216 research outputs found
A connection between star formation activity and cosmic rays in the starburst galaxy M 82
Although Galactic cosmic rays (protons and nuclei) are widely believed to be
dominantly accelerated by the winds and supernovae of massive stars, definitive
evidence of this origin remains elusive nearly a century after their discovery
[1]. The active regions of starburst galaxies have exceptionally high rates of
star formation, and their large size, more than 50 times the diameter of
similar Galactic regions, uniquely enables reliable calorimetric measurements
of their potentially high cosmic-ray density [2]. The cosmic rays produced in
the formation, life, and death of their massive stars are expected to
eventually produce diffuse gamma-ray emission via their interactions with
interstellar gas and radiation. M 82, the prototype small starburst galaxy, is
predicted to be the brightest starburst galaxy in gamma rays [3, 4]. Here we
report the detection of >700 GeV gamma rays from M 82. From these data we
determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or
about 500 times the average Galactic density. This result strongly supports
that cosmic-ray acceleration is tied to star formation activity, and that
supernovae and massive-star winds are the dominant accelerators.Comment: 18 pages, 4 figures; published in Nature; Version is prior to
Nature's in-house style editing (differences are minimal
Evidence for long-term Gamma-ray and X-ray variability from the unidentified TeV source HESS J0632+057
HESS J0632+057 is one of only two unidentified very-high-energy gamma-ray
sources which appear to be point-like within experimental resolution. It is
possibly associated with the massive Be star MWC 148 and has been suggested to
resemble known TeV binary systems like LS I +61 303 or LS 5039. HESS J0632+057
was observed by VERITAS for 31 hours in 2006, 2008 and 2009. During these
observations, no significant signal in gamma rays with energies above 1 TeV was
detected from the direction of HESS J0632+057. A flux upper limit corresponding
to 1.1% of the flux of the Crab Nebula has been derived from the VERITAS data.
The non-detection by VERITAS excludes with a probability of 99.993% that HESS
J0632+057 is a steady gamma-ray emitter. Contemporaneous X-ray observations
with Swift XRT reveal a factor of 1.8+-0.4 higher flux in the 1-10 keV range
than earlier X-ray observations of HESS J0632+057. The variability in the
gamma-ray and X-ray fluxes supports interpretation of the ob ject as a
gamma-ray emitting binary.Comment: 8 pages, 3 figures, Accepted for publication in The Astrophysical
Journa
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
Detection of Extended VHE Gamma Ray Emission from G106.3+2.7 with VERITAS
We report the detection of very-high-energy (VHE) gamma-ray emission from
supernova remnant (SNR) G106.3+2.7. Observations performed in 2008 with the
VERITAS atmospheric Cherenkov gamma-ray telescope resolve extended emission
overlapping the elongated radio SNR. The 7.3 sigma (pre-trials) detection has a
full angular extent of roughly 0.6deg by 0.4deg. Most notably, the centroid of
the VHE emission is centered near the peak of the coincident 12CO (J = 1-0)
emission, 0.4deg away from the pulsar PSR J2229+6114, situated at the northern
end of the SNR. Evidently the current-epoch particles from the pulsar wind
nebula are not participating in the gamma-ray production. The VHE energy
spectrum measured with VERITAS is well characterized by a power law dN/dE =
N_0(E/3 TeV)^{-G} with a differential index of G = 2.29 +/- 0.33stat +/-
0.30sys and a flux of N_0 = (1.15 +/- 0.27stat +/- 0.35sys)x 10^{-13} cm^{-2}
s^{-1} TeV^{-1}. The integral flux above 1 TeV corresponds to ~5 percent of the
steady Crab Nebula emission above the same energy. We describe the observations
and analysis of the object and briefly discuss the implications of the
detection in a multiwavelength context.Comment: 5 pages, 2 figure
VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275
The recent detection by the Fermi gamma-ray space telescope of high-energy
gamma-rays from the radio galaxy NGC 1275 makes the observation of the very
high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly
interesting, especially for the understanding of active galactic nuclei (AGN)
with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently
observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE
gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence
level upper limit of 2.1% of the Crab Nebula flux level is obtained at the
decorrelation energy of approximately 340 GeV, corresponding to 19% of the
power-law extrapolation of the Fermi Large Area Telescope (LAT) result.Comment: Accepted for publication in ApJ Letter
VERITAS Search for VHE Gamma-ray Emission from Dwarf Spheroidal Galaxies
Indirect dark matter searches with ground-based gamma-ray observatories
provide an alternative for identifying the particle nature of dark matter that
is complementary to that of direct search or accelerator production
experiments. We present the results of observations of the dwarf spheroidal
galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by VERITAS. These
galaxies are nearby dark matter dominated objects located at a typical distance
of several tens of kiloparsecs for which there are good measurements of the
dark matter density profile from stellar velocity measurements. Since the
conventional astrophysical background of very high energy gamma rays from these
objects appears to be negligible, they are good targets to search for the
secondary gamma-ray photons produced by interacting or decaying dark matter
particles. No significant gamma-ray flux above 200 GeV was detected from these
four dwarf galaxies for a typical exposure of ~20 hours. The 95% confidence
upper limits on the integral gamma-ray flux are in the range 0.4-2.2x10^-12
photons cm^-2s^-1. We interpret this limiting flux in the context of pair
annihilation of weakly interacting massive particles and derive constraints on
the thermally averaged product of the total self-annihilation cross section and
the relative velocity of the WIMPs. The limits are obtained under conservative
assumptions regarding the dark matter distribution in dwarf galaxies and are
approximately three orders of magnitude above the generic theoretical
prediction for WIMPs in the minimal supersymmetric standard model framework.
However significant uncertainty exists in the dark matter distribution as well
as the neutralino cross sections which under favorable assumptions could
further lower the limits.Comment: 21 pages, 2 figures, updated to reflect version published in ApJ.
NOTE: M.D. Wood added as autho
VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275
The recent detection by the Fermi gamma-ray space telescope of high-energy
gamma-rays from the radio galaxy NGC 1275 makes the observation of the very
high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly
interesting, especially for the understanding of active galactic nuclei (AGN)
with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently
observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE
gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence
level upper limit of 2.1% of the Crab Nebula flux level is obtained at the
decorrelation energy of approximately 340 GeV, corresponding to 19% of the
power-law extrapolation of the Fermi Large Area Telescope (LAT) result.Comment: Accepted for publication in ApJ Letter
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