207 research outputs found
Time-Variable Emission from Transiently Accreting Neutron Stars In Quiescence due to Deep Crustal Heating
Transiently accreting neutron stars in quiescence (Lx<10^34 erg/s) have been
observed to vary in intensity by factors of few, over timescales of days to
years. If the quiescent luminosity is powered by a hot NS core, the core
cooling timescale is much longer than the recurrence time, and cannot explain
the observed, more rapid variability. However, the non-equilibrium reactions
which occur in the crust during outbursts deposit energy in iso-density shells,
from which the thermal diffusion timescale to the photosphere is days to years.
The predicted magnitude of variability is too low to explain the observed
variability unless - as is widely believed - the neutrons beyond the
neutron-drip density are superfluid. Even then, variability due to this
mechanism in models with standard core neutrino cooling processes is less than
50 per cent - still too low to explain the reported variability. However,
models with rapid core neutrino cooling can produce variability by a factor as
great as 20, on timescales of days to years following an outburst. Thus, the
factors of few intensity variability observed from transiently accreting
neutron stars can be accounted for by this mechanism only if rapid core cooling
processes are active.Comment: 12 pages, 5 figures, submitted to MNRA
Statistical Re-examination of Reported Emission Lines in the X-ray Afterglow of GRB 011211
(abridged) A 0.2-12 keV spectrum obtained with the XMM EPIC/pn instrument of
GRB 011211 was found by Reeves et al. (2002) to contain emission lines which
were interpreted to be from Mg XI, Si XIV, S XVI, Ar XVIII, and Ca XX, at a
lower redshift (z_{obs}=1.88) than the host galaxy (z_{host}=2.14). We examine
the spectrum independently, and find that the claimed lines would not be
discovered in a blind search. Specifically, Monte Carlo simulations show that
they would be observed in 10% of featureless spectra with the same
signal-to-noise. Imposing a model in which the two brightest lines would be Si
XIV and S XVI K-alpha emission possibly velocity shifted to between
z=1.88--2.40, such features would be found in between ~1.2-2.6% of observed
featureless spectra. We find the detection significances to be insufficient to
justify the claim of detection and the model put forth to explain them. K-alpha
line complexes are also found at z=1.2 and z=2.75 of significance equal to or
greater than that at z=1.88. If one adopts the z=1.88 complex as significant,
one must also adopt the other two complexes to be significant. The
interpretation of these data in the context of the model proposed by Reeves et
al. is therefore degenerate, and cannot be resolved by these data alone.Comment: 23 pages, 4 figures. MNRAS, accepted. Expanded discussio
XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-ray sources
are quantitatively cross-associated with near-infrared (NIR) sources from the
Two Micron All Sky Survey Point Source Catalog (2MASS/PSC). An association
catalog is presented, listing the most likely counterpart for each RASS/BSC
source, the probability Pid that the NIR source and X-ray source are uniquely
associated, and the probability Pnoid that none of the 2MASS/PSC sources are
associated with the X-ray source. The catalog includes 3853 high quality
(Pid>0.98) X-ray--NIR matches, 2280 medium quality (0.98>Pid>0.9) matches, and
4153 low quality (0.9>Pid>0.5) matches. Of the high quality matches, 1418 are
associations that are not listed in the SIMBAD database, and for which no high
quality match with a USNO-A2 optical source was presented for the RASS/BSC
source in previous work. The present work offers a significant number of new
associations with RASS/BSC objects that will require optical/NIR spectroscopy
for classification. For example, of the 6133 Pid>0.9 2MASS/PSC counterparts
presented in the association catalog, 2411 have no classification listed in the
SIMBAD database. These 2MASS/PSC sources will likely include scientifically
useful examples of known source classes of X-ray emitters (white dwarfs,
coronally active stars, active galactic nuclei), but may also contain
previously unknown source classes. It is determined that all coronally active
stars in the RASS/BSC should have a counterpart in the 2MASS/PSC, and that the
unique association of these RASS/BSC sources with their NIR counterparts thus
is confusion limited.Comment: 14 pages, 13 figures, 5 table
Measurement of the Radius of Neutron Stars with High S/N Quiescent Low-mass X-ray Binaries in Globular Clusters
This paper presents the measurement of the neutron star (NS) radius using the
thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) inside globular
clusters (GCs). Recent observations of NSs have presented evidence that cold
ultra dense matter -- present in the core of NSs -- is best described by
"normal matter" equations of state (EoSs). Such EoSs predict that the radii of
NSs, Rns, are quasi-constant (within measurement errors, of ~10%) for
astrophysically relevant masses (Mns > 0.5 Msun). The present work adopts this
theoretical prediction as an assumption, and uses it to constrain a single Rns
value from five qLMXB targets with available high signal-to-noise X-ray
spectroscopic data. Employing a Markov-Chain Monte-Carlo approach, we produce
the marginalized posterior distribution for Rns, constrained to be the same
value for all five NSs in the sample. An effort was made to include all
quantifiable sources of uncertainty into the uncertainty of the quoted radius
measurement. These include the uncertainties in the distances to the GCs, the
uncertainties due to the Galactic absorption in the direction of the GCs, and
the possibility of a hard power-law spectral component for count excesses at
high photon energy, which are observed in some qLMXBs in the Galactic plane.
Using conservative assumptions,we found that the radius, common to the five
qLMXBs and constant for a wide range of masses, lies in the low range of
possible NS radii, Rns=9.1(+1.3)(-1.5) km (90%-confidence). Such a value is
consistent with low-res equations of state. We compare this result with
previous radius measurements of NSs from various analyses of different types of
systems. In addition, we compare the spectral analyses of individual qLMXBs to
previous works.Comment: Accepted to Apj. 31 pages, 17 figures, 8 table
A Method for Distinguishing Between Transiently Accreting Neutron Stars and Black Holes, in Quiescence
We fit hydrogen atmosphere models to the X-ray data for four neutron stars
(three from a previous paper, plus 4U 2129+47) and six black hole candidates
(A0620-00, GS 2000+25, GS 1124-68, GS 2023+33, GRO J1655-40, and GRO J0422+32).
While the neutron stars are similar in their intrinsic X-ray spectra (similar
effective temperatures and emission area radii ~10 km), the spectra of two
black hole candidates are significantly different, and the spectra of the
remaining four are consistent with a very large parameter space that includes
the neutron stars. The spectral differences between the neutron stars and black
hole candidates favors the interpretation that the quiescent neutron star
emission is predominantly thermal emission from the neutron star surface. Our
work suggests that an X-ray spectral comparison in quiescence provides an
additional means for distinguishing between neutron stars and black holes. The
faint X-ray sources in globular clusters are also a class of objects which can
be investigated in this manner.Comment: 33 pages, including 3 ps figures, LaTeX. To appear in Ap
On Estimating the QSO Transmission Power Spectrum
We present new methods to minimize the systematic and random errors for
measuring the transmission power spectrum from the Lyman-alpha forest. Sources
of systematic errors explored include metal line contamination and
continuum-fitting. We advocate the technique of trend-removal in place of
traditional continuum-fitting -- here, a spectrum is normalized by its
(smoothly varying) mean rather than its continuum -- this method is easily
automated and removes biases introduced by continuum-fitting. Trend- removal
can be easily applied to spectra where continuum-fitting is difficult, such as
when the resolution or signal-to-noise is low, or for spectra at high
redshifts. Furthermore, a measurement of the continuum power spectrum using
trend-removal, from either low redshift quasar spectra or the red-side of
Lyman-alpha, allows in principle the removal of spurious power introduced by
the continuum and thereby expanding scales probed to larger ones. We also
derive expressions for the shot-noise bias and variance of the power spectrum
estimate, taking into account the non-Poissonian nature of the shot-noise and
the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance
weighting of the data is given. Finally, we give practical suggestions on
observing strategy: the desired resolution and S/N for different purposes, and
how to distribute one's finite observing time among quasar targets. Also
discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey,
which has the potential to measure the power spectrum at z ~ 2-4 accurate to
better than 1 % per mode -- the techniques presented here will be useful for
tackling the anticipated issues of shot-noise and continuum contamination.Comment: 35 pages, 14 figures, submitted to Ap
Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the nu F_nu Spectrum of Gamma Ray Bursts
We present results of correlation studies, examining the association between
the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's
energy fluence and photon peak flux. We discuss methods to account for data
truncation in E_p and fluence or flux when performing the correlation analyses.
However, because bursts near the detector threshold are not usually able to
provide reliable spectral parameters, we focus on results for the brightest
bursts in which we can better understand the selection effects relevant to E_p
and burst strength.
We find that there is a strong correlation between total fluence and E_p. We
discuss these results in terms of both cosmological and intrinsic effects.
In particular, we show that for realistic distributions of the burst
parameters, cosmological expansion alone cannot account for the correlation
between E_p and total fluence; the observed correlation is likely a result of
an intrinsic relation between the burst rest-frame peak energy and the total
radiated energy. We investigate this latter scenario in the context of
synchrotron radiation from external and internal shock models of GRBs. We find
that the internal shock model is consistent with our interpretation of the
correlation, while the external shock model cannot easily explain this
intrinsic relation between peak energy and burst radiated energy.Comment: 23 pages, including 8 postscript figures. Submitted to Ap
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