27 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
ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac)
We present ASCA observations of the radio-selected BL Lacertae objects
1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept and
Nov, respectively. The ASCA spectra of both sources can be described as a first
approximation by a power law with photon index Gamma ~ 2. This is flatter than
for most X-ray-selected BL Lacs observed with ASCA, in agreement with the
predictions of current blazar unification models. While 1749+096 exhibits
tentative evidence for spectral flattening at low energies, a concave continuum
is detected for 2200+420: the steep low-energy component is consistent the
high-energy tail of the synchrotron emission responsible for the longer
wavelengths, while the harder tail at higher energies is the onset of the
Compton component. The spectral energy distributions from radio to gamma-rays
are consistent with synchrotron-self Compton emission from a single homogeneous
region shortward of the IR/optical wavelengths, with a second component in the
radio domain related to a more extended emission region. For 2200+420,
comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we find
that models requiring inverse Compton scattering of external photons provide a
viable mechanism for the production of the highest (GeV) energies during the
flare. An increase of the external radiation density and of the power injected
in the jet can reproduce the flat gamma-ray continuum observed in 1997 July. A
directly testable prediction of this model is that the line luminosity in
2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for
publication in the Astrophysical Journa
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
RXJ 1821.6+6827: a Cool Cluster at z=0.81 from the ROSAT NEP Survey
We present an analysis of the properties of the cluster of galaxies RXJ
1821.6+6827, or NEP 5281, at a redshift z=0.816+/-0.001. RXJ 1821.6+6827 was
discovered during the optical identification of the X-ray sources in the North
Ecliptic Pole (NEP) region of the ROSAT All-Sky Survey and it is the highest
redshift cluster of galaxies of the NEP survey. We have measured spectroscopic
redshifts for twenty cluster galaxies using the Keck-I and the
Canada-France-Hawai'i (CFH) telescopes. The value for the cluster velocity
dispersion is sigma_V=775(+182,-113) km s-1. The cluster was also observed by
XMM-Newton. Both the optical and X-ray data are presented in this paper. The
cluster has an unabsorbed X-ray flux in the 2-10 keV energy band of F(2-10
keV)=1.24(+0.16,-0.23) x 10-13 erg cm2 s-1 and a K-corrected luminosity in the
same band of L(2-10 keV)=6.32(+76,-0.73)x10^44 h_50^-2 erg s-1 (90% confidence
level). The cluster X-ray bolometric luminosity is
L(BOL,X)=1.35(+0.08,-0.21)x10^45 h_50^-2 erg s-1. The data do not allow fitting
both metal abundance and temperature at the same time. The abundance is
unconstrained and can vary in the range 0.28-1.42 Z_sun while the best fit
X-ray temperature is T=4.7(+1.2,-0.7) keV. This emission weighted X-ray
temperature is a little lower, barely within the uncertainties, than the
predicted temperature, T=6.34(+0.13,-0.35) keV, from the L_X-T_X relation of
local clusters published in the literature. The optically measured
velocitydispersion is consistent with the velocity dispersion expected from the
sigma_V-T_X relationship. We also examine the point X-ray source
RXJ1821.9+6818, or NEP 5330, located to the south east of the cluster which was
identified as a QSO at z=1.692+/-0.008 in the ROSAT NEP survey.Comment: 13 pages including 5 postscript figures and 3 tables. Accepted for
publication in the A&A main Journal. A postscript version with Fig. 2 can be
downloaded from http://www.ira.cnr.it/~gioia/PUB/publications.htm
Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign
Optical, near-infrared, and radio observations of the BL Lac object
PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray
monitoring campaign in 1994 May. Further optical observations were gathered
throughout most of 1994. The radio, millimeter, and near-infrared data show no
strong correlations with the higher energies. The optical light curves exhibit
flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer
timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are
the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases
in the V and R bands occur simultaneously with a flare seen at higher energies.
All optical wavebands (UBVRI) track each other well over the period of
observation with no detectable delay. For most of the period the average colors
remain relatively constant, although there is a tendency for the colors (in
particular B-V) to vary more when the source fades. In polarized light, PKS
2155-304 showed strong color dependence and the highest optical polarization (U
= 14.3%) ever observed for this source. The polarization variations trace the
flares seen in the ultraviolet flux.Comment: 45 pages, latex file with encapsulated postscript, accepted to the
Astrophysical Journa