29 research outputs found
Detection of the BL Lac Object H1426+428 at TeV Gamma Ray Energies
A very high energy (VHE) gamma-ray signal has been detected at the 5.4 sigma
level from H1426+428, an x-ray selected BL Lacertae object at a redshift of
0.129. The object was monitored from 1995 - 1998 with the Whipple 10m imaging
atmospheric Cherenkov telescope as part of a general blazar survey; the results
of these observations, although not statistically significant, were
consistently positive. X-ray observations of H1426+428 during 1999 with the
BeppoSAX instrument revealed that the peak of its synchrotron spectrum occurs
at > 100 keV, leading to the prediction of observable TeV emission from this
object. H1426+428 was monitored extensively at the Whipple Observatory during
the 1999, 2000, and 2001 observing seasons. The strongest TeV signals were
detected in 2000 and 2001. During 2001, an integral flux of 2.04 +/- 0.35
x10e-11 cm-2 s-1 above 280 GeV was recorded from H1426+428. The detection of
H1426+428 supports the idea that, as also seen in Markarian 501 and
1ES2344+514, BL Lacertae objects with extremely high synchrotron peak
frequencies produce gamma rays in the TeV range.Comment: 35 pages, 7 figures, accepted by ApJ Two upper limits in Table 3
(upper limits for 1995 and 1997) are different from the ApJ versio
Search for High Energy Gamma Rays from an X-ray Selected Blazar Sample
Our understanding of blazars has been greatly increased in recent years by
extensive multi-wavelength observations, particularly in the radio, X-ray and
gamma-ray regions. Over the past decade the Whipple 10m telescope has
contributed to this with the detection of 5 BL Lacertae objects at very high
gamma-ray energies. The combination of multi-wavelength data has shown that
blazars follow a well-defined sequence in terms of their broadband spectral
properties. Together with providing constraints on emission models, this
information has yielded a means by which potential sources of TeV emission may
be identified and predictions made as to their possible gamma-ray flux. We have
used the Whipple telescope to search for TeV gamma-ray emission from eight
objects selected from a list of such candidates. No evidence has been found for
VHE emission from the objects in our sample, and upper limits have been derived
for the mean gamma-ray flux above 390GeV. These flux upper limits are compared
with the model predictions and the implications of our results for future
observations are discussed.Comment: 15 pages, 2 figures, Accepted for publication in Ap
Observations of 54 Active Galactic Nuclei with the HEGRA System of Cherenkov Telescopes
A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with
the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in
the TeV energy regime. The observations were motivated by the positive results
obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary
over a large range of redshifts between z = 0.004 and z = 0.7. Among the
observed AGN are the meanwhile established TeV-emitting BL Lac type objects
H1426+428 and 1ES1959+650. Furthermore the BL Lac object 1ES2344+514 and the
radio galaxy M87 show evidence for a signal on a 4 sigma level. The observation
of 1ES2344+514 together with the Whipple results firmly establishes this AGN as
a TeV source. Several objects (PKS2155-304, BL Lacertae, 3C066A) that have been
claimed as TeV gamma-ray emitters by other groups are included in this data
sample but could not be confirmed using data analysed here. The upper limits of
several AGN included in this analysis are compared with predictions in the
frame-work of SSC models.Comment: 9 pages, 2 figures, 7 tables, submitted to A&
Receptor Complementation and Mutagenesis Reveal SR-BI as an Essential HCV Entry Factor and Functionally Imply Its Intra- and Extra-Cellular Domains
HCV entry into cells is a multi-step and slow process. It is believed that the
initial capture of HCV particles by glycosaminoglycans and/or lipoprotein
receptors is followed by coordinated interactions with the scavenger receptor
class B type I (SR-BI), a major receptor of high-density lipoprotein (HDL), the
CD81 tetraspanin, and the tight junction protein Claudin-1, ultimately leading
to uptake and cellular penetration of HCV via low-pH endosomes.
Several reports have indicated that HDL promotes HCV entry through interaction
with SR-BI. This pathway remains largely elusive, although it was shown that HDL
neither associates with HCV particles nor modulates HCV binding to SR-BI. In
contrast to CD81 and Claudin-1, the importance of SR-BI has only been addressed
indirectly because of lack of cells in which functional complementation assays
with mutant receptors could be performed. Here we identified for the first time
two cell types that supported HCVpp and HCVcc entry upon ectopic SR-BI
expression. Remarkably, the undetectable expression of SR-BI in rat hepatoma
cells allowed unambiguous investigation of human SR-BI functions during HCV
entry. By expressing different SR-BI mutants in either cell line, our results
revealed features of SR-BI intracellular domains that influence HCV infectivity
without affecting receptor binding and stimulation of HCV entry induced by
HDL/SR-BI interaction. Conversely, we identified positions of SR-BI ectodomain
that, by altering HCV binding, inhibit entry. Finally, we characterized
alternative ectodomain determinants that, by reducing SR-BI cholesterol uptake
and efflux functions, abolish HDL-mediated infection-enhancement. Altogether, we
demonstrate that SR-BI is an essential HCV entry factor. Moreover, our results
highlight specific SR-BI determinants required during HCV entry and
physiological lipid transfer functions hijacked by HCV to favor infection
The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar
We report on observations of BL Lacertae during the first 18 months of
Fermi-LAT science operations and present results from a 48-day multifrequency
coordinated campaign from 2008 August 19 to 2008 October 7. The radio to
gamma-ray behavior of BL Lac is unveiled during a low activity state thanks to
the coordinated observations of radio-band (Metsahovi and VLBA),
near-IR/optical (Tuorla, Steward, OAGH and MDM) and X-ray (RXTE and Swift)
observatories. No variability was resolved in gamma-rays during the campaign,
and the brightness level was 15 times lower than the level of the 1997 EGRET
outburst. Moderate and uncorrelated variability has been detected in UV and
X-rays. The X-ray spectrum is found to be concave indicating the transition
region between the low and high energy component of the spectral energy
distribution (SED). VLBA observation detected a synchrotron spectrum
self-absorption turnover in the innermost part of the radio jet appearing to be
elongated and inhomogeneous, and constrained the average magnetic field there
to be less than 3 G. Over the following months BL Lac appeared variable in
gamma-rays, showing flares (in 2009 April and 2010 January). There is no
evidence for correlation of the gamma-rays with the optical flux monitored from
the ground in 18 months. The SED may be described by a single zone or two zone
synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation
Compton (ERC) model seems preferred based on the observed variability and the
fact that it provides a fit closest to equipartition.Comment: 14 pages, apj-style emulated, 10 figures. In press by The
Astrophysical Journa
Observation of M87 at 400 GeV with the Whipple 10 meter telescope
We present results from observations taken with the Whipple 10 m very high energy gamma-ray telescope with maximal sensitivity at 400 GeV during 39 hr between 2000 and 2003 in the direction of the giant radio galaxy M87. Using the entire data set, we derive a 99% confidence level upper limit on the flux of gamma-ray emission above 400 GeV from M87 to be less than or equal to 6: 9; 10(-1)2 cm(-2) s(-1). This suggests variability at the 90% confidence level when compared to the flux measured by the HEGRA collaboration in 1999 if the differential spectrum is steeper than a power law of index 3.75. Our search for a correlation between the Rossi X-Ray Timing Explorer all-sky monitor observation and a potential gamma-ray signal is inconclusive