be aligned, then it
probably formed coplanar. This reinforces the importance of Rossiter-McLaughlin
effect observations in determining the degree of spin-orbit alignment in
transiting systems.Comment: 6 pages, 2 figures, to appear in IAU 259 Conference Proceeding
Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient
We analyse the two-dimensional MHD configurations characterising the steady
state of the accretion disk on a highly magnetised neutron star. The model we
describe has a local character and represents the extension of the crystalline
structure outlined in Coppi (2005), dealing with a local model too, when a
specific accretion rate is taken into account. We limit our attention to the
linearised MHD formulation of the electromagnetic back-reaction characterising
the equilibrium, by fixing the structure of the radial, vertical and azimuthal
profiles. Since we deal with toroidal currents only, the consistency of the
model is ensured by the presence of a small collisional effect,
phenomenologically described by a non-zero constant Nernst coefficient (thermal
power of the plasma). Such an effect provides a proper balance of the electron
force equation via non zero temperature gradients, related directly to the
radial and vertical velocity components.
We show that the obtained profile has the typical oscillating feature of the
crystalline structure, reconciled with the presence of viscosity, associated to
the differential rotation of the disk, and with a net accretion rate. In fact,
we provide a direct relation between the electromagnetic reaction of the disk
and the (no longer zero) increasing of its mass per unit time. The radial
accretion component of the velocity results to be few orders of magnitude below
the equatorial sound velocity. Its oscillating-like character does not allow a
real matter in-fall to the central object (an effect to be searched into
non-linear MHD corrections), but it accounts for the out-coming of steady
fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics
Letters
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
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