ABSTRACT
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.
ABSTRACT The COSMOS-Legacy survey is a 4.6 Ms Chandra program that has imaged 2.2 deg2 of the COSMOS field with an effective exposure of ks over the central 1.5 deg2 and of ks in the remaining area. ...The survey is the combination of 56 new observations obtained as an X-ray Visionary Project with the previous C-COSMOS survey. We describe the reduction and analysis of the new observations and the properties of 2273 point sources detected above a spurious probability of 2 × 10−5. We also present the updated properties of the C-COSMOS sources detected in the new data. The whole survey includes 4016 point sources (3814, 2920 and 2440 in the full, soft, and hard band). The limiting depths are 2.2 × 10−16, 1.5 × 10−15, and 8.9 × 10−16 in the 0.5-2, 2-10, and 0.5-10 keV bands, respectively. The observed fraction of obscured active galactic nuclei with a column density >1022 cm−2 from the hardness ratio (HR) is ∼50 %. Given the large sample we compute source number counts in the hard and soft bands, significantly reducing the uncertainties of 5%-10%. For the first time we compute number counts for obscured (HR > −0.2) and unobscured (HR < −0.2) sources and find significant differences between the two populations in the soft band. Due to the unprecedent large exposure, COSMOS-Legacy area is three times larger than surveys at similar depths and its depth is three times fainter than surveys covering similar areas. The area-flux region occupied by COSMOS-Legacy is likely to remain unsurpassed for years to come.
ABSTRACT We present the catalog of optical and infrared counterparts of the Chandra COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 deg2 of the COSMOS field, combination of 56 new ...overlapping observations obtained in Cycle 14 with the previous C-COSMOS survey. In this Paper we report the i, K, and 3.6 m identifications of the 2273 X-ray point sources detected in the new Cycle 14 observations. We use the likelihood ratio technique to derive the association of optical/infrared (IR) counterparts for 97% of the X-ray sources. We also update the information for the 1743 sources detected in C-COSMOS, using new K and 3.6 m information not available when the C-COSMOS analysis was performed. The final catalog contains 4016 X-ray sources, 97% of which have an optical/IR counterpart and a photometric redshift, while 54% of the sources have a spectroscopic redshift. The full catalog, including spectroscopic and photometric redshifts and optical and X-ray properties described here in detail, is available online. We study several X-ray to optical (X/O) properties: with our large statistics we put better constraints on the X/O flux ratio locus, finding a shift toward faint optical magnitudes in both soft and hard X-ray band. We confirm the existence of a correlation between X/O and the the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the analysis of the correlation between the fraction of obscured AGNs and the hard band luminosity, finding a different behavior between the optically and X-ray classified obscured fraction.
The Clustering of X-Ray Luminous Quasars Powell, M. C.; Urry, C. M.; Cappelluti, N. ...
Astrophysical journal/The Astrophysical journal,
03/2020, Volume:
891, Issue:
1
Journal Article
Peer reviewed
Open access
The clustering of active galactic nuclei (AGNs) 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 AGNs 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 AGNs in two luminosity bins, erg s−1 at z ∼ 0.8 and LX ≥ 1044.5 erg s−1 at z ∼ 1.8, 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 AGNs, 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 ∼1013 , which is similar to previous measurements of moderate-luminosity X-ray AGNs 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.
ABSTRACT We analyze high-quality NuSTAR observations of the local (z = 0.011) Seyfert 2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the ...first broadband X-ray spectral analysis of the source, spanning nearly two decades in energy (0.5-30 keV). Previous X-ray observations of the source below 10 keV indicated strong reflection/obscuration on the basis of a pronounced iron fluorescence line at 6.4 keV. The hard X-ray energy coverage of NuSTAR, together with self-consistent toroidal reprocessing models, enables direct broadband constraints on the obscuring column density of the source. We find the source to be heavily Compton-thick (CTK) with an obscuring column in excess of cm−2, unconstrained at the upper end. We further find an intrinsic 2-10 keV luminosity of to 90% confidence, almost 400 times the observed flux, and consistent with various multiwavelength diagnostics. Such a high ratio of intrinsic to observed flux, in addition to an Fe-K fluorescence line equivalent width exceeding 2 keV, is extreme among known bona fide CTK AGNs, which we suggest are both due to the high level of obscuration present around IC 3639. Our study demonstrates that broadband spectroscopic modeling with NuSTAR enables large corrections for obscuration to be carried out robustly and emphasizes the need for improved modeling of AGN tori showing intense iron fluorescence.
ABSTRACT To provide the census of the sources contributing to the X-ray background peak above 10 keV, Nuclear Spectroscopic Telescope Array (NuSTAR) is performing extragalactic surveys using a ...three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity, and medium area tier, covering 1.7 deg2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ∼3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9, and 6.4 × 10−14 in the 3-24, 3-8, and 8-24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 1042-1045.5 luminosities and redshift z = 0.04-2.5. Thirty-two sources are detected in the 8-24 keV band with fluxes ∼100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but 4 are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick active galactic nuclei measured from the hardness ratio is between 13%-20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z = 0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 1024 cm−2, implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.
NGC 1448 is one of the nearest luminous galaxies (L8-1000 m > 109 L ) to ours (z = 0.00390), and yet the active galactic nucleus (AGN) it hosts was only recently discovered, in 2009. In this paper, ...we present an analysis of the nuclear source across three wavebands: mid-infrared (MIR) continuum, optical, and X-rays. We observed the source with the Nuclear Spectroscopic Telescope Array (NuSTAR), and combined these data with archival Chandra data to perform broadband X-ray spectral fitting ( 0.5-40 keV) of the AGN for the first time. Our X-ray spectral analysis reveals that the AGN is buried under a Compton-thick (CT) column of obscuring gas along our line of sight, with a column density of NH(los) 2.5 × 1024 cm−2. The best-fitting torus models measured an intrinsic 2-10 keV luminosity of L (3.5-7.6) × 1040 erg s−1, making NGC 1448 one of the lowest luminosity CTAGNs known. In addition to the NuSTAR observation, we also performed optical spectroscopy for the nucleus in this edge-on galaxy using the European Southern Observatory New Technology Telescope. We re-classify the optical nuclear spectrum as a Seyfert on the basis of the Baldwin-Philips-Terlevich diagnostic diagrams, thus identifying the AGN at optical wavelengths for the first time. We also present high spatial resolution MIR observations of NGC 1448 with Gemini/T-ReCS, in which a compact nucleus is clearly detected. The absorption-corrected 2-10 keV luminosity measured from our X-ray spectral analysis agrees with that predicted from the optical O iiiλ5007 emission line and the MIR 12 m continuum, further supporting the CT nature of the AGN.
Heliospheric plasma sheets Crooker, N. U.; Huang, C.-L.; Lamassa, S. M. ...
Journal of Geophysical Research - Space Physics,
March 2004, Volume:
109, Issue:
A3
Journal Article
Peer reviewed
Open access
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.
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.
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