151 research outputs found
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
The Lower Main Sequence and the Orbital Period Distribution of Cataclysmic Variable Stars
The color-magnitude diagram of the lower main sequence, as measured from a
volume-limited sample of nearby stars, shows an abrupt downward jump between Mv
= 12 and 13. This jump indicates that the observed mass-radius relationship
steepens between 0.3 and 0.2 solar masses, but theoretical models show no such
effect. It is difficult to isolate the source of this disagreement: the
observational mass-radius relationship relies upon transformations that may not
be sufficiently accurate, while the theoretical relationship relies upon
stellar models that may not be sufficiently complete, particularly in their
treatment of the complex physics governing the interior equation-of-state.
If the features in the observationally derived mass-radius relationship are
real, their existence provides a natural explanation for the well-known gap in
the orbital period distribution of cataclysmic variables. This explanation
relies only upon the observed mass-radius relationship of low-mass stars, and
does not require ad hoc changes in magnetic braking or in the structure of
cataclysmic variable secondaries. If correct, it will allow broader application
of cataclysmic variable observations to problems of basic stellar physics.Comment: 27 pages, 10 postscript figures, AASTeX (aaspp4), accepted by Ap
XMM-Newton observations of two black hole X-ray transients in quiescence
We report on XMM-Newton observations of GRO J1655-40 and GRS 1009-45, which
are two black hole X-ray transients currently in their quiescent phase. GRO
J1655-40 was detected with a 0.5 - 10 keV luminosity of 5.9 10^{31} erg/s. This
luminosity is comparable to a previous Chandra measurement, but ten times lower
than the 1996 ASCA value, most likely obtained when the source was not yet in a
true quiescent state. Unfortunately, XMM-Newton failed to detect GRS 1009-45. A
stringent upper limit of 8.9 10^{30} erg/s was derived by combining data from
the EPIC-MOS and PN cameras.
The X-ray spectrum of GRO J1655-40 is very hard as it can be fitted with a
power law model of photon index ~ 1.3 +/- 0.4. Similarly hard spectra have been
observed from other systems; these rule out coronal emission from the secondary
or disk flares as the origin of the observed X-rays. On the other hand, our
observations are consistent with the predictions of the disc instability model
in the case that the accretion flow forms an advection dominated accretion flow
(ADAF) at distances less than a fraction ~ 0.1 - 0.3) of the circularization
radius. This distance corresponds to the greatest extent of the ADAF that is
thought to be possible.Comment: 6 pages, 4 figures. Submitted to Astronomy and Astrophysic
The Mini AGN at the Center of the Elliptical Galaxy NGC 4552 with HST
The complex phenomenology shown by the UV-bright, variable spike first
detected with the Hubble Space Telescope (HST) at the center of the otherwise
normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and
spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near UV
have been analyzed in a homogeneous fashion, showing that the central spike has
brightened by a factor ~4.5 between 1991 and 1993, and has decreased its
luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending
from the near UV to the red side of the optical spectrum reveals a strong UV
continuum over the spectrum of the underlying galaxy, along with several
emission lines in both the UV and the optical ranges. In spite of the low
luminosity of the UV continuum of the spike (~3*10^5 Lsolar), the spike is
definitely placed among AGNs by current diagnostics based on the emission line
intensity ratios, being just on the borderline between Seyferts and LINERs.
Line profiles are very broad, and both permitted and forbidden lines are best
modelled with a combination of broad and narrow components, with FWHM of ~3000
km s^-1 and ~700 km s^-1, respectively. This evidence argues for the variable
central spike being produced by a modest accretion event onto a central massive
black hole (BH), with the accreted material having possibly being stripped from
a a star in a close fly by with the BH. The 1996 broad Halpha luminosity of
this mini-AGN is ~5.6*10^37 erg s^-1, about a factor of two less than that of
the nucleus of NGC 4395, heretofore considered to be the faintest known AGN.Comment: 40 pages, LaTeX, with 12 PostScript figures. Accepted for publication
in the Astrophysical Journa
A Detection of an Anti-correlated Hard X-ray Lag in AM Herculis
Context {Earlier cross-correlation studies for AM Her were performed in
various energy range from optical to X-ray and suggested that it mostly shows a
high level of correlation but on occasion it shows a low level of correlation
or uncorrelation.} Aims {To investigate the degree of correlation between soft
(2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of
the X-ray data sets of AM Her obtained with {\it RXTE}.}
Methods {We cross-correlate the background-subtracted soft and hard X-ray
light curves using the XRONOS program crosscor and fit a model to the obtained
cross-correlation functions.}
Results {We detect a hard X-ray lag of s in a specific section of
energy-dependent light curve, where the soft X-ray (2-4 keV) intensity
decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral
analysis, we find that the X-ray emission temperature increases during the
anti-correlated intensity variation. In two other observations, the
cross-correlation functions show a low level of correlation, which is
consistent with the earlier results performed in a different energy range.}
Conclusions {We report a detection of an anti-correlated hard X-ray lag of
190 s from the proto-type polar AM Her. The hard X-ray lag is detected
for the first time in the given energy range, and it is the longest lag among
those reported in magnetic cataclysmic variables. We discuss the implications
of our findings regarding the origin of the hard X-ray lag and the
anti-correlated intensity variation.}Comment: Accepted in A&A, 4 page
On the Nature of the Bright Short-Period X-ray Source in the Circinus Galaxy Field
The spectrum and light curve of the bright X-ray source CG X-1 in the field
of the Circinus galaxy are re-examined. Previous analyses have concluded that
the source is an accreting black hole of about 50 solar masses although it was
noted that the light curve resembles that of an AM Her system. Here we show
that the short period and an assumed main sequence companion constrain the mass
of the companion to less than one solar mass. Further a possible eclipse seen
during one of the Chandra observations and a subsequent XMM-Newton observation
constrains the mass of the compact object to less than about 60 solar masses.
If such a system lies in the Circinus galaxy, then the accreting object must
either radiate anisotropically or strongly violate the Eddington limit. Even if
the emission is beamed, then the companion star which intercepts this flux
during eclipse will be driven out of thermal equilibrium and evaporate within
about 1000 years. We find that the observations cannot rule out an AM Her
system in the Milky Way and that such a system can account for the variations
seen in the light curve.Comment: 25 pages, 8 figures, accepted for publication in the Astrophysical
Journa
The accretion flow in the discless intermediate polar V2400 Ophiuchi
RXTE observations confirm that the X-ray lightcurve of V2400 Oph is pulsed at
the beat cycle, as expected in a discless intermediate polar. There are no
X-ray modulations at the orbital or spin cycles, but optical line profiles vary
with all three cycles. We construct a model for line-profile variations in a
discless accretor, based on the idea that the accretion stream flips from one
magnetic pole to the other, and show that this accounts for the observed
behaviour over the spin and beat cycles. The minimal variability over the
orbital cycle implies that 1) V2400 Oph is at an inclination of only ~10 deg,
and 2) much of the accretion flow is not in a coherent stream, but is circling
the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss
the light this sheds on disc formation in intermediate polars.Comment: 10 pages, 12 figures, To appear in MNRAS, includes low-res figures to
reduce siz
The Birth of an Ultra-Luminous X-ray Source in M83
A previously undetected X-ray source (L_X<10**36 erg/s) in the strongly
star-forming galaxy M83 entered an ultraluminous state between August 2009 and
December 2010. It was first seen with Chandra on 23 December 2010 at L_X ~ 4
10**39 ergs/s, and has remained ultraluminous through our most recent
observations in December 2011, with typical flux variation of a factor of two.
The spectrum is well fitted by a combination of absorbed power-law and disk
black-body models. While the relative contributions of the models varies with
time, we have seen no evidence for a canonical state transition. The luminosity
and spectral properties are consistent with accretion powered by a black hole
with M_BH ~ 40-100 solar masses. In July 2011 we found a luminous, blue optical
counterpart which had not been seen in deep HST observations obtained in August
2009. These optical observations suggest that the donor star is a low-mass star
undergoing Roche-lobe overflow, and that the blue optical emission seen during
the outburst is coming from an irradiated accretion disk. This source shows
that ultraluminous X-ray sources (ULXs) with low-mass companions are an
important component of the ULX population in star-forming galaxies, and
provides further evidence that the blue optical counterparts of some ULXs need
not indicate a young, high-mass companion, but rather that they may indicate
X-ray reprocessing.Comment: 40 pages, 7 figures, accepted for publication in Ap
X-Ray Flares and Oscillations from the Black Hole Candidate X-Ray Transient XTE J1650-500 at Low Luminosity
We report on X-ray observations made with the Rossi X-ray Timing Explorer of
the black hole candidate (BHC) transient XTE J1650-500 at the end of its first,
and currently only, outburst. By monitoring the source at low luminosities over
several months, we found 6 bright ~100 second X-ray flares and long time scale
oscillations of the X-ray flux. The oscillations are aperiodic with a
characteristic time scale of 14.2 days and an order of magnitude variation in
the 2.8-20 keV flux. The oscillations may be related to optical
"mini-outbursts" that have been observed at the ends of outbursts for other
short orbital period BHC transients. The X-ray flares have durations between 62
and 215 seconds and peak fluxes that are 5-24 times higher than the persistent
flux. The flares have non-thermal energy spectra and occur when the persistent
luminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for the
brightest flare demonstrates that physical models for BHC systems must be able
to account for the situation where the X-ray flux increases by a factor of up
to 24 on a time scale of seconds. We discuss the flares in the context of
observations and theory of Galactic BHCs and compare the flares to those
detected from Sgr A*, the super-massive black hole at the Galactic center. We
also compare the flares to X-ray bursts that are seen in neutron star systems.
While some of the flare light curves are similar to those of neutron star
bursts, the flares have non-thermal energy spectra in contrast to the blackbody
spectra exhibited in bursts. This indicates that X-ray bursts should not be
taken as evidence that a given system contains a neutron star unless the
presence of a blackbody component in the burst spectrum can be demonstrated.Comment: 9 pages, accepted by Ap
Advection-Dominated Flows and the X-ray Delay in the Outburst of GRO J1655-40
We show that the time delay between the optical and X-ray outbursts of the
black-hole soft X-ray transient source GRO J1655-40, observed in April 1996,
requires that the accretion flow in this object must consist of two components:
a cold outer accretion disk and an extremely hot inner advection-dominated
accretion flow (ADAF). In quiescence, the model predicts a spectrum which is in
good agreement with observations, with most of the observed radiation coming
from the ADAF. By fitting the observed spectrum, we estimate the mass accretion
rate of the quiescent system and the transition radius between the disk and the
ADAF. We present a detailed numerical simulation of a dwarf-nova type
instability in the outer disk. The resulting heat front reaches the ADAF cavity
promptly; however, it must then propagate inward slowly on a viscous time
scale, thereby delaying the onset of the X-ray flux. The model reproduces the
observed optical and X-ray light curves of the April 1996 outburst, as well as
the 6-day X-ray delay. Further, the model gives an independent estimate of the
quiescent mass accretion rate which is in very good agreement with the rate
estimated from fitting the quiescent spectrum. We show that a pure thin disk
model without an ADAF zone requires significant tuning to explain the X-ray
delay; moreover, such a model does not explain the quiescent X-ray emission of
GRO J1655-40.Comment: 13 pages, 4 figures Submitted to The Astrophysical Journa
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