55 research outputs found
Four New BL Lac Surveys: Sampling New Populations
The advent of large area deep radio and X-ray surveys is leading to the
creation of many new BL Lac samples. In particular, the ROSAT All-Sky, Green
Bank and FIRST surveys are proving to be rich sources of new BL Lacs. We will
discuss the methods used in four independent BL Lac searches based on these
surveys. Comparison of the broadband spectral energy distributions of these BL
Lacs with those of previously known objects clearly points to the existence of
a large previously unrecognized population of objects with characteristics
intermediate between those exhibited by Low and High energy peaked BL Lacs.Comment: 4 pages, 3 postscript figures, To be published in the Proceedings of
the conference "BL Lac Phenomenon" held in Turku, Finland, June 22-26, 199
Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. II. The IUE Campaign
PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was
monitored with the IUE satellite at ~1 hour time-resolution for ten nearly
uninterrupted days in May 1994. The campaign, which was coordinated with EUVE,
ROSAT, and ASCA monitoring, along with optical and radio observations from the
ground, yielded the largest set of spectra and the richest short time scale
variability information ever gathered for a blazar at UV wavelengths. The
source flared dramatically during the first day, with an increase by a factor
~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly
constant level for ~5 days, then flared with ~35% amplitude for two days. The
same variability was seen in both short- and long-wavelength IUE light curves,
with zero formal lag (~<2 hr), except during the rapid initial flare, when the
variations were not resolved. Spectral index variations were small and not
clearly correlated with flux. The flux variability observed in the present
monitoring is so rapid that for the first time, based on the UV emission alone,
the traditional Delta L/Delta t limit indicating relativistic beaming is
exceeded. The most rapid variations, under the likely assumption of synchrotron
radiation, lead to a lower limit of 1 G on the magnetic field strength in the
UV emitting region. These results are compared with earlier intensive
monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux
variations had completely different characteristics.Comment: 45 pages, Latex, 11 PostScript figures, to appear in The
Astrophysical Journa
The milliarcsecond-scale jet of PKS 0735+178 during quiescence
We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae
object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous
and later observations suggests that the overall kinematic and structural
properties of the jet are greatly influenced by its activity. Time intervals of
enhanced activity, as reported before 1993 and after 2000 by other studies, are
followed by highly superluminal motion along a rectilinear jet. In contrast the
less active state in which we performed our observations, shows subluminal or
slow superluminal jet features propagating through a twisted jet with two sharp
bends of about 90 deg. within the innermost three-milliarcsecond jet structure.
Proper motion estimates from the data presented here allow us to constrain the
jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between
2 and 4.Comment: 11 pages, 12 figures. Accepted for publication in A&
Radio-loud Active Galaxies in the Northern ROSAT All-Sky Survey III: New Spectroscopic Identifications from the RGB BL Lac Survey
We present new spectroscopic identifications for 169 objects in the
RASS-Green Bank (RGB) catalog of radio- and X-ray-emitting AGN. These data
significantly increase the fraction of bright RGB objects with classifications.
Specifically, we report and discuss the classification of 66 radio-loud
quasars, 53 BL Lacs, 33 Broad Line Radio Galaxies, 5 Narrow Line Radio
Galaxies, 1 Seyfert I galaxy and 11 galaxies or galaxies in clusters. Over 78%
of the identifications we present here are new. The observations we report were
undertaken as part of our targeted search program to identify a new, large
unbiased sample of BL Lac Objects and we therefore discuss the BL Lac sample
extensively. Unlike many previous surveys, we impose no selection criteria
based on optical morphology, color or broadband spectral energy distribution.
Our classifications are based solely on a carefully defined set of
self-consistent spectroscopic classification criteria. We show the 53 RGB
presented here exhibit transitional properties between normal galaxies and BL
Lacs discovered previously. We show there is no clear separation in CaII break
strength between RGB BL Lacs and galaxies, with the distribution of break
strengths varying smoothly between 0% and 50%. We also show that the newly
discovered RGB BL Lacs reside in a "zone of avoidance" in the log(S_x/S_r) vs.
log(S_o/S_r) diagram. This has important implications for BL Lac search
strategies since it shows that RASS BL Lac samples will be severely incomplete
if candidates are chosen only from among those objects with the highest S_x/S_r
flux ratios.Comment: 21 pages text, 189 Figures, 4 tables, LaTeX2E, 4.2MB tar file
(compressed); special style file paper.sty provide
Spectroscopy of BL Lac Objects: new redshifts and mis-identified sources
We are carrying out a program of high signal to noise optical spectroscopy of
BL Lacs with unknown or tentative redshift. Here we report some preliminary
results. New redshifts are measured for PKS0754+100 (z=0.266) and 1ES0715-259
(z=0.464) . From lineless spectra of PG1553+113 and PKS1722+119 we set a lower
limit of z>0.3 for both sources. In two cases (UM493 and 1620+103) stellar
spectra indicate a wrong classification.Comment: 4 pages; Conference proceeding "High Energy Blazar Astronomy", Tuorla
Observatory, Finland, 17-21 June 2002; to be published in the PASP conference
serie
The North Ecliptic Pole Wide survey of AKARI: a near- and mid-infrared source catalog
We present a photometric catalog of infrared (IR) sources based on the North
Ecliptic PoleWide field (NEP-Wide) survey of AKARI, which is an infrared space
telescope launched by Japan. The NEP-Wide survey covered 5.4 deg2 area, a
nearly circular shape centered on the North Ecliptic Pole, using nine
photometric filter-bands from 2 - 25 {\mu}m of the Infrared Camera (IRC).
Extensive efforts were made to reduce possible false objects due to cosmic ray
hits, multiplexer bleeding phenomena around bright sources, and other
artifacts. The number of detected sources varied depending on the filter band:
with about 109,000 sources being cataloged in the near-IR bands at 2 - 5
{\mu}m, about 20,000 sources in the shorter parts of the mid-IR bands between 7
- 11 {\mu}m, and about 16,000 sources in the longer parts of the mid-IR bands,
with \sim 4,000 sources at 24 {\mu}m. The estimated 5? detection limits are
approximately 21 magnitude (mag) in the 2 - 5 {\mu}m bands, 19.5 - 19 mag in
the 7 - 11 {\mu}m, and 18.8 - 18.5 mag in the 15 - 24 {\mu}m bands in the AB
magnitude scale. The completenesses for those bands were evaluated as a
function of magnitude: the 50% completeness limits are about 19.8 mag at 3
{\mu}m, 18.6 mag at 9 {\mu}m, and 18 mag at 18 {\mu}m band, respectively. To
construct a reliable source catalog, all of the detected sources were examined
by matching them with those in other wavelength data, including optical and
ground-based near-IR bands. The final band-merged catalog contains about
114,800 sources detected in the IRC filter bands. The properties of the sources
are presented in terms of the distributions in various color-color diagrams.Comment: Accepted for publication in A&A, 23 pages, 27 figure
The stellar content of low redshift BL Lac host galaxies from multicolour imaging
We present B-band imaging of 18 low redshift (z<0.3) BL Lac objects for which
their host galaxies were previously resolved in the R-band and the
near-infrared H-band. For a subset of the objects, also U- and V-band imaging
is presented. These data are used to investigate the blue-red-near-infrared
colours and the colour gradients of the host galaxies of BL Lacs in comparison
with other elliptical galaxies with and without nuclear activity. In all cases
galaxies are well represented by an elliptical model, with average absolute
magnitude M_B=-21.6+-0.7 and average scale length R_e=7.6+-3.2 kpc. The
best-fit B-band Kormendy relation is in reasonable agreement with that obtained
for normal ellipticals and radio galaxies. This structural and dynamical
similarity indicates that all massive elliptical galaxies can experience
nuclear activity without significant perturbation of their global structure.
The distributions of the integrated blue/near-infrared colour (with average
B-H=3.5+-0.5) and colour gradient (with average Delta(B-R)/Delta(log
r)=-0.14+-0.75) of the BL Lac hosts are much wider than those for normal
ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour
gradients than those in normal ellipticals. The blue colours are likely caused
by a young stellar population component, and indicates a link between star
formation caused by an interaction/merging event and the onset of the nuclear
activity. This result is corroborated by stellar population modelling,
indicating a presence of young/intermediate age populations in the majority of
the sample, in agreement with low redshift quasar hosts. The lack of strong
signs of interaction may require a significant time delay between the event
with associated star formation episodes and the start of the nuclear activity.Comment: 16 pages, 10 figure
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