234 research outputs found
Suprathermal electron isotropy in high-beta solar wind and its role in heat flux dropouts
[1] Time variations in plasma beta and a parameter which measures isotropy in suprathermal electron pitch angle distributions show a remarkably close correspondence throughout the solar wind. The finding implies that high-beta plasma, with its multiple magnetic holes and sharp field and plasma gradients, is conducive to electron pitch-angle scattering, which reduces heat flux from the Sun without field-line disconnection. Thus the finding impacts our understanding of signatures we use to determine magnetic topology in the heliosphere
Heliospheric plasma sheets
[1] As a high-beta feature on scales of hours or less, the heliospheric plasma sheet (HPS) encasing the heliospheric current sheet shows a high degree of variability. A study of 52 sector boundaries identified in electron pitch angle spectrograms in Wind data from 1995 reveals that only half concur with both high-beta plasma and current sheets, as required for an HPS. The remaining half lack either a plasma sheet or current sheet or both. A complementary study of 37 high-beta events reveals that only 5 contain sector boundaries while nearly all (34) contain local magnetic field reversals, however brief. We conclude that high-beta plasma sheets surround current sheets but that most of these current sheets are associated with fields turned back on themselves. The findings are consistent with the hypothesis that high-beta plasma sheets, both at and away from sector boundaries, are the heliospheric counterparts of the small coronal transients observed at the tips of helmet streamers, in which case the proposed mechanism for their release, interchange reconnection, could be responsible for the field inversions
Photometric redshifts for X-ray-selected active galactic nuclei in the eROSITA era
With the launch of eROSITA (extended Roentgen Survey with an Imaging Telescope Array), successfully occurred on 2019 July 13, we are facing the challenge of computing reliable photometric redshifts for 3 million of active galactic nuclei (AGNs) over the entire sky, having available only patchy and inhomogeneous ancillary data. While we have a good understanding of the photo-z quality obtainable for AGN using spectral energy distribution (SED)-fitting technique, we tested the capability of machine learning (ML), usually reliable in computing photo-z for QSO in wide and shallow areas with rich spectroscopic samples. Using MLPQNA as example of ML, we computed photo-z for the X-ray-selected sources in Stripe 82X, using the publicly available photometric and spectroscopic catalogues. Stripe 82X is at least as deep as eROSITA will be and wide enough to include also rare and bright AGNs. In addition, the availability of ancillary data mimics what can be available in the whole sky. We found that when optical, and near- and mid-infrared data are available, ML and SED fitting perform comparably well in terms of overall accuracy, realistic redshift probability density functions, and fraction of outliers, although they are not the same for the two methods. The results could further improve if the photometry available is accurate and including morphological information. Assuming that we can gather sufficient spectroscopy to build a representative training sample, with the current photometry coverage we can obtain reliable photo-z for a large fraction of sources in the Southern hemisphere well before the spectroscopic follow-up, thus timely enabling the eROSITA science return. The photo-z catalogue is released here
X-ray constraints on the fraction of obscured AGN at high accretion luminosities
The wide-area XMM-XXL X-ray survey is used to explore the fraction of
obscured AGN at high accretion luminosities, , and out to redshift . The sample covers an area
of about and provides constraints on the space density of
powerful AGN over a wide range of neutral hydrogen column densities extending
beyond the Compton-thick limit, . The fraction
of obscured Compton-thin () AGN is estimated
to be for luminosities
independent of redshift. For less luminous sources the fraction of obscured
Compton-thin AGN increases from at to at
. Studies that select AGN in the infrared via template fits to the
observed Spectral Energy Distribution of extragalactic sources estimate space
densities at high accretion luminosities consistent with the XMM-XXL
constraints. There is no evidence for a large population of AGN (e.g. heavily
obscured) identified in the infrared and missed at X-ray wavelengths. We
further explore the mid-infrared colours of XMM-XXL AGN as a function of
accretion luminosity, column density and redshift. The fraction of XMM-XXL
sources that lie within the mid-infrared colour wedges defined in the
literature to select AGN is primarily a function of redshift. This fraction
increases from about 20-30% at z=0.25 to about 50-70% at .Comment: MNRAS accepte
The Clustering of X-ray Luminous Quasars
The clustering of active galactic nuclei (AGN) sheds light on their typical
large (Mpc-scale) environments, which can constrain the growth and evolution of
supermassive black holes. Here we measure the clustering of luminous
X-ray-selected AGN in the Stripe 82X and XMM-XXL-North surveys around the peak
epoch of black hole growth, in order to investigate the dependence of
luminosity on large-scale AGN environment. We compute the auto-correlation
function of AGN in two luminosity bins, erg
s at and erg s at ,
and calculate the AGN bias taking into account the redshift distribution of the
sources using three different methods. Our results show that while the less
luminous sample has an inferred typical halo mass that is smaller than for the
more luminous AGN, the host halo mass may be less dependent on luminosity than
suggested in previous work. Focusing on the luminous sample, we calculate a
typical host halo mass of M, which is similar
to previous measurements of moderate-luminosity X-ray AGN and significantly
larger than the values found for optical quasars of similar luminosities and
redshifts. We suggest that the clustering differences between different AGN
selection techniques are dominated by selection biases, and not due to a
dependence on AGN luminosity. We discuss the limitations of inferring AGN
triggering mechanisms from halo masses derived by large-scale bias.Comment: Accepted for publication in Ap
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
AGN Feedback and Evolution of Radio Sources: Discovery of an X-ray Cluster Associated with z=1 Quasar
We report the first significant detection of an X-ray cluster associated with
a powerful (L(bol) ~1e47 erg/sec) radio-loud quasar at high redshift (z=1.06).
Diffuse X-ray emission is detected out to ~120 kpc from the CSS quasar 3C 186.
A strong Fe-line emission at the z(rest)=1.06 confirms its thermal nature. We
find that the CSS radio source is highly overpressured with respect to the
thermal cluster medium by ~2-3 orders of magnitude. This provides direct
observational evidence that the radio source is not thermally confined as
posited in the ``frustrated'' scenario for CSS sources. Instead, the radio
source may be young and at an early stage of its evolution. This source
provides the first detection of the AGN in outburst in the center of a cooling
flow cluster. Powerful radio sources are thought to be triggered by the cooling
flows. The evidence for the AGN activity and intermittent outbursts comes from
the X-ray morphology of low redshift clusters, which usually do not harbour
quasars. 3C186 is a young active radio source which can supply the energy into
the cluster and potentially prevent its cooling. We discuss energetics related
to the quasar activity and the cluster cooling flow, and possible feedback
between the evolving radio source and the cluster.Comment: 5 pages, 3 figures, to appear in Proc. "The X-ray Universe 2005", San
Lorenzo de El Escorial, Spain, September 200
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
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