250 research outputs found
The swansong in context: long-timescale X-ray variability of NGC 4051
On 9-11 May 1998, the highly-variable, low luminosity Seyfert 1 galaxy
NGC4051 was observed in an unusual low flux state by BeppoSAX (Guainazzi et al.
1998) RXTE and EUVE. We present fits of the 4-15 keV RXTE spectrum and BeppoSAX
MECS spectrum obtained during this observation, which are consistent with the
interpretation that the source had switched off, leaving only the spectrum of
pure reflection from distant cold matter. We place this result in context by
showing the X-ray lightcurve of NGC4051 obtained by our RXTE monitoring
campaign over the past two and a half years, which shows that the low state
lasted for ~150 days before the May observations (implying that the reflecting
material is > 10^17 cm from the continuum source) and forms part of a
lightcurve showing distinct variations in long-term average flux over
timescales > months. We show that the long-timescale component to X-ray
variability is intrinsic to the primary continuum and is probably distinct from
the variability at shorter timescales, possibly associated with variations in
the accretion flow of matter onto the central black hole. As the source
approaches the low state, the variability process becomes non-linear. NGC4051
may represent a microcosm of all X-ray variability in radio quiet active
galactic nuclei (AGNs), displaying in a few years a variety of flux states and
variability properties which more luminous AGNs may pass through on timescales
of decades to thousands of years.Comment: 5 pages, accepted for publication in MNRA
Chandra View of the Dynamically Young Cluster of Galaxies A1367 I. Small-Scale Structures
The 40 ks \emph{Chandra} ACIS-S observation of A1367 provides new insights
into small-scale structures and point sources in this dynamically young
cluster. Here we concentrate on small-scale extended structures. A ridge-like
structure around the center (``the ridge'') is significant in the \chandra\
image. The ridge, with a projected length of 8 arcmin (or 300
h kpc), is elongated from northwest (NW) to southeast (SE), as is
the X-ray surface brightness distribution on much larger scales ( 2
h Mpc). The ridge is cooler than its western and southern
surroundings while the differences from its eastern and northern surroundings
are small. We also searched for small-scale structures with sizes
arcmin. Nine extended features, with sizes from 0.5 to 1.5, were
detected at significance levels above 4 . Five of the nine features are
located in the ridge and form local crests. The nine extended features can be
divided into two types. Those associated with galaxies (NGC 3860B, NGC 3860 and
UGC 6697) are significantly cooler than their surroundings (0.3 - 0.9 keV vs. 3
- 4.5 keV). The masses of their host galaxies are sufficient to bind the
extended gas. These extended features are probably related to thermal halos or
galactic superwinds of their host galaxies. The existence of these relatively
cold halos imply that galaxy coronae can survive in cluster environment (e.g.,
Vikhlinin et al. 2001). Features of the second type are not apparently
associated with galaxies. Their temperatures may not be significantly different
from those of their surroundings. This class of extended features may be
related to the ridge. We consider several possibilities for the ridge and the
second type of extended features. The merging scenario is preferred.Comment: To appear in ApJ, Vol 576, 2002, Sep., a high-resolution version is
in http://cfa160.harvard.edu/~sunm/a1367_a.ps.g
Bright ULXs from Ring Galaxies
Collisional Ring Galaxies (RiGs), with their expanding ring of gas and stars and their high star formation rate are unique objects where to study the most massive products of star formation and shed some light on the nature of ultra luminous X-ray sources (ULXs) found on their rings. We have recently constructed the X-ray luminosity function of ULXs in RiGs which is mostly consistent with previous results but shows two bright objects at the highest luminosity above the extrapolation of previous models for ULXs. They could be the best cases of Intermediate Mass Black Holes, or further ULXs powered by neutron stars, or even something different (e.g Supernovae). We will describe in detail the RiGs and their properties and compare the derived XLF with previous results
The Chandra survey of the COSMOS field II: source detection and photometry
The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program,
that covers the central contiguous ~0.92 deg^2 of the COSMOS field. C-COSMOS is
the result of a complex tiling, with every position being observed in up to six
overlapping pointings (four overlapping pointings in most of the central ~0.45
deg^2 area with the best exposure, and two overlapping pointings in most of the
surrounding area, covering an additional ~0.47 deg^2). Therefore, the full
exploitation of the C-COSMOS data requires a dedicated and accurate analysis
focused on three main issues: 1) maximizing the sensitivity when the PSF
changes strongly among different observations of the same source (from ~1
arcsec up to ~10 arcsec half power radius); 2) resolving close pairs; and 3)
obtaining the best source localization and count rate. We present here our
treatment of four key analysis items: source detection, localization,
photometry, and survey sensitivity. Our final procedure consists of a two step
procedure: (1) a wavelet detection algorithm, to find source candidates, (2) a
maximum likelihood Point Spread Function fitting algorithm to evaluate the
source count rates and the probability that each source candidate is a
fluctuation of the background. We discuss the main characteristics of this
procedure, that was the result of detailed comparisons between different
detection algorithms and photometry tools, calibrated with extensive and
dedicated simulations.Comment: Accepted for publication in The Astrophysical Journal Supplement
Serie
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in
MNRA
The Chandra-COSMOS survey IV: X-ray spectra of the bright sample
We present the X-ray spectral analysis of the 390 brightest extragalactic
sources in the Chandra-COSMOS catalog, showing at least 70 net counts in the
0.5-7 keV band. This sample has a 100% completeness in optical-IR
identification, with 75% of the sample having a spectroscopic redshift and 25%
a photometric redshift. Our analysis allows us to accurately determine the
intrinsic absorption, the broad band continuum shape ({\Gamma}) and intrinsic
L(2-10) distributions, with an accuracy better than 30% on the spectral
parameters for 95% of the sample. The sample is equally divided in type-1
(49.7%) and type-2 AGN (48.7%) plus few passive galaxies at low z. We found a
significant difference in the distribution of {\Gamma} of type-1 and type-2,
with small intrinsic dispersion, a weak correlation of {\Gamma} with L(2-10)
and a large population (15% of the sample) of high luminosity, highly obscured
(QSO2) sources. The distribution of the X ray/Optical flux ratio (Log(FX /Fi))
for type-1 is narrow (0 < X/O < 1), while type-2 are spread up to X/O = 2. The
X/O correlates well with the amount of X-ray obscuration. Finally, a small
sample of Compton thick candidates and peculiar sources is presented. In the
appendix we discuss the comparison between Chandra and XMM-Newton spectra for
280 sources in common. We found a small systematic difference, with XMM-Newton
spectra that tend to have softer power-laws and lower obscuration.Comment: 20 pages, 16 figures. Accepted for Pubblication in MNRAS, 2013
February
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