393 research outputs found
Transits and Lensing by Compact Objects in the Kepler Field: Disrupted Stars Orbiting Blue Stragglers
Kepler's first major discoveries are two hot objects orbiting stars in its
field. These may be the cores of stars that have each been eroded or disrupted
by a companion star. The companion, which is the star monitored today, is
likely to have gained mass from its now-defunct partner, and can be considered
to be a blue straggler. KOI-81 is almost certainly the product of stable mass
transfer; KOI-74 may be as well, or it may be the first clear example of a blue
straggler created throughthree-body interactions.
We show that mass transfer binaries are common enough that Kepler should
discover ~1000 white dwarfs orbiting main sequence stars. Most, like KOI-74 and
KOI-81, will be discovered through transits, but many will be discovered
through a combination of gravitational lensing and transits, while lensing will
dominate for a subset. In fact, some events caused by white dwarfs will have
the appearance of "anti-transits" --i.e., short-lived enhancements in the
amount of light received from the monitored star. Lensing and other mass
measurements methods provide a way to distinguish white dwarf binaries from
planetary systems. This is important for the success of Kepler's primary
mission, in light of the fact that white dwarf radii are similar to the radii
of terrestrial planets, and that some white dwarfs will have orbital periods
that place them in the habitable zones of their stellar companions. By
identifying transiting and/or lensing white dwarfs, Kepler will conduct
pioneering studies of white dwarfs and of the end states of mass transfer. It
may also identify orbiting neutron stars or black holes. The calculations
inspired by the discovery of KOI-74 and KOI-81 have implications for
ground-based wide-field surveys as well as for future space-based surveys.Comment: 29 pages, 6 figures, 1 table; submitted to The Astrophysical Journa
Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?
Observations of hundreds of supersoft x-ray sources (SSSs) in external
galaxies have shed light on the diversity of the class and on the natures of
the sources. SSSs are linked to the physics of Type Ia supernovae and
accretion-induced collapse, ultraluminous x-ray sources and black holes, the
ionization of the interstellar medium, and tidal disruption by supermassive
black holes. The class of SSSs has an extension to higher luminosities:
ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an
extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with
energies above 1 eV, but few or none with energies above 2 keV. Finally, a
significant fraction of the SSSs found in external galaxies switch states
between observations, becoming either quasisoft or hard. For many systems
``supersoft'' refers to a temporary state; SSSs are sources, possibly including
a variety of fundamentally different system types, that pass through such a
state. We review those results derived from extragalactic data and related
theoretical work that are most surprising and that suggest directions for
future research.Comment: submitted to Astron.Nachr.; latex, 6 figure
An Ultraluminous Supersoft X-ray Source in M81: An Intermediate-Mass Black Hole?
Ultraluminous supersoft X-ray sources (ULSSS) exhibit supersoft spectra with
blackbody temperatures of 50-100 eV and bolometric luminosities above
erg s, and are possibly intermediate mass black holes (IMBHs) of
or massive white dwarfs that are progenitors of type Ia
supernovae. In this letter we report our optical studies of such a source in
M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a
point-like object, the spectral energy distribution of which reveals a blue
component in addition to the companion of an AGB star. The blue component is
consistent with the power-law as expected from the geometrically-thin accretion
disk around an IMBH accretor, but inconsistent with the power-law as expected
from the X-ray irradiated flared accretion disk around a white dwarf accretor.
This result is strong evidence that M81-ULS1 is an IMBH instead of a white
dwarf.Comment: 12 pages, 1 table, 3 figure
A Third Hot White Dwarf Companion Detected by Kepler
We have found a system listed in the Kepler Binary Catalog (3.273 day period;
Prsa et al. 2010) that we have determined is comprised of a low-mass,
thermally-bloated, hot white dwarf orbiting an A star of about 2.3 solar
masses. In this work we designate the object, KIC 10657664, simply as KHWD3. We
use the transit depth of ~0.66%, the eclipse depth of ~1.9%, and regular smooth
periodic variations at the orbital frequency and twice the orbital frequency to
analyze the system parameters. The smooth periodic variations are identified
with the classical ellipsoidal light variation and illumination effects, and
the newly utilized Doppler boosting effect. Given the measured values of R/a
and inclination angle of the binary, both the ELV and DB effects are mostly
sensitive to the mass ratio, q = M_2/M_1, of the binary. The two effects yield
values of q which are somewhat inconsistent - presumably due to unidentified
systematic effects - but which nonetheless provide a quite useful set of
possibilities for the mass of the white dwarf (either 0.18 +/- 0.03 M_Sun or
0.37 +/- 0.08 M_Sun). All of the other system parameters are determined fairly
robustly. In particular, we show that the white dwarf has a radius of 0.15 +/-
0.01 R_Sun which is extremely bloated over the radius it would have as a fully
degenerate object, and an effective temperature T_eff = 14,100 +/- 350 K.
Binary evolution scenarios and models for this system are discussed. We suggest
that the progenitor binary was comprised of a primary of mass ~2.2 M_Sun (the
progenitor of the current hot white dwarf) and a secondary of mass ~1.4 M_Sun
(the progenitor of the current A star in the system). We compare this new
system with three other white dwarfs in binaries that likely were formed via
stable Roche-lobe overflow (KOI-74, KOI-81, and Regulus).Comment: Accepted for publication in Ap
New ephemeris of the ADC source 2A 1822-371: a stable orbital-period derivative over 30 years
We report on a timing of the eclipse arrival times of the low mass X-ray
binary and X-ray pulsar 2A 1822-371 performed using all available observations
of the Proportional Counter Array on board the Rossi X-ray Timing Explorer,
XMM-Newton pn, and Chandra. These observations span the years from 1996 to
2008. Combining these eclipse arrival time measurements with those already
available covering the period from 1977 to 1996, we obtain an orbital solution
valid for more than thirty years. The time delays calculated with respect to a
constant orbital period model show a clear parabolic trend, implying that the
orbital period in this source constantly increases with time at a rate s/s. This is 3 orders of magnitude larger than
what is expected from conservative mass transfer driven by magnetic braking and
gravitational radiation. From the conservation of the angular momentum of the
system we find that to explain the high and positive value of the orbital
period derivative the mass transfer rate must not be less than 3 times the
Eddington limit for a neutron star, suggesting that the mass transfer has to be
partially non-conservative. With the hypothesis that the neutron star accretes
at the Eddington limit we find a consistent solution in which at least 70% of
the transferred mass has to be expelled from the system.Comment: Published by A&
KOI 1224, a Fourth Bloated Hot White Dwarf Companion Found With Kepler
We present an analysis and interpretation of the Kepler binary system KOI
1224. This is the fourth binary found with Kepler that consists of a thermally
bloated, hot white dwarf in a close orbit with a more or less normal star of
spectral class A or F. As we show, KOI 1224 contains a white dwarf with Teff =
14400 +/- 1100 K, mass = 0.20 +/- 0.02 Msun, and radius = 0.103 +/- 0.004 Rsun,
and an F-star companion of mass = 1.59 +/- 0.07 Msun that is somewhat beyond
its terminal-age main sequence. The orbital period is quite short at 2.69802
days. The ingredients that are used in the analysis are the Kepler binary light
curve, including the detection of the Doppler boosting effect; the NUV and FUV
fluxes from the Galex images of this object; an estimate of the spectral type
of the F-star companion; and evolutionary models of the companion designed to
match its effective temperature and mean density. The light curve is modelled
with a new code named Icarus which we describe in detail. Its features include
the full treatment of orbital phase-resolved spectroscopy, Doppler boosting,
irradiation effects and transits/eclipses, which are particularly suited to
irradiated eclipsing binaries. We interpret the KOI 1224 system in terms of its
likely evolutionary history. We infer that this type of system, containing a
bloated hot white dwarf, is the direct descendant of an Algol-type binary. In
spite of this basic understanding of the origin of KOI 1224, we discuss a
number of problems associated with producing this type of system with this
short of an short orbital period.Comment: 14 pages, 8 figures, 2 tables, submitted to Ap
Searching for pulsed emission from XTE J0929-314 at high radio frequencies
The aim of this work is to search for radio signals in the quiescent phase of
accreting millisecond X-ray pulsars, in this way giving an ultimate proof of
the recycling model, thereby unambiguously establishing that accreting
millisecond X-ray pulsars are the progenitors of radio millisecond pulsars.
To overcome the possible free-free absorption caused by matter surrounding
accreting millisecond X-ray pulsars in their quiescence phase, we performed the
observations at high frequencies. Making use of particularly precise orbital
and spin parameters obtained from X-ray observations, we carried out a deep
search for radio-pulsed emission from the accreting millisecond X-ray pulsar
XTE J0929-314 in three steps, correcting for the effect of the dispersion due
to the interstellar medium, eliminating the orbital motions effects, and
finally folding the time series.
No radio pulsation is present in the analyzed data down to a limit of 68
microJy at 6.4 GHz and 26 microJy at 8.5 GHz.
We discuss several mechanisms that could prevent the detection, concluding
that beaming factor and intrinsic low luminosity are the most likely
explanations.Comment: 7 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Quantifying Long-Term Changes in Carbon Stocks and Forest Structure from Amazon Forest Degradation
Despite sustained declines in Amazon deforestation, forest degradation from logging and firecontinues to threaten carbon stocks, habitat, and biodiversity in frontier forests along the Amazon arcof deforestation. Limited data on the magnitude of carbon losses and rates of carbon recoveryfollowing forest degradation have hindered carbon accounting efforts and contributed to incompletenational reporting to reduce emissions from deforestation and forest degradation (REDD+). Wecombined annual time series of Landsat imagery and high-density airborne lidar data to characterizethe variability, magnitude, and persistence of Amazon forest degradation impacts on abovegroundcarbon density (ACD) and canopy structure. On average, degraded forests contained 45.1% of thecarbon stocks in intact forests, and differences persisted even after 15 years of regrowth. Incomparison to logging, understory fires resulted in the largest and longest-lasting differences in ACD.Heterogeneity in burned forest structure varied by fire severity and frequency. Forests with a historyof one, two, and three or more fires retained only 54.4%, 25.2%, and 7.6% of intact ACD,respectively, when measured after a year of regrowth. Unlike the additive impact of successive fires,selective logging before burning did not explain additional variability in modeled ACD loss andrecovery of burned forests. Airborne lidar also provides quantitative measures of habitat structure thatcan aid the estimation of co-benefits of avoided degradation. Notably, forest carbon stocks recoveredfaster than attributes of canopy structure that are critical for biodiversity in tropical forests, includingthe abundance of tall trees. We provide the first comprehensive look-up table of emissions factors forspecific degradation pathways at standard reporting intervals in the Amazon. Estimated carbon lossand recovery trajectories provide an important foundation for assessing the long-term contributionsfrom forest degradation to regional carbon cycling and advance our understanding of the currentstate of frontier forests
Formation of the planet around the millisecond pulsar J1719-1438
Context. Recently the discovery of PSR J1719-1438, a 5.8 ms pulsar with a
companion in a 2.2 hr orbit, was reported. The combination of this orbital
period and the very low mass function is unique. The discoverers, Bailes et
al., proposed an ultracompact X-ray binary (UCXB) as the progenitor system.
However, the standard UCXB scenario would not produce this system as the time
required to reach this orbital period exceeds the current estimate of the age
of the Universe. The detached state of the system aggravates the problem. Aims.
We want to understand the evolutionary history of PSR J1719-1438, and determine
under which circumstances it could have evolved from an UCXB. Methods. We model
UCXB evolution varying the donor size and investigate the effect of a wind mass
loss from the donor, and compare the results with the observed characteristics
of PSR J1719-1438. Results. An UCXB can reach a 2.2 hr orbit within the age of
the Universe, provided that 1) the millisecond pulsar can significantly heat
and expand the donor by pulsar irradiation, or 2) the system loses extra
orbital angular momentum, e.g. via a fast wind from the donor. Conclusions. The
most likely scenario for the formation of PSR J1719-1438 is UCXB evolution
driven by angular momentum loss via the usual gravitational wave emission,
which is enhanced by angular momentum loss via a donor wind of ~3x10^-13
Msun/yr. Depending on the size of the donor during the evolution, the companion
presently probably has a mass of ~1-3 Jupiter masses, making it a very low mass
white dwarf as proposed by Bailes et al. Its composition can be either helium
or carbon-oxygen. A helium white dwarf companion makes the long (for an UCXB)
orbital period easier to explain, but the required inclination makes it a
priori less likely than a carbon-oxygen white dwarf.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysics. v2: Updated a referenc
Secular spin-down of the AMP XTE J1751-305
Context. Of the 13 known accreting millisecond pulsars, only a few showed
more than one outburst during the RXTE era. XTE J1751-305 showed, after the
main outburst in 2002, other three dim outbursts. We report on the timing
analysis of the latest one, occurred on October 8, 2009 and serendipitously
observed from its very beginning by RXTE. Aims. The detection of the pulsation
during more than one outburst permits to obtain a better constraint of the
orbital parameters and their evolution as well as to track the secular spin
frequency evolution of the source. Methods. Using the RXTE data of the last
outburst of the AMP XTE J1751-305, we performed a timing analysis to improve
the orbital parameters. Because of the low statistics, we used an epoch folding
search technique on the whole data set to improve the local estimate of the
time of ascending node passage. Results. Using this new orbital solution we
epoch folded data obtaining three pulse phase delays on a time span of 1.2
days, that we fitted using a constant spin frequency model. Comparing this
barycentric spin frequency with that of the 2002 outburst, we obtained a
secular spin frequency derivative of -0.55(12) x 10^{-14} Hz s^{-1}. In the
hypothesis that the secular spin-down is due to a rotating magneto-dipole
emission, consistently with what is assumed for radio pulsars, we estimate the
pulsar's magnetic dipole value. We derive an estimate of the magnetic field
strength at the polar cap of B_{PC} = 4.0(4) x 10^8 Gauss, for a neutron star
mass of 1.4M\odot, assuming the Friedman Pandharipande Skyrme equation of
state.Comment: 7 pages, 4 figures, accepted for publication on A&
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