103 research outputs found
Discovery of a Classic FR-II Broad Absorption Line Quasar from the FIRST Survey
We have discovered a remarkable quasar, FIRST J101614.3+520916, whose optical
spectrum shows unambiguous broad absorption features while its double-lobed
radio morphology and luminosity clearly indicate a classic Fanaroff-Riley Type
II radio source. Its radio luminosity places it at the extreme of the recently
established class of radio-loud broad absorption line quasars (Becker et al.
1997, 2000; Brotherton et al. 1998). Because of its hybrid nature, we speculate
that FIRST J101614.3+520916 is a typical FR-II quasar which has been
rejuvenated as a broad absorption line (BAL) quasar with a Compact Steep
Spectrum core. The direction of the jet axis of FIRST J101614.3+520916 can be
estimated from its radio structure and optical brightness, indicating that we
are viewing the system at a viewing angle of > 40 degrees. The position angles
of the radio jet and optical polarization are not well-aligned, differing by 20
to 30 degrees. When combined with the evidence presented by Becker et al.
(2000) for a sample of 29 BAL quasars showing that at least some BAL quasars
are viewed along the jet axis, the implication is that no preferred viewing
orientation is necessary to observe BAL systems in a quasar's spectrum. This,
and the probable young nature of compact steep spectrum sources, leads
naturally to the alternate hypothesis that BALs are an early stage in the lives
of quasars.Comment: 14 pages, 6 postscript figures; accepted for publication in the
Astrophysical Journa
What Determines the Depth of BALs? Keck HIRES Observations of BALQSO 1603+300
We find that the depth and shape of the broad absorption lines (BALs) in
BALQSO 1603+3002 are determined largely by the fraction of the emitting source
which is covered by the BAL flow. In addition, the observed depth of the BALs
is poorly correlated with their real optical depth. The implication of this
result is that abundance studies based on direct extraction of column densities
from the depth of the absorption troughs are unreliable. Our conclusion is
based on analysis of unblended absorption features of two lines from the same
ion (in this case the Si IV doublet), which allows unambiguous separation of
covering factor and optical depth effects. The complex morphology of the
covering factor as a function of velocity suggests that the BALs are produced
by several physically separated outflows. The covering factor is ion dependent
in both depth and velocity width. We also find evidence that in BALQSO
1603+3002 the flow does not cover the broad emission line region.Comment: 13 pages, 2 figures, accepted for publication in Ap
Unveiling the nature of INTEGRAL objects through optical spectroscopy. IV. A study of six new hard X-ray sources
We present further results from our ongoing optical spectrophotometric
campaign at the Astronomical Observatory of Bologna in Loiano (Italy) on
unidentified hard X-ray sources detected by INTEGRAL. We observed
spectroscopically the putative optical counterparts of the INTEGRAL sources IGR
J00234+6141, IGR J01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR
J20286+2544. We find that the first two are Galactic objects, namely a
Cataclysmic Variable at a distance of about 300 pc and a Be/X transient
High-Mass X-ray Binary (HMXB) located at about 6.4 kpc, respectively, whereas
the last one is identified with MCG +04-48-002, a Starburst/HII galaxy at
redshift z = 0.013 hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 as
the (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we
classify as an X-ray Bright, Optically Normal Galaxy; this is the first example
of this type of object to be detected by INTEGRAL, and one of the closest of
this class. We moreover confirm the possible Be/X nature of IGR J06074+2205,
and we estimate it to be at a distance of about 1 kpc. We also reexamine the
spectrum of the z = 0.087 elliptical radio galaxy PKS 0352-686, the possible
counterpart of the INTEGRAL source IGR J03532-6829, and we find that it is a BL
Lac. Physical parameters for these sources are also evaluated by discussing our
findings in the context of the available multiwavelength information. These
identifications further stress the importance of INTEGRAL in the study of the
hard X-ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables.Comment: 11 pages, 3 figures, 2 tables, accepted for publication on A&A, main
journal. The quality of Fig. 1 was degraded to fit the arXiv uploads size
limits. Revised version matches the A&A corrected proof
The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later
We have used the VLA FIRST survey and the APM catalog of the POSS-I plates as
the basis for constructing a new radio-selected sample of optically bright
quasars. This is the first radio-selected sample that is competitive in size
with current optically selected quasar surveys. Using only two basic criteria,
radio-optical positional coincidence and optical morphology, quasars and BL
Lacs can be identified with 60% selection efficiency; the efficiency increases
to 70% for objects fainter than magnitude 17. We show that a more sophisticated
selection scheme can predict with better than 85% reliability which candidates
will turn out to be quasars.
This paper presents the second installment of the FIRST Bright Quasar Survey
with a catalog of 636 quasars distributed over 2682 square degrees. The quasar
sample is characterized and all spectra are displayed. The FBQS detects both
radio-loud and radio-quiet quasars out to a redshift z>3. We find a large
population of objects of intermediate radio-loudness; there is no evidence in
our sample for a bimodal distribution of radio characteristics. The sample
includes ~29 broad absorption line quasars, both high and low ionization, and a
number of new objects with remarkable optical spectra.Comment: 41 pages plus 39 gifs which contain all quasar spectra. Accepted for
publication in the Astrophysical Journal Supplement Serie
Identification of A-colored Stars and Structure in the Halo of the Milky Way from SDSS Commissioning Data
A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main
sequence A stars has been selected from 370 square degrees of Sloan Digital Sky
Survey (SDSS) commissioning observations. The data is from two long, narrow
stripes, each with an opening angle of greater than 60 deg, at Galactic
latitudes 36 < abs(b) < 63 on the celestial equator. An examination of the
sample's distribution shows that these stars trace considerable substructure in
the halo. Large overdensities of A-colored stars in the North at (l,b,R) =
(350, 50, 46 kpc) and in the South at (157, -58, 33 kpc) and extending over
tens of degrees are present in the halo of the Milky Way. Using photometry to
separate the stars by surface gravity, both structures are shown to contain a
sequence of low surface gravity stars consistent with identification as a blue
horizontal branch (BHB). Both structures also contain a population of high
surface gravity stars two magnitudes fainter than the BHB stars, consistent
with their identification as blue stragglers (BSs). From the numbers of
detected BHB stars, lower limits to the implied mass of the structures are
6x10^6 M_sun and 2x10^6 M_sun. The fact that two such large clumps have been
detected in a survey of only 1% of the sky indicates that such structures are
not uncommon in the halo. Simple spheroidal parameters are fit to a complete
sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to
a halo distribution with flattening (c/a = 0.65+/-0.2) and a density falloff
exponent of alpha = -3.2+/-0.3.Comment: AASTeX v5_0, 26 pages, 1 table, 20 figures, ApJ accepte
Using the Rosat Catalogue to find Counterparts for Unidentified Objects in the 1st Fermi/LAT Catalogue
There are a total of 1451 gamma-ray emitting objects in the Fermi Large Area
Telescope First Source Catalogue. The point source location accuracy of
typically a few arcminutes has allowed the counterparts for many of these
sources to be found at other wavelengths, but even so there are 630 which are
described as having no plausible counterpart at 80% confidence. In order to
help identify the unknown objects, we have cross-correlated the positions of
these sources with the Rosat All Sky Survey Bright Source Catalogue. In this
way, for Fermi sources which have a possible counterpart in soft X-rays, we can
use the, much smaller, Rosat error box to search for identifications. We find a
strong correlation between the two samples and calculate that there are about
60 sources with a Rosat counterpart. Using the Rosat error boxes we provide
tentative associations for half of them, demonstrate that the majority of these
are either blazars or blazar candidates and give evidence that most belong to
the BL Lac class. Given that they are X-ray selected and most are high
synchrotron peaked objects, which indicates the presence of high energy
electrons, these sources are also good candidates for TeV emission, and
therefore good probes of the extragalactic background light.Comment: 9 pages, 1 figure; Accepted for publication in MNRA
A Viewing Angle - Kinetic Luminosity Unification Scheme For BL Lacertae Objects
We propose a unified classification for BL Lac objects (BLs), focusing on the
synchrotron peak frequency of the spectral energy distribution. The unification
scheme is based on the angle Theta that describes the orientation of the
relativistic jet and on the electron kinetic luminosity Lambda of the jet. We
assume that Lambda scales with the size of the jet r in a self-similar fashion
(Lambda propto r^2), as supported by observational data. The jets are
self-similar in geometry and have the same pressure and median magnetic field
at the inlet, independent of size. The self-similarity is broken for the
highest energy electrons, which radiate mainly at high frequencies, since for
large sources they suffer more severe radiative energy losses over a given
fraction of the jet length. We calculate the optically thin synchrotron
spectrum using an accelerating inner jet model based on simple relativistic gas
dynamics and show that it can fit the observed infrared to X-ray spectrum of
PKS 2155--304. We couple the accelerating jet model to the unification scheme
and compare the results to complete samples of
BLs. The negative apparent evolution of X-ray selected BLs is explained as a
result of positive evolution of the jet electron kinetic luminosity
. We review observational arguments in favor of the existence of
scaled-down accretion disks and broad emission-line regions in BLs. The
proposed unification scheme can explain the lack of observed broad emission
lines in X-ray selected BLs, as well as the existence of those lines
preferentially in luminous radio-selected BLs. Finally, we review observational
arguments that suggest the extension of this unification scheme to all blazars.Comment: 32 pages, 8 figures, to be published in the ApJ (Oct 20, 1998
The Properties of the Radio-Selected 1Jy Sample of BL Lacertae Objects
We present new optical and near-IR spectroscopy as well as new high dynamic
range, arcsecond-resolution VLA radio maps of BL Lacs from the complete
radio-selected "1 Jansky" (1Jy) sample (RBLs) for which such data were not
previously available. Unlike BL Lacs from the complete X-ray-selected Einstein
Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but
moderately luminous emission lines. And whereas nearly all XBLs have extended
power levels consistent with FR-1s, more than half of the RBLs have extended
radio power levels too luminous to be beamed FR-1 radio galaxies. In fact, we
find evidence for and examples of three distinct mechanisms for creating the BL
Lac phenomenon in the 1Jy sample: beamed FR-1s, beamed FR-2s and possibly a few
gravitationally-lensed quasars. The v/v_max determined for the 1Jy sample is
0.614+/-0.047, which is markedly different from the negative evolution seen in
the EMSS and other XBL samples. A correlation between logarithmic X-ray to
radio flux ratio and v/v_max is observed across the EMSS and 1Jy samples, from
negative evolution in the more extreme XBLs to positive evolution in the more
extreme RBLs. There is evidence that the selection criteria chosen by Stickel
et al. eliminates some BL Lac objects from the 1Jy sample, although how many is
unknown. And several objects currently in the sample have exhibited strong
emission lines in one or more epochs, suggesting they should be reclassified as
FSRQs. However these selection effects cannot account for the observed
discrepancy in XBL and RBL properties. From these observational properties we
conclude that RBLs and XBLs cannot be related by viewing angle alone, and that
RBLs are more closely related to FSRQs.Comment: 29 pages, 47 figures, accepted A
BL Lac Identification for the Ultraluminous X-ray Source Observed in the Direction of NGC 4698
We report the identification of the optical and radio counterparts of the
ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC4698-ULX1). The
optical spectrum taken with the VLT yields a redshift of z=0.43, which implies
that the ULX is not associated with the nearby galaxy NGC4698. The spectral
energy distribution calculated from the available data indicates that the
source is likely to be a BL Lac object. The possible synchrotron peak at X-ray
energies suggests that this source may be a gamma-ray emitter.Comment: 6 pages, 5 figures. Accepted for publication by Astronomy and
Astrophysic
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