1,337 research outputs found
High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)
We present multi-frequency, high resolution VLBA circular polarization images
of the radio source 3C 84 in the center of NGC 1275. Our images reveal a
complex distribution of circular polarization in the inner parsec of the radio
jet, with local levels exceeding 3% polarization, the highest yet detected with
VLBI techniques. The circular polarization changes sign along the jet, making
3C 84 also the first radio jet to show both signs of circular polarization
simultaneously. The spectrum and changing sign of the circular polarization
indicate that it is unlikely to be purely intrinsic to the emitted synchrotron
radiation. The Faraday conversion process makes a significant and perhaps
dominant contribution to the circular polarization, and the observed spectrum
suggests the conversion process is near saturation. The sign change in the
circular polarization along the jet may result from this saturation or may be
due to a change in magnetic field order after an apparent bend in the jet. From
the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels
of circular polarization inferred for the intra-day variable source PKS
1519-273, we suggest a connection between small spatial scales and efficient
production of circular polarization.Comment: 4 pages, accepted in ApJ Letter
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
Search for IR Emission from Intracluster Dust in A2029
We have searched for IR emission from the intracluster dust (ICD) in the
galaxy cluster A2029. Weak signals of enhanced extended emission in the cluster
are detected at both 24 and 70 micron. However, the signals are
indistinguishable from the foreground fluctuations. The 24 versus 70 micron
color map does not discriminate the dust emission in the cluster from the
cirrus emission. After excluding the contamination from the point sources, we
obtain upper limits for the extended ICD emission in A2029, 5 x 10^3 Jy/sr at
24 micron and 5 x 10^4 Jy/sr at 70 micron. The upper limits are generally
consistent with the expectation from theoretical calculations and support a
dust deficiency in the cluster compared to the ISM in our galaxy. Our results
suggest that even with the much improved sensitivity of current IR telescopes,
a clear detection of the IR emission from ICD may be difficult due to cirrus
noise.Comment: 5 pages, 4 figures, accepted by ApJ
Peptic and Tryptic Digestion Products as Inexpensive Culture Mediums for Routine Bacteriologic Work
n/
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
Performance of three-photon PET imaging: Monte Carlo simulations
We have recently introduced the idea of making use of three-photon positron
annihilations in positron emission tomography. In this paper the basic
characteristics of the three-gamma imaging in PET are studied by means of Monte
Carlo simulations and analytical computations. Two typical configurations of
human and small animal scanners are considered. Three-photon imaging requires
high energy resolution detectors. Parameters currently attainable by CdZnTe
semiconductor detectors, the technology of choice for the future development of
radiation imaging, are assumed. Spatial resolution is calculated as a function
of detector energy resolution and size, position in the field of view, scanner
size, and the energies of the three gamma annihilation photons. Possible ways
to improve the spatial resolution obtained for nominal parameters: 1.5 cm and
3.2 mm FWHM for human and small animal scanners, respectively, are indicated.
Counting rates of true and random three-photon events for typical human and
small animal scanning configurations are assessed. A simple formula for minimum
size of lesions detectable in the three-gamma based images is derived.
Depending on the contrast and total number of registered counts, lesions of a
few mm size for human and sub mm for small animal scanners can be detected
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
Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant
We present measurements of bi-directional motions in the jets of the radio
galaxy 1946+708 at z=0.101. This is a Compact Symmetric Object with striking
S-symmetry. Sensitive 15 GHz observations reveal a compact component at the
center of symmetry with a strongly inverted spectrum, that we identify as the
core. From five 4.9 GHz observations spread over 4 years we have determined the
velocities of four compact jet components. If simple kinematic models can be
applied then the inclination of the source and the bulk jet velocity can be
directly determined for any assumed value of the Hubble constant. Conversely,
the measurements already place constraints on the Hubble constant, and we show
how further observations of 1946+708 can yield an increasingly accurate
determination of H_0.Comment: in press at ApJ Letters, 12 page LaTex document includes 5 postscript
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