128 research outputs found
The Future of X-ray Time Domain Surveys
Modern X-ray observatories yield unique insight into the astrophysical time
domain. Each X-ray photon can be assigned an arrival time, an energy and a sky
position, yielding sensitive, energy-dependent light curves and enabling
time-resolved spectra down to millisecond time-scales. Combining those with
multiple views of the same patch of sky (e.g., in the Chandra and XMM-Newton
deep fields) so as to extend variability studies over longer baselines, the
spectral timing capacity of X-ray observatories then stretch over 10 orders of
magnitude at spatial resolutions of arcseconds, and 13 orders of magnitude at
spatial resolutions of a degree. A wealth of high-energy time-domain data
already exists, and indicates variability on timescales ranging from
microseconds to years in a wide variety of objects, including numerous classes
of AGN, high-energy phenomena at the Galactic centre, Galactic and
extra-Galactic X-ray binaries, supernovae, gamma-ray bursts, stellar flares,
tidal disruption flares, and as-yet unknown X-ray variables. This workshop
explored the potential of strategic X-ray surveys to probe a broad range of
astrophysical sources and phenomena. Here we present the highlights, with an
emphasis on the science topics and mission designs that will drive future
discovery in the X-ray time domain.Comment: 8 pages, 1 figure, Conference proceedings for IAU Symposium 285, "New
Horizons in Time Domain Astronomy," Oxford, UK, Sep 19-23, 2011. To be
published by IA
Chandra Observations of A2670 and A2107: A Comet Galaxy and cDs with Large Peculiar Velocities
We present an analysis of Chandra observations of the galaxy clusters A2670
and A2107. Their cD galaxies have large peculiar velocities (>200km/s) and thus
the clusters appear to be undergoing mergers. In A2670, we find a comet-like
structure around one of the brightest galaxies. At the leading edge of the
structure, there is a cold front. The mass of the X-ray gas in the comet-like
structure suggests that the galaxy was in a small cluster or group, and its
intracluster medium (ICM) is being stripped by ram-pressure. The regions of
cool interstellar medium (ISM) of the cD galaxies in A2670 and A2107 are very
compact. This is similar to the brightest galaxies in the Coma cluster, which
is also a merging cluster. In each galaxy, the short cooling time of the ISM
requires a heating source; the compact nature of the ISM makes it unlikely that
the heating source is a central active galactic nucleus (AGN).Comment: PASJ in pres
Luminous X-ray Flares from Low Mass X-ray Binary Candidates in the Early-Type Galaxy NGC 4697
We report results of the first search specifically targeting short-timescale
X-ray flares from low-mass X-ray binaries in an early-type galaxy. A new method
for flare detection is presented. In NGC 4697, the nearest, optically luminous,
X-ray faint elliptical galaxy, 3 out of 157 sources are found to display flares
at >99.95% probability, and all show more than one flare. Two sources are
coincident with globular clusters and show flare durations and luminosities
similar to (but larger than) Type-I X-ray superbursts found in Galactic neutron
star (NS) X-ray binaries (XRBs). The third source shows more extreme flares.
Its flare luminosity (~6E39 erg/s) is very super-Eddington for an NS and is
similar to the peak luminosities of the brightest Galactic black hole (BH)
XRBs. However, the flare duration (~70 s) is much shorter than are typically
seen for outbursts reaching those luminosities in Galactic BH sources.
Alternative models for the flares are considered.Comment: Astrophysical Journal Letters, accepted: 4 page
2MASS Reveals a Large Intrinsic Fraction of BALQSOs
The intrinsic fraction of broad absorption line quasars (BALQSOs) is
important in constraining geometric and evolutionary models of quasars. We
present the fraction of BALQSOs in 2MASS detected quasars within the SDSS DR3
sample in the redshift range of 1.7 < z < 4.38. The fraction of BALQSOs is
40.4^{+3.4}_{-3.3}% in the 2MASS 99% database K_s band completeness sample, and
38.5^{+1.7}_{-1.7}% in the larger 2MASS sample extending below the completeness
limit. These fractions are significantly higher than the 26% reported in the
optical bands for the same parent sample. We also present the fraction of
BALQSOs as functions of apparent magnitudes, absolute magnitudes, and redshift
in the 2MASS and SDSS bands. The 2MASS fractions are consistently higher than
the SDSS fractions in every comparison, and the BALQSO fractions steadily
increase with wavelength from the SDSS u to the 2MASS K_s bands. Furthermore,
the i - K_s color distributions of BALQSOs and non-BALQSOs indicate that
BALQSOs are redder than non-BALQSOs, with a K-S test probability of 2e-12.
These results are consistent with the spectral difference between BALQSOs and
non-BALQSOs including both the absorption troughs and dust extinction in
BALQSOs, which leads to significant selection biases against BALQSOs in the
optical bands. Using a simple simulation incorporating the luminosity function
of quasars and the amount of obscuration for BALQSOs, we simultaneously fit the
BALQSO fractions in the SDSS and 2MASS bands. We obtain a true BALQSO fraction
of 43\pm2% for luminous quasars (M_{K_s} \lesssim -30.1 mag).Comment: 20 pages, 6 figures, accepted by Ap
Deep Chandra Observations of the Compact Starburst Galaxy Henize 2-10: X-rays from the Massive Black Hole
We present follow-up X-ray observations of the candidate massive black hole
(BH) in the nucleus of the low-mass, compact starburst galaxy Henize 2-10.
Using new high-resolution observations from the Chandra X-ray Observatory
totaling 200 ks in duration, as well as archival Chandra observations from
2001, we demonstrate the presence of a previously unidentified X-ray point
source that is spatially coincident with the known nuclear radio source in
Henize 2-10 (i.e., the massive BH). We show that the hard X-ray emission
previously identified in the 2001 observation is dominated by a source that is
distinct from the nucleus, with the properties expected for a high-mass X-ray
binary. The X-ray luminosity of the nuclear source suggests the massive BH is
radiating significantly below its Eddington limit (~10^-6 L_Edd), and the soft
spectrum resembles other weakly accreting massive BHs including Sagittarius A*.
Analysis of the X-ray light curve of the nucleus reveals the tentative
detection of a ~9-hour periodicity, although additional observations are
required to confirm this result. Our study highlights the need for sensitive
high-resolution X-ray observations to probe low-level accretion, which is the
dominant mode of BH activity throughout the Universe.Comment: Accepted for publication in ApJ Letters. 7 pages, 4 figure
Chandra Observations of Low Mass X-ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and NGC 4382 (M85)
(Abridged) We used the Chandra X-ray Observatory ACIS S3 to image the X-ray
faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The
observations resolve much of the X-ray emission into 99 and 58 sources,
respectively, most of which are low-mass X-ray binaries (LMXBs) associated with
each of the galaxies. We identify 18 out of the 37 X-ray sources in a central
field in NGC 4365 with globular clusters. The luminosity functions of the
resolved sources for both galaxies are best fit with cutoff power-laws whose
cutoff luminosity is ergs s. These
luminosities are much larger than those previously measured for similar
galaxies; we do not find evidence for a break in the luminosity function at the
Eddington luminosity of a 1.4 neutron star. The spatial distributions
of the resolved sources for both galaxies are broader than the distribution of
optical stars. In both galaxies, a hard power-law model fits the summed
spectrum of all of the sources. The unresolved emission is best fit by the sum
of a soft mekal model representing emission from diffuse gas, and a hard
power-law, presumed to be from unresolved LMXBs. A standard beta model fits the
radial distribution of the diffuse gas in both galaxies. In the elliptical NGC
4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has
a larger core radius. This may indicate that the gas in NGC 4382 is rotating
significantly.Comment: Astrophysical Journal, accepted: 38 pages with 20 embedded reduced
resolution Postscript figure
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