135 research outputs found
Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425
We have obtained a 110 ksec Chandra ACIS-S exposure of UM425, a pair of QSOs
at z=1.47 separated by 6.5 arcsec, which show remarkably similar emission and
broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray
spectrum of UM425A (the brighter component) is well-fit with a power law
(photon spectral index Gamma=2.0) partially covered by a hydrogen column of
3.8x10^22 cm^-2. The underlying power-law slope for this object and for other
recent samples of BALQSOs is typical of radio-quiet quasars, lending credence
to the hypothesis that BALs exist in every quasar. Assuming the same Gamma for
the much fainter image of UM425B, we detect an obscuring column 5 times larger.
We search for evidence of an appropriately large lensing mass in our Chandra
image and find weak diffuse emission near the quasar pair, with an X-ray flux
typical of a group of galaxies at redshift z ~ 0.6. From our analysis of
archival HST WFPC2 and NICMOS images, we find no evidence for a luminous
lensing galaxy, but note a 3-sigma excess of galaxies in the UM425 field with
plausible magnitudes for a z=0.6 galaxy group. However, the associated X-ray
emission does not imply sufficient mass to produce the observed image
splitting. The lens scenario thus requires a dark (high M/L ratio) lens, or a
fortuitous configuration of masses along the line of sight. UM425 may instead
be a close binary pair of BALQSOs, which would boost arguments that
interactions and mergers increase nuclear activity and outflows.Comment: 13 pages, 9 figures, Accepted for publication in the Astrophysical
Journa
Discovery of a Jet-Like Structure at the High Redshift QSO CXOMP J084128.3+131107
The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like
structure associated with a newly recognized QSO at redshift z=1.866. The
system was 9.4 arcmin off-axis during an observation of 3C 207. Although
significantly distorted by the mirror PSF, we use both a raytrace and a nearby
bright point source to show that the X-ray image must arise from some
combination of point and extended sources, or else from a minimum of three
distinct point sources. We favor the former situation, as three unrelated
sources would have a small probability of occurring by chance in such a close
alignment. We show that interpretation as a jet emitting X-rays via inverse
Compton (IC) scattering on the cosmic microwave background (CMB) is plausible.
This would be a surprising and unique discovery of a radio-quiet QSO with an
X-ray jet, since we have obtained upper limits of 100 microJy on the QSO
emission at 8.46 GHz, and limits of 200 microJy for emission from the putative
jet.Comment: 12 pages including 4 figures. Accepted for publication by ApJ Letter
Chandra Observation of 3C 212: a New Look at the X-ray and UV Absorbers
The red quasar 3C 212 (z=1.049) is one of the most distant and most luminous
AGN which has shown evidence of an X-ray warm absorber. In order to further
investigate this unusual quasar, we used Chandra/ACIS-S to observe 3C 212 for
19.5 ksec, resulting in a net detection of ~4000 counts. The Chandra data
confirm the presence of an excess absorbing column N_H ~ 4 x 10^21 cm^-2 at the
quasar redshift, but we find no compelling evidence for a warm absorber. Using
both the Chandra and archival ROSAT PSPC data, we obtain very good fits for
both a partially covered neutral absorber and a low-ionization (U = 0.03)
photo-ionized absorber. In the ultraviolet, 3C 212 shows a strong associated
MgII absorber. Based on a moderate resolution (80 km/s) MMT spectrum we show
that the absorber is highly saturated and has a covering fraction less than
60%, implying that the absorber is truly intrinsic to the quasar.
Photo-ionization modeling of the MgII absorber yields a constraint on the
ionization parameter of U < 0.03, inconsistent with a warm UV/X-ray absorber.
In addition to our spectral analysis, we find evidence in the ACIS image data
for weak extended emission surrounding the quasar as well as emission
corresponding to the radio lobes at a distance of 5 arcsec from 3C 212. The
statistical significance of these features is low, but we briefly explore the
implications if the detections are valid.Comment: 8 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
The 300 kpc Long X-ray Jet in PKS 1127-145, z=1.18 Quasar: Constraining X-ray Emission Models
We present a ~100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with alpha_X = 0.66 +/-0.15 and steep in the outer jet with alpha_X = 1.0 +/-0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ``one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology
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
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
(abridged) We have investigated 136 Chandra extragalactic sources without
broad optical emission lines, including 93 galaxies with narrow emission lines
(NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray
spectral hardness and optical emission line diagnostics, we have conservatively
classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We
found no statistically significant evolution in Lx/LB, within the limited z
range. We have built log(N)-log(S), after correcting for completeness based on
a series of simulations. The best-fit slope is -1.5 for both S and B energy
bands, which is considerably steeper than that of the AGN-dominated cosmic
background sources, but slightly flatter than the previous estimate, indicating
normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1
cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear
to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting
for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1,
making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG -
X-ray bright, optically normal galaxy candidates) do not appear to be
significantly absorbed, we found two heavily obscured objects, which could be
as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A
galaxy candidates with strong Balmer absorption lines, but no [OII] line. The
X-ray spectra of both galaxies are soft and one of them has a nearby close
companion galaxy, supporting the merger/interaction scenario rather than the
dusty starburst hypothesis.Comment: 31 pages, 9 figures, accepted for publication in ApJ (20 June 2006,
v644), replaced with minor correction
Optical photometric GTC/OSIRIS observations of the young massive association Cygnus OB2
In order to fully understand the gravitational collapse of molecular clouds,
the star formation process and the evolution of circumstellar disks, these
phenomena must be studied in different Galactic environments with a range of
stellar contents and positions in the Galaxy. The young massive association
Cygnus OB2, in the Cygnus-X region, is an unique target to study how star
formation and the evolution of circumstellar disks proceed in the presence of a
large number of massive stars. We present a catalog obtained with recent
optical observations in r,i,z filters with OSIRIS, mounted on the GTC
telescope, which is the deepest optical catalog of Cyg OB2 to date.
The catalog consist of 64157 sources down to M=0.15 solar masses at the
adopted distance and age of Cyg OB2. A total of 38300 sources have good
photometry in all three bands. We combined the optical catalog with existing
X-ray data of this region, in order to define the cluster locus in the optical
diagrams. The cluster locus in the r-i vs. i-z diagram is compatible with an
extinction of the optically selected cluster members in the 2.64<AV<5.57 range.
We derive an extinction map of the region, finding a median value of AV=4.33 in
the center of the association, decreasing toward the north-west. In the
color-magnitude diagrams, the shape of the distribution of main sequence stars
is compatible with the presence of an obscuring cloud in the foreground at
about 850+/-25 pc from the Sun.Comment: Accepted for publication ApJS 201
A First Look at the Nuclear Region of M31 with Chandra
We report on the first observation of the nuclear region of M31 with the
Chandra X-ray Observatory. The nuclear source seen with the Einstein and ROSAT
HRIs is resolved into five point sources. One of these sources is within 1'' of
the M31 central super-massive black hole. As compared to the other point
sources in M31, this nuclear source has an unusual x-ray spectrum. Based on the
spatial coincidence we identify this source with the central black hole, and
note that the unusual spectrum is a challenge to current theories. A bright
transient is detected ~26'' to the west of the nucleus, which may be associated
with a stellar mass black hole.Comment: Submitted to ApJ Letters, 4 pages, 4 figures. email:
garcia,ssm,fap,wrf,jem,cjf, @head-cfa.harvard.ed
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