131 research outputs found
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
Gamma-ray observations of Tycho's SNR with VERITAS and Fermi
High-energy gamma-ray emission from supernova remnants (SNRs) has provided a
unique perspective for studies of Galactic cosmic-ray acceleration. Tycho's SNR
is a particularly good target because it is a young, type Ia SNR that is
well-studied over a wide range of energies and located in a relatively clean
environment. Since the detection of gamma-ray emission from Tycho's SNR by
VERITAS and Fermi-LAT, there have been several theoretical models proposed to
explain its broadband emission and high-energy morphology. We report on an
update to the gamma-ray measurements of Tycho's SNR with 147 hours of VERITAS
and 84 months of Fermi-LAT observations, which represents about a factor of two
increase in exposure over previously published data. About half of the VERITAS
data benefited from a camera upgrade, which has made it possible to extend the
TeV measurements toward lower energies. The TeV spectral index measured by
VERITAS is consistent with previous results, but the expanded energy range
softens a straight power-law fit. At energies higher than 400 GeV, the
power-law index is . It
is also softer than the spectral index in the GeV energy range, , measured by this study using
Fermi--LAT data. The centroid position of the gamma-ray emission is coincident
with the center of the remnant, as well as with the centroid measurement of
Fermi--LAT above 1 GeV. The results are consistent with an SNR shell origin of
the emission, as many models assume. The updated spectrum points to a lower
maximum particle energy than has been suggested previously.Comment: Accepted for publication in The Astrophysical Journa
Very-high-energy observations of the binaries V 404 Cyg and 4U 0115+634 during giant X-ray outbursts
Transient X-ray binaries produce major outbursts in which the X-ray flux can
increase over the quiescent level by factors as large as . The low-mass
X-ray binary V 404 Cyg and the high-mass system 4U 0115+634 underwent such
major outbursts in June and October 2015, respectively. We present here
observations at energies above hundreds of GeV with the VERITAS observatory
taken during some of the brightest X-ray activity ever observed from these
systems. No gamma-ray emission has been detected by VERITAS in 2.5 hours of
observations of the microquasar V 404 Cyg from 2015, June 20-21. The upper flux
limits derived from these observations on the gamma-ray flux above 200 GeV of F
cm s correspond to a tiny fraction (about
) of the Eddington luminosity of the system, in stark contrast to that
seen in the X-ray band. No gamma rays have been detected during observations of
4U 0115+634 in the period of major X-ray activity in October 2015. The flux
upper limit derived from our observations is F cm
s for gamma rays above 300 GeV, setting an upper limit on the ratio of
gamma-ray to X-ray luminosity of less than 4%.Comment: Accepted for publication in the Astrophysical Journa
A Search for Very High-Energy Gamma Rays from the Missing Link Binary Pulsar J1023+0038 with VERITAS
The binary millisecond radio pulsar PSR J1023+0038 exhibits many
characteristics similar to the gamma-ray binary system PSR B1259--63/LS 2883,
making it an ideal candidate for the study of high-energy non-thermal emission.
It has been the subject of multi-wavelength campaigns following the
disappearance of the pulsed radio emission in 2013 June, which revealed the
appearance of an accretion disk around the neutron star. We present the results
of very high-energy gamma-ray observations carried out by VERITAS before and
after this change of state. Searches for steady and pulsed emission of both
data sets yield no significant gamma-ray signal above 100 GeV, and upper limits
are given for both a steady and pulsed gamma-ray flux. These upper limits are
used to constrain the magnetic field strength in the shock region of the PSR
J1023+0038 system. Assuming that very high-energy gamma rays are produced via
an inverse-Compton mechanism in the shock region, we constrain the shock
magnetic field to be greater than 2 G before the disappearance of the
radio pulsar and greater than 10 G afterwards.Comment: 7 pages, 3 figures, accepted for publication in Ap
Very-High-Energy -Ray Observations of the Blazar 1ES 2344+514 with VERITAS
We present very-high-energy -ray observations of the BL Lac object
1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array
System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a
statistical significance above background of in hours
(livetime) of observations, making this the most comprehensive very-high-energy
study of 1ES 2344+514 to date. Using these observations the temporal properties
of 1ES 2344+514 are studied on short and long times scales. We fit a constant
flux model to nightly- and seasonally-binned light curves and apply a
fractional variability test, to determine the stability of the source on
different timescales. We reject the constant-flux model for the 2007-2008 and
2014-2015 nightly-binned light curves and for the long-term seasonally-binned
light curve at the level. The spectra of the time-averaged emission
before and after correction for attenuation by the extragalactic background
light are obtained. The observed time-averaged spectrum above 200 GeV is
satisfactorily fitted () by a power-law function with
index and extends to at least 8
TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit
() by a power-law function with index while an F-test indicates that the power-law with
exponential cutoff function provides a marginally-better fit ( =
) at the 2.1 level. The source location is found to be
consistent with the published radio location and its spatial extent is
consistent with a point source.Comment: 7 pages, 2 figures. Published in Monthly Notices of the Royal
Astronomical Societ
Gamma-ray Observations Under Bright Moonlight with VERITAS
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive
photomultiplier tube (PMT) cameras. Exposure to high levels of background
illumination degrades the efficiency of and potentially destroys these
photo-detectors over time, so IACTs cannot be operated in the same
configuration in the presence of bright moonlight as under dark skies. Since
September 2012, observations have been carried out with the VERITAS IACTs under
bright moonlight (defined as about three times the night-sky-background (NSB)
of a dark extragalactic field, typically occurring when Moon illumination >
35%) in two observing modes, firstly by reducing the voltage applied to the
PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to
the cameras. This has allowed observations at up to about 30 times previous NSB
levels (around 80% Moon illumination), resulting in 30% more observing time
between the two modes over the course of a year. These additional observations
have already allowed for the detection of a flare from the 1ES 1727+502 and for
an observing program targeting a measurement of the cosmic-ray positron
fraction. We provide details of these new observing modes and their performance
relative to the standard VERITAS observations
Discovery of Very High Energy Gamma Rays from 1ES 1440+122
The BL Lacertae object 1ES 1440+122 was observed in the energy range from 85
GeV to 30 TeV by the VERITAS array of imaging atmospheric Cherenkov telescopes.
The observations, taken between 2008 May and 2010 June and totalling 53 hours,
resulted in the discovery of -ray emission from the blazar, which has a
redshift =0.163. 1ES 1440+122 is detected at a statistical significance of
5.5 standard deviations above the background with an integral flux of
(2.8) 10
cm s (1.2\% of the Crab Nebula's flux) above 200 GeV. The
measured spectrum is described well by a power law from 0.2 TeV to 1.3 TeV with
a photon index of 3.1 0.4 0.2.
Quasi-simultaneous multi-wavelength data from the Fermi Large Area Telescope
(0.3--300 GeV) and the Swift X-ray Telescope (0.2--10 keV) are additionally
used to model the properties of the emission region. A synchrotron self-Compton
model produces a good representation of the multi-wavelength data. Adding an
external-Compton or a hadronic component also adequately describes the data.Comment: 8 pages, 4 figures. Accepted for publication in MNRA
VERITAS and Multiwavelength Observations of the BL Lacertae Object 1ES 1741+196
We present results from multiwavelength observations of the BL Lacertae
object 1ES 1741+196, including results in the very-high-energy -ray
regime using the Very Energetic Radiation Imaging Telescope Array System
(VERITAS). The VERITAS time-averaged spectrum, measured above 180 GeV, is
well-modelled by a power law with a spectral index of
. The integral flux above 180
GeV is
m s, corresponding to 1.6% of the Crab Nebula flux on average.
The multiwavelength spectral energy distribution of the source suggests that
1ES 1741+196 is an extreme-high-frequency-peaked BL Lacertae object. The
observations analysed in this paper extend over a period of six years, during
which time no strong flares were observed in any band. This analysis is
therefore one of the few characterizations of a blazar in a non-flaring state.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Dark Matter Constraints from a Joint Analysis of Dwarf Spheroidal Galaxy Observations with VERITAS
We present constraints on the annihilation cross section of WIMP dark matter
based on the joint statistical analysis of four dwarf galaxies with VERITAS.
These results are derived from an optimized photon weighting statistical
technique that improves on standard imaging atmospheric Cherenkov telescope
(IACT) analyses by utilizing the spectral and spatial properties of individual
photon events. We report on the results of 230 hours of observations of
five dwarf galaxies and the joint statistical analysis of four of the dwarf
galaxies. We find no evidence of gamma-ray emission from any individual dwarf
nor in the joint analysis. The derived upper limit on the dark matter
annihilation cross section from the joint analysis is at 1 TeV for the bottom quark () final state,
at 1 TeV for the tau lepton
() final state and at 1 TeV for the gauge boson () final state.Comment: 14 pages, 9 figures, published in PRD, Ascii tables containing
annihilation cross sections limits are available for download as ancillary
files with readme.txt file description of limit
- …