4,362 research outputs found
A deep cluster survey in Chandra archival data. First results
I present the first results of a search for clusters of galaxies in Chandra
ACIS pointed observations at high galactic latitude with exposure times larger
than 10 ks. The survey is being carried out using the Voronoi Tessellation and
Percolation technique, which is particularly suited for the detection and
accurate quantification of extended and/or low surface brightness emission in
X-ray imaging observations. A new catalogue of 36 cluster candidates has been
created from 5.55 square degrees of surveyed area. Five of these candidates
have already been associated to visible enhancements of the projected galaxy
distribution in low deepness DSS-II fields and are probably low-to moderate
redshift systems. Three of the candidates have been identified in previous
ROSAT-based surveys. I show that a significative fraction (30-40%) of the
candidate clusters are probably intermediate to high redshift systems. In this
paper I publish the catalogue of these first candidate clusters. I also derive
the number counts of clusters and compare it with the results of deep
ROSAT-based cluster surveys.Comment: 28 pages, 20 figure
AMUSE-Virgo I. Super-massive black holes in low-mass spheroids
We present the first results from the AGN Multiwavelength Survey of
Early-type galaxies in the Virgo cluster (AMUSE-Virgo). This large program
targets 100 early-type galaxies with the Advanced CCD Imaging Spectrometer on
board the Chandra X-ray Observatory and the Multi-band Imaging Photometer on
board the Spitzer Space Telescope, with the aim of providing an unbiased census
of low-level super-massive black hole (SMBH) activity in the local universe.
Here we report on the Chandra observations of the first 16 targets, and combine
them with results from archival data of another, typically more massive, 16
targets. Point-like X-ray emission from a position coincident with the optical
nucleus is detected in 50% of the galaxies (down to our completeness limit of
~4E+38 erg/sec). Two of the X-ray nuclei are hosted by galaxies (VCC1178=N4464
and VCC1297=N4486B) with absolute B magnitudes fainter than -18, where nuclear
star clusters are known to become increasingly common. After carefully
accounting for possible contamination from low mass X-ray binaries, we argue
that the detected nuclear X-ray sources are most likely powered by low-level
accretion on to a SMBH, with a <11% chance contamination in VCC1178, where a
star cluster is barely resolvable in archival Hubble Space Telescope images.
Based on black hole mass estimates from the global properties of the host
galaxies, all the detected nuclei are highly sub-Eddington, with luminosities
in the range -8.4<log(L_0.3-10keV/L_Edd)<-5.9. The incidence of nuclear X-ray
activity increases with the stellar mass M_star of the host galaxy: only
between 3-44% of the galaxies with M_star<1E+10 M_Sun harbor an X-ray active
SMBH. The fraction rises to between 49-87% in galaxies with stellar mass above
1E+10 M_Sun (at the 95% confidence level).Comment: Revised version, accepted by Ap
The Chandra Deep Group Survey -- cool core evolution in groups and clusters of galaxies
We report the results of a study which assembles deep observations with the
ACIS-I instrument on the Chandra Observatory to study the evolution in the core
properties of a sample of galaxy groups and clusters out to redshifts . A search for extended objects within these fields yields a total of 62
systems for which redshifts are available, and we added a further 24
non-X-ray-selected clusters, to investigate the impact of selection effects and
improve our statistics at high redshift. Six different estimators of cool core
strength are applied to these data: the entropy (K) and cooling time
() within the cluster core, the cooling time as a fraction of the age
of the Universe (), and three estimators based on the
cuspiness of the X-ray surface brightness profile. A variety of statistical
tests are used to quantify evolutionary trends in these cool core indicators.
In agreement with some previous studies, we find that there is significant
evolution in , but little evolution in , suggesting
that gas is accumulating within the core, but that the cooling time deep in the
core is controlled by AGN feedback. We show that this result extends down to
the group regime and appears to be robust against a variety of selection biases
(detection bias, archival biases and biases due to the presence of central
X-ray AGN) which we consider.Comment: Accepted by MNRAS, 24 pages, 11 figure
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Finding Rare AGN: X-ray Number Counts of Chandra Sources in Stripe 82
We present the first results of a wide area X-ray survey within the Sloan
Digital Sky Survey (SDSS) Stripe 82, a 300 deg region of the sky with a
substantial investment in multi-wavelength coverage. We analyzed archival {\it
Chandra} observations that cover 7.5 deg within Stripe 82 ("Stripe 82
ACX"), reaching 4.5 flux limits of 7.9,
3.4 and 1.8 erg s cm in the soft
(0.5-2 keV), hard (2-7 keV) and full (0.5-7 keV) bands, to find 774, 239 and
1118 X-ray sources, respectively. Three hundred twenty-one sources are detected
only in the full band and 9 sources are detected solely in the soft band.
Utilizing data products from the {\it Chandra} Source Catalog, we construct
independent Log-Log relationships, detailing the number density of X-ray
sources as a function of flux, which show general agreement with previous {\it
Chandra} surveys. We compare the luminosity distribution of Stripe 82 ACX with
the smaller, deeper CDF-S + E-CDFS surveys and with {\it Chandra}-COSMOS,
illustrating the benefit of wide-area surveys in locating high luminosity AGN.
We also investigate the differences and similarities of X-ray and optical
selection to uncover obscured AGN in the local Universe. Finally, we estimate
the population of AGN we expect to find with increased coverage of 100 deg
or 300 deg, which will provide unprecedented insight into the high
redshift, high luminosity regime of black hole growth currently
under-represented in X-ray surveys.Comment: Accepted for publication in MNRAS, 15 pages, 6 Figures, 2 Table
Two fast X-ray transients in archival Chandra data
We present the discovery of two new X-ray transients in archival Chandra
data. The first transient, XRT 110103, occurred in January 2011 and shows a
sharp rise of at least three orders of magnitude in count rate in less than 10
s, a flat peak for about 20 s and decays by two orders of magnitude in the next
60 s. We find no optical or infrared counterpart to this event in preexisting
survey data or in an observation taken by the SIRIUS instrument at the Infrared
Survey Facility 2.1 yr after the transient, providing limiting magnitudes of
J>18.1, H>17.6 and Ks>16.3. This event shows similarities to the transient
previously reported in Jonker et al. which was interpreted as the possible
tidal disruption of a white dwarf by an intermediate mass black hole. We
discuss the possibility that these transients originate from the same type of
event. If we assume these events are related a rough estimate of the rates
gives 1.4*10^5 per year over the whole sky with a peak 0.3-7 keV X-ray flux
greater than 2*10^-10 erg cm^-2 s^-1 . The second transient, XRT 120830,
occurred in August 2012 and shows a rise of at least three orders of magnitude
in count rate and a subsequent decay of around one order of magnitude all
within 10 s, followed by a slower quasi-exponential decay over the remaining 30
ks of the observation. We detect a likely infrared counterpart with magnitudes
J=16.70+/-0.06, H=15.92+/-0.04 and Ks=15.37+/-0.06 which shows an average
proper motion of 74+/-19 milliarcsec per year compared to archival 2MASS
observations. The JHKs magnitudes, proper motion and X-ray flux of XRT 120830
are consistent with a bright flare from a nearby late M or early L dwarf.Comment: Accepted for publication in MNRAS, 6 pages, 5 figure
First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies
We present the first measurement of the AGN fraction in high-redshift
clusters of galaxies (z~0.6) with spectroscopy of one cluster and archival data
for three additional clusters. We identify 8 AGN in all four of these clusters
from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV)
luminosity L_{X,H} > 10^43 erg/s in host galaxies more luminous than a rest
frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X,H}
> 10^43 erg/s we discovered in our earlier study of eight low-redshift clusters
with z=0.06-0.31 (average z~0.2). Three of the four high-redshift cluster
datasets are sensitive to nearly L_{X,H} > 10^42 erg/s and we identify seven
AGN above this luminosity limit, compared to two in eight, low-redshift
clusters. Based on membership estimates for each cluster, we determine that the
AGN fraction at z~0.6 is f_A(L_X>10^42;M_R<-20) = 0.028 (+0.019/-0.012) and
f_A(L_X>10^43;M_R<-20) = 0.020 (+0.012/-0.008). These values are approximately
a factor of 20 greater than the AGN fractions in lower-redshift (average z~0.2)
clusters of galaxies and represent a substantial increase over the factors of
1.5 and 3.3 increase, respectively, in the measured space density evolution of
the hard X-ray luminosity function over this redshift range. Potential
systematic errors would only increase the significance of our result. The
cluster AGN fraction increases more rapidly with redshift than the field and
the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler
Effect.Comment: ApJL Accepted, 5 pages, 2 figure
X-ray emission from optical novae in M 31
The first supersoft source (SSS) identification with an optical nova in M 31
was based on ROSAT observations. Twenty additional X-ray counterparts (mostly
identified as SSS by their hardness ratios) were detected using archival ROSAT,
XMM-Newton and Chandra observations obtained before July 2002. Based on these
results optical novae seem to constitute the major class of SSS in M 31. An
analysis of archival Chandra HRC-I and ACIS-I observations obtained from July
2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months
to about 10 years. Several novae showed short X-ray outbursts starting within
50 d after the optical outburst and lasting only two to three months. The
fraction of novae detected in soft X-rays within a year after the optical
outburst was more than 30%. Ongoing optical nova monitoring programs, optical
spectral follow-up and an up-to-date nova catalogue are essential for the X-ray
work. Re-analysis of archival nova data to improve positions and find
additional nova candidates are urgently needed for secure recurrent nova
identifications. Dedicated XMM-Newton/Chandra monitoring programs for X-ray
emission from optical novae covering the center area of M 31 continue to
provide interesting new results (e.g. coherent 1105s pulsations in the SSS
counterpart of nova M31N 2007-12b). The SSS light curves of novae allow us -
together with optical information - to estimate the mass of the white dwarf, of
the ejecta and the burned mass in the outburst. Observations of the central
area of M 31 allow us - in contrast to observations in the Galaxy - to monitor
many novae simultaneously and proved to be prone to find many interesting SSS
and nova types.Comment: 6 pages, 4 figures, accepted for publication in Aston.Nach
- …