1,303 research outputs found
Evidence for anisotropic motion of the clouds in broad-line regions of BL Lacertae objects
The masses of central massive black holes in BL Lac objects are estimated
from their host galaxy absolute magnitude at R-band by using the empirical
relation between absolute magnitude of host galaxy and black hole mass. Only a
small fraction of BL Lac objects exhibit weak broad-line emission, and we
derive the sizes of the broad-line regions (BLRs) in these BL Lac objects from
the widths of their broad emission lines on the assumption of the clouds being
virilized in BLRs. It is found that the sizes of the BLRs in these sources are
usually 2-3 orders of magnitude larger than that expected by the empirical
correlation between BLR size and optical luminosity defined by a sample of
Seyfert galaxies and quasars. We discuss a variety of possibilities and suggest
it may probably be attributed to anisotropic motion of the BLR clouds in these
BL Lac objects. If the BLR geometry of these sources is disk-like, the viewing
angles between the axis and the line of sight are in the range of 2-12 degrees,
which is consistent with the unification schemes.Comment: 5 pages, accepted for publication in Ap
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
An Investigation of Gravitational Lensing in the Southern BL Lac PKS 0537-441
The BL-Lac family of active galaxies possess almost featureless spectra and
exhibit rapid variability over their entire spectral range. A number of models
have been developed to explain these extreme properties, several of which have
invoked the action of microlensing by sub-stellar mass objects in a foreground
galaxy; this not only introduces variability, but also amplifies an otherwise
normal quasar source. Here we present recent spectroscopy and photometry of the
southern BL Lac PKS 0537-441; with an inferred redshift of z~0.9 it represents
one of the most distant and most luminous members of the BL Lac family. The
goal of the observations was not only to confirm the redshift of PKS~0537-441,
but also to determine the redshift of a putative galaxy along the line of sight
to the BL-Lac; it has been proposed that this galaxy is the host of
microlensing stars that account for PKS 0537-441's extreme properties. While
several observations have failed to detect any extended emission in PKS
0537-441, the HST imaging data presented here indicate the presence of a
galactic component, although we fail to identify any absorption features that
reveal the redshift of the emission. It is also noted that PKS 0537-441 is
accompanied by several small, but extended companions, located a few arcseconds
from the point-like BL-Lac source. Two possibilities present themselves; either
they represent true companions of PKS 0537-441, or are themselves
gravitationally lensed images of more distant sources.Comment: 13 Pages with 4 Figures; Accepted for Publication by the
Astrophysical Journa
The nature of close companions of the BL Lac Objects 1ES 0502+675 and 1ES 1440+122
We report on deep radio images and optical spectroscopy of two BL Lac objects
that have very close compact companions. The two targets, 1ES 0502+675 and 1ES
1440+122, were selected from the HST imaging survey of 110 BL Lacs as
candidates for possible gravitational lensing. The new observations clearly
demonstrate that the companion objects are not secondary images of the active
nuclei but, in spite of the relatively low chance projection probability,
foreground Galactic stars. Gravitational lensing appears to be unimportant to
the BL Lac phenomenon. We discuss the radio properties of the BL Lac objects in
the context of standard beaming models, and show they are as expected for
beamed FRI radio galaxies.Comment: 13 pages, 5 figures, ApJ in pres
Redshifts and Neutral Hydrogen Observations of Compact Symmetric Objects in the COINS Sample
Compact Symmetric Objects (CSOs) are young radio galaxies whose jet axes lie
close to the plane of the sky, and whose appearance is therefore not dominated
by relativistic beaming effects. The small linear sizes of CSOs make them
valuable for studies of both the evolution of radio galaxies and testing
unified schemes for active galactic nuclei (AGN). A parsec-scale region of gas
surrounding the central engine is predicted by both accretion and obscuration
scenarios. Working surfaces, or ``hot spots,'' and the radio jets of CSOs are
close enough to the central engines that this circumnuclear gas can be seen in
absorption.
The CSOs Observed in the Northern Sky (COINS) sample is comprised of 52 CSO
candidates identified in three VLBI surveys. Of these, 27 have now been
confirmed as CSOs. Optical redshifts are available in the literature for 28 of
the CSO candidates, and HI absorption has been detected toward four. We present
new optical spectroscopic redshifts for three of the candidates and summarize
the current status of optical identifications. We further report on the
discovery of HI in absorption towards the CSO J1816+3457 and summarize the
results of neutral hydrogen absorption studies of the sources in this sample.Comment: 12 pages, Accepted for publication in Ap
X-ray Flares from Markarian 501
Motivated by the recent finding of hierarchical X-ray flaring phenomenon in
Mrk 421, we conducted a systematic search for X-ray flares from Mrk 501,
another well-known TeV blazar, by making use of the rich {\em RXTE} archival
database. We detected flares over a wide range of timescales, from months down
to minutes, as in the case of Mrk 421. However, the flares do not seem to occur
nearly as frequently in Mrk 501 as in Mrk 421 on any of the timescales. The
flaring hierarchy also seems apparent in Mrk 501, suggesting that it might be
common among TeV blazars. The results seem to imply a scale-invariant physical
origin of the flares (large or small). The X-ray spectrum of the source shows a
general trend of hardening toward the peak of long-duration flares, with
indication of spectral hysteresis, which is often seen in TeV blazars. However,
the data are not of sufficient quality to allow us to draw definitive
conclusions about spectral variability associated with more rapid but weaker
flares. We critically examine a reported sub-hour X-ray flare from Mrk 501, in
light of intense background flaring activity at the time of the observation,
and concluded that the flare is likely an artifact. On the other hand, we did
identify a rapid X-ray flare that appears to be real. It lasted only for about
15 minutes, during which the flux of the source varied by about 30%.
Sub-structures are apparent in its profile, implying variabilities on even
shorter timescales. Such rapid variabilities of Mrk 501 place severe
constraints on the physical properties of the flaring region in the jet, which
have serious implications on the emission models proposed for TeV blazars.Comment: 23 pages, 11 figures, accepted for publication in Ap
Evidence for the evolutionary sequence of blazars: different types of accretion flows in BL Lac objects
The limits on the mass of the black hole in 23 BL Lac objects are obtained
from their luminosities of the broad emission line H\beta on the assumption
that broad emission lines are emitted from clouds ionized by the radiation of
the accretion disk surrounding a black hole. The distribution of line
luminosity L_{H\beta} of all these BL Lac objects suggests a bimodal nature,
although this cannot be statistically proven on the basis of the present,
rather small sample. We found that standard thin disks are probably in the
sources with L_{H\beta}>10^{41} erg s^{-1}. The central black holes in these
sources have masses of 10^{8-10} M_\odot, if the matter is accreting at the
rate of 0.025 {\dot M}_{\rm Edd}. For the sources with L_{H\beta}<10^{41} erg
s^{-1}, the accretion flows have transited from standard thin disk type to the
ADAF type. The lower limits on the mass of the black hole in these sources are
in the range of 1.66-24.5\times 10^{8} M_\odot. The results support the
evolutionary sequence of blazars: FSRQ\to LBL\to HBL.Comment: 5 pages, accepted for publication in ApJ Letter
An Excess of Mg II Absorbers in BL Lac Objects
Two new Mg II absorbers are presented (z=1.340 in S5 0454+844 and z=1.117 in
PKS 2029+121), bringing the total number of Mg II systems in the 1 Jy
radio-selected BL Lac sample to 10. Five of the ten absorption systems are at W
> 1A; this is a factor of four to five greater than the number expected based
upon quasar sightlines, and is 2.5 to 3 sigma greater than the expectation
value. Interpretations of this possible excess include either that some of the
Mg II absorbers might be intrinsic to the BL Lac or that there is a correlation
between the presence of absorbing gas in the foreground and the nearly
featureless spectra of these BL Lac Objects compared to quasars. Such a
correlation can be created by gravitational microlensing as suggested by
Ostriker & Vietri. The similarity between the optical spectra of BL Lacs with
Mg II absorption and the spectrum of the Gamma-ray burst source GRB 970508
suggests that models of Gamma-ray bursts as microlensed AGN should be
investigated.Comment: LaTeX, AASTeX, 3 figures: jy0138.eps, jy0454.eps, jy2029.eps ApJL,
Accepted September 2nd, 199
Jet formation in BL Lacertae objects with different accretion modes
We estimate the masses of massive black holes in BL Lac objects from their
host galaxy luminosity. The power of jets and central optical ionizing
luminosity for a sample of BL Lac objects are derived from their extended radio
emission and the narrow-line emission, respectively. The maximal jet power can
be extracted from a standard thin accretion disk/spinning black hole is
calculated as a function of dimensionless accretion rate
(). Comparing with the derived jet power, we
find that the accretion disks in most BL Lac objects should not be standard
accretion disks. For a pure advection dominated accretion flow (ADAF), there is
an upper limit on its optical continuum luminosity due to the existence of an
upper limit on the accretion rate. It is found that a pure
ADAF is too faint to produce the optical ionizing luminosity of BL Lac objects
derived from their narrow-line luminosity. We propose that an ADAF is present
in the inner region of the disk and it becomes a standard thin disk in the
outer region in most BL Lac objects, i.e., ADAF+SD(standard disk) scenario.
This ADAF+SD scenario can explain both the jet power and optical ionizing
continuum emission of these BL Lac objects. The inferred transition radii
between the inner ADAF and outer SD are in the range of ,
if the disks are accreting at the rate .Comment: accepted by Ap
The jet power, radio loudness and black hole mass in radio loud AGNs
The jet formation is thought to be closely connected with the mass of central
supermassive black hole in Active Galactic Nuclei. The radio luminosity
commonly used in investigating this issue is merely an indirect measure of the
energy transported through the jets from the central engine, and severely
Doppler boosted in core-dominated radio quasars. In this work, we investigate
the relationship between the jet power and black hole mass, by estimating the
jet power using extrapolated extended 151 MHz flux density from the VLA 5 GHz
extended radio emission, for a sample of 146 radio loud quasars complied from
literature. After removing the effect of relativistic beaming in the radio and
optical emission, we find a significant intrinsic correlation between the jet
power and black hole mass. It strongly implies that the jet power, so as jet
formation, is closely connected with the black hole mass.To eliminate the
beaming effect in the conventional radio loudness, we define a new radio
loudness as the ratio of the radio extended luminosity to the optical
luminosity estimated from the broad line luminosity.In a tentatively combined
sample of radio quiet with our radio loud quasars, the apparent gap around the
conventional radio loudness R=10 is not prominent for the new-defined radio
loudness. In this combined sample, we find a significant correlation between
the black hole mass and new-defined radio loudness.Comment: 35 pages, 10 figures. accepted by Ap
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