118 research outputs found
Photometric Redshift of X-Ray Sources in the Chandra Deep Field South
Based on the photometry of 10 near-UV, optical, and near-infrared bands of
the Chandra Deep Field South, we estimate the photometric redshifts for 342
X-ray sources, which constitute ~99% of all the detected X-ray sources in the
field. The models of spectral energy distribution are based on galaxies and a
combination of power-law continuum and emission lines. Color information is
useful for source classifications: Type-I AGN show non-thermal spectral
features that are distinctive from galaxies and Type-II AGN. The hardness ratio
in X-ray and the X-ray-to-optical flux ratio are also useful discriminators.
Using rudimentary color separation techniques, we are able to further refine
our photometric redshift estimations. Among these sources, 137 have reliable
spectroscopic redshifts, which we use to verify the accuracy of photometric
redshifts and to modify the model inputs. The average relative dispersion in
redshift distribution is ~8%, among the most accurate for photometric surveys.
The high reliability of our results is attributable to the high quality and
broad coverage of data as well as the applications of several independent
methods and a careful evaluation of every source. We apply our redshift
estimations to study the effect of redshift on broadband colors and to study
the redshift distribution of AGN. Our results show that both the hardness ratio
and U-K color decline with redshift, which may be the result of a K-correction.
The number of Type-II AGN declines significantly at z>2 and that of galaxies
declines at z>1. However, the distribution of Type-I AGN exhibits less redshift
dependence. As well, we observe a significant peak in the redshift distribution
at z=0.6. We demonstrate that our photometric redshift estimation produces a
reliable database for the study of X-ray luminosity of galaxies and AGN.Comment: 40 pages, 11 figures. Accepted for publication in the Astrophysical
Journa
A Puzzling X-Ray Source Found in the chandra Deep Field South
In this letter we report the detection of an extremely strong X-ray emission
line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The
source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0
keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was
detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of
4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the
rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of
11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV,
rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We
reject the possibility that the line is due to a statistical or instrumental
artifact. The line is most likely due to blueshifted Fe-K emission from an
relativistic outflow, probably an inner X-ray jet, with velocities of the order
of ~ 0.6-0.7c. Other possible explanations are also discussed
XMM-Newton observation of the Lockman Hole II. Spectral analysis
We present the results of the X-ray spectral analysis of the first deep X-ray
survey with the XMM-Newton observatory during Performance Verification. We
restrict the analysis to the sample of 98 sources with more than 70 net counts
(flux limit in the [0.5-7] keV band of 1.6 10^{-15} erg cm^{-2} s^{-1}) of
which 61 have redshift identification. We find no correlation between the
spectral index Gamma and the intrinsic absorption column density N_H and, for
both the Type-1 and Type-2 AGN populations, we obtain ~2. The
progressive hardening of the mean X-ray source spectrum with decreasing flux is
essentially due to an increase in intrinsic absorption. The marked separation
between the two AGN populations in several diagnostics diagrams, involving
X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is
also a consequence of different absorption column densities and enables the
classification of optically faint obscured AGN. About 27% of the subsample with
R-K' colour are EROs (R-K>5) and most of these 18 X-ray selected EROs contain
anobscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios.
There are six sources in our sample with L_X[0.5-10]>10^44 erg s^{-1} and
log(N_H)>10^22 cm^{-2}: which are likely Type-2 QSOs and we thus derive a
density of ~69 objects of this class per square degree.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 10 figure
Discovery of the Most-Distant Double-Peaked Emitter at z=1.369
We report the discovery of the most-distant double-peaked emitter, CXOECDFS
J033115.0-275518, at z=1.369. A Keck/DEIMOS spectrum shows a clearly
double-peaked broad Mg II emission line, with FWHM 11000 km/s for
the line complex. The line profile can be well fit by an elliptical
relativistic Keplerian disk model. This is one of a handful of double-peaked
emitters known to be a luminous quasar, with excellent multiwavelength coverage
and a high-quality X-ray spectrum. CXOECDFS J033115.0-275518 is a radio-loud
quasar with two radio lobes (FR II morphology) and a radio loudness of f_{5
GHz}/f_{4400 \AA}~429. The X-ray spectrum can be modeled by a power law with
photon index 1.72 and no intrinsic absorption; the rest-frame 0.5-8.0 keV
luminosity is erg/s. The spectral energy distribution (SED)
of CXOECDFS J033115.0-275518 has a shape typical for radio-loud quasars and
double-peaked emitters at lower redshift. The local viscous energy released
from the line-emitting region of the accretion disk is probably insufficient to
power the observed line flux, and external illumination of the disk appears to
be required. The presence of a big blue bump in the SED along with the
unexceptional X-ray spectrum suggest that the illumination cannot arise from a
radiatively inefficient accretion flow.Comment: 6 pages, 5 figures, ApJ in pres
Sub-mm detection of a high redshift Type 2 QSO
We report on the first SCUBA detection of a Type 2 QSO at z=3.660 in the
Chandra Deep Field South. This source is X-ray absorbed, shows only narrow
emission lines in the optical spectrum and is detected in the sub-mm: it is the
ideal candidate in an evolution scheme for AGN (e.g. Fabian (1999); Page et al.
(2004)) of an early phase corresponding to the main growth of the host galaxy
and formation of the central black hole. The overall photometry (from the radio
to the X-ray energy band) of this source is well reproduced by the spectral
energy distribution (SED) of NGC 6240, while it is incompatible with the
spectrum of a Type 1 QSO (3C273) or a starburst galaxy (Arp 220). Its sub-mm
(850 \mu m) to X-ray (2 keV) spectral slope (alpha_SX) is close to the
predicted value for a Compton-thick AGN in which only 1% of the nuclear
emission emerges through scattering. Using the observed flux at 850 \mu m we
have derived a SFR=550--680 M_dust=4.2 10^8 M_odotComment: 6 Pages, 5 Figures, accepted for publication in MNRA
The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C
The detection of GeV photons from gamma-ray bursts (GRBs) has important
consequences for the interpretation and modelling of these most-energetic
cosmological explosions. The full exploitation of the high-energy measurements
relies, however, on the accurate knowledge of the distance to the events. Here
we report on the discovery of the afterglow and subsequent redshift
determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray
Space Telescope with high significance detection of photons at >0.1 GeV.
Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO
telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4m telescope in South
Africa, and the GMOS instrument at Gemini-S. The afterglow photometric redshift
of z=4.35+-0.15, based on simultaneous 7-filter observations with the Gamma-Ray
Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5%
most distant GRBs, and makes it the most energetic GRB known to date. The
detection of GeV photons from such a distant event is rather surprising.
The observed gamma-ray variability in the prompt emission together with the
redshift suggests a lower limit for the Lorentz factor of the
ultra-relativistic ejecta of Gamma > 1090. This value rivals any previous
measurements of Gamma in GRBs and strengthens the extreme nature of GRB
080916C.Comment: 6 pages, 5 figures; subm. to A&
The Great Observatories Origins Deep Survey - VLT/VIMOS Spectroscopy in the GOODS-South Field: Part II
We present the full data set of the VIMOS spectroscopic campaign of the
ESO/GOODS program in the CDFS, which complements the FORS2 ESO/GOODS
spectroscopic campaign. The GOODS/VIMOS spectroscopic campaign is structured in
two separate surveys using two different VIMOS grisms. The VIMOS Low Resolution
Blue (LR-Blue) and Medium Resolution (MR) orange grisms have been used to cover
different redshift ranges. The LR-Blue campaign is aimed at observing galaxies
mainly at 1.8<z<3.5, while the MR campaign mainly aims at galaxies at z<1 and
Lyman Break Galaxies (LBGs) at z>3.5. The full GOODS/VIMOS spectroscopic
campaign consists of 20 VIMOS masks. This release adds 8 new masks to the
previous release (12 masks, Popesso et al. 2009). In total we obtained 5052
spectra, 3634 from the 10 LR-Blue masks and 1418 from the 10 MR masks. A
significant fraction of the extracted spectra comes from serendipitously
observed sources: ~21% in the LR-Blue and ~16% in the MR masks. We obtained
2242 redshifts in the LR-Blue campaign and 976 in the MR campaign for a total
success rate of 62% and 69% respectively, which increases to 66% and 73% if
only primary targets are considered. The typical redshift uncertainty is
estimated to be ~0.0012 (~255 km/s) for the LR-Blue grism and ~0.00040 (~120
km/s) for the MR grism. By complementing our VIMOS spectroscopic catalog with
all existing spectroscopic redshifts publicly available in the CDFS, we
compiled a redshift master catalog with 7332 entries, which we used to
investigate large scale structures out to z~3.7. We produced stacked spectra of
LBGs in a few bins of equivalent width (EW) of the Ly-alpha and found evidence
for a lack of bright LBGs with high EW of the Ly-alpha. Finally, we obtained
new redshifts for 12 X-ray sources of the CDFS and extended-CDFS.Comment: 22 pages, 20 figures, accepted for publication on Astronomy and
Astrophysics, catalogs and data products are available at
http://archive.eso.org/cms/eso-data/data-packages/goods-vimos-spectroscopy-data-release-version-2.0/,
for ESO-GOODS related material consult
http://www.eso.org/sci/activities/projects/goods
Relativistic outflow in CXO CDFS J033260.0-274748
In this letter we report the detection of a strong and extremely blueshifted
X-ray absorption feature in the 1 Ms Chandra spectrum of CXO CDFS
J033260.0-274748, a quasar at z = 2.579 with L_2-10keV ~ 4x10^44 ergs/s. The
broad absorption feature at ~ 6.3 keV in the observed frame can be fitted
either as an absorption edge at 20.9 keV or as a broad absorption line at 22.2
keV rest frame. The absorber has to be extremely ionized with an ionization
parameter \xi ~ 10^4, and a high column density N_H >5x10^23 cm^-2. We reject
the possibility of a statistical or instrumental artifact. The most likely
interpretation is an extremely blueshifted broad absorption line or absorption
edge, due to H or He--like iron in a relativistic jet-like outflow with bulk
velocity of ~ 0.7-0.8 c. Similar relativistic outflows have been reported in
the X-ray spectra of several other AGNs in the past few years.Comment: ApJ letter accepte
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