944 research outputs found
Chandra imaging of the kpc extended outflow in 1H 0419-577
The Seyfert 1 galaxy 1H 0419-577 hosts a kpc extended outflow that is
evident in the [\ion{O}{iii}] image and that is also detected as a warm
absorber in the UV/X-ray spectrum. Here, we analyze a 30 ks Chandra-ACIS
X-ray image, with the aim of resolving the diffuse extranuclear X-ray emission
and of investigating its relationship with the galactic outflow. Thanks to its
sub-arcsecond spatial resolution, Chandra resolves the circumnuclear X-ray
emission, which extends up to a projected distance of at least 16 kpc
from the center. The morphology of the diffuse X-ray emission is spherically
symmetrical. We could not recover a morphological resemblance between the soft
X-ray emission and the ionization bicone that is traced by the [\ion{O}{iii}]
outflow. we argue that the photoionized gas nebula must be distributed mostly
along the polar directions, outside our line of sight. In this geometry, the
X-ray/UV warm absorber must trace a different gas component, physically
disconnected from the emitting gas, and located closer to the equatorial plane.Comment: accepted for publications A&
On the high coherence of kilo-Hz Quasi-Periodic Oscillations
We have carried out a systematic study of the properties of the kilo-Hertz
quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron
star low-mass X-ray binary 4U1608-52, using archival data obtained with the
Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the
oscillations (defined as the ratio of the frequency of the QPO peak to its full
width at half maximum). In order to minimise the effect of long-term frequency
drifts, power spectra were computed over the shortest times permitted by the
data statistics. We show that the high Q of ~200 reported by Berger et al.
(1996) for the lower frequency kilo-Hz QPO in one of their observations is by
no means exceptional, as we observe a mean Q value in excess of 150 in 14 out
of the 21 observations analysed and Q can remain above 200 for thousands of
seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we
find a systematic trend for the coherence time of the QPO, estimated as tau=Q
/(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz,
beyond which it appears to decrease, at frequencies where the QPO weakens.
There is a more complex relationship between tau and the QPO root mean squared
amplitude (RMS), in which positive and negative correlations can be found. A
higher-frequency QPO, revealed by correcting for the frequency drift of the
560-890 Hz one, has a much lower Q (~10) which does not follow the same
pattern. We discuss these results in the framework of competing QPO models and
show that those involving clumps orbiting within or above the accretion disk
are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table
On the connection between turbulent motions and particle acceleration in galaxy clusters
Giant radio halos are Mpc-scale diffuse radio sources associated with the
central regions of galaxy clusters. The most promising scenario to explain the
origin of these sources is that of turbulent re-acceleration, in which MeV
electrons injected throughout the formation history of galaxy clusters are
accelerated to higher energies by turbulent motions mostly induced by cluster
mergers. In this Letter, we use the amplitude of density fluctuations in the
intracluster medium as a proxy for the turbulent velocity and apply this
technique to a sample of 51 clusters with available radio data. Our results
indicate a segregation in the turbulent velocity of radio halo and radio quiet
clusters, with the turbulent velocity of the former being on average higher by
about a factor of two. The velocity dispersion recovered with this technique
correlates with the measured radio power through the relation , which implies that the radio power is
nearly proportional to the turbulent energy rate. Our results provide an
observational confirmation of a key prediction of the turbulent re-acceleration
model and possibly shed light on the origin of radio halos.Comment: Submitted to ApJ Letter
The stripping of a galaxy group diving into the massive cluster A2142
Structure formation in the current Universe operates through the accretion of
group-scale systems onto massive clusters. The detection and study of such
accreting systems is crucial to understand the build-up of the most massive
virialized structures we see today. We report the discovery with XMM-Newton of
an irregular X-ray substructure in the outskirts of the massive galaxy cluster
Abell 2142. The tip of the X-ray emission coincides with a concentration of
galaxies. The bulk of the X-ray emission of this substructure appears to be
lagging behind the galaxies and extends over a projected scale of at least 800
kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of
~4 lower than the surrounding medium and is typical of the virialized plasma of
a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret
this structure as a galaxy group in the process of being accreted onto the main
dark-matter halo. The X-ray structure trailing behind the group is due to gas
stripped from its original dark-matter halo as it moves through the
intracluster medium (ICM). This is the longest X-ray trail reported to date.
For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has
been surviving in the presence of the hot ICM for at least 600 Myr, which
exceeds the Spitzer conduction timescale in the medium by a factor of >~400.
Such a strong suppression of conductivity is likely related to a tangled
magnetic field with small coherence length and to plasma microinstabilities.
The long survival time of the low-entropy intragroup medium suggests that the
infalling material can eventually settle within the core of the main cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
EZ: A Tool for Automatic Redshift Measurement
We present EZ (Easy redshift), a tool we have developed within the VVDS
project to help in redshift measurement from otpical spectra. EZ has been
designed with large spectroscopic surveys in mind, and in its development
particular care has been given to the reliability of the results obtained in an
automatic and unsupervised mode. Nevertheless, the possibility of running it
interactively has been preserved, and a graphical user interface for results
inspection has been designed. EZ has been successfully used within the VVDS
project, as well as the zCosmos one. In this paper we describe its architecture
and the algorithms used, and evaluate its performances both on simulated and
real data. EZ is an open source program, freely downloadable from
http://cosmos.iasf-milano.inaf.it/pandora.Comment: accepted for publication in Publications of the Astronomical Society
of the Pacifi
AGN variability time scales and the discrete-event model
We analyse the ultraviolet variability time scales in a sample of 15 Type 1
Active Galactic Nuclei (AGN) observed by IUE. Using a structure function
analysis, we demonstrate the existence in most objects of a maximum variability
time scale of the order of 0.02-1.00 year. We do not find any significant
dependence of these maximum variability time scales on the wavelength, but we
observe a weak correlation with the average luminosity of the objects. We also
observe in several objects the existence of long-term variability, which seems
decoupled from the short-term one. We interpret the existence of a maximum
variability time scale as a possible evidence that the light curves of Type 1
AGN are the result of the superimposition of independent events. In the
framework of the so-called discrete-event model, we study the event energy and
event rate as a function of the object properties. We confront our results to
predictions from existing models based on discrete events. We show that models
based on a fixed event energy, like supernova explosions, can be ruled out. In
their present form, models based on magnetic blobs are also unable to account
for the observed relations. Stellar collision models, while not completely
satisfactory, cannot be excluded.Comment: 13 pages, 10 figures. Accepted for publication in A&
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