17 research outputs found
Multi-wavelength observations of 2HWC J1928+177: dark accelerator or new TeV gamma-ray binary?
2HWC J1928+177 is a Galactic TeV gamma-ray source detected by the High
Altitude Water Cherenkov (HAWC) Observatory up to ~ 56 TeV. The HAWC source,
later confirmed by H.E.S.S., still remains unidentified as a dark accelerator
since there is no apparent supernova remnant or pulsar wind nebula detected in
the lower energy bands. The radio pulsar PSR J1928+1746, coinciding with the
HAWC source position, has no X-ray counterpart. Our SED modeling shows that
inverse Compton scattering in the putative pulsar wind nebula can account for
the TeV emission only if the unseen nebula is extended beyond r ~ 4 [arcmin].
Alternatively, TeV gamma rays may be produced by hadronic interactions between
relativistic protons from an undetected supernova remnant associated with the
radio pulsar and a nearby molecular cloud G52.9+0.1. NuSTAR and Chandra
observations detected a variable X-ray point source within the HAWC error
circle, potentially associated with a bright IR source. The X-ray spectra can
be fitted with an absorbed power-law model with cm and and exhibit
long-term X-ray flux variability over the last decade. If the X-ray source,
possibly associated with the IR source (likely an O star), is the counterpart
of the HAWC source, it may be a new TeV gamma-ray binary powered by collisions
between the pulsar wind and stellar wind. Follow-up X-ray observations are
warranted to search for diffuse X-ray emission and determine the nature of the
HAWC source.Comment: accepted to ApJ, 8 pages, 7 figure
Limits on Supersymmetric Dark Matter From EGRET Observations of the Galactic Center Region
In most supersymmetic models, neutralino dark matter particles are predicted
to accumulate in the Galactic center and annihilate generating, among other
products, gamma rays. The EGRET experiment has made observations in this
region, and is sensitive to gamma rays from 30 MeV to 30 GeV. We have
used an improved point source analysis including an energy dependent point
spread function and an unbinned maximum likelihood technique, which has allowed
us to significantly lower the limits on gamma ray flux from the Galactic
center. We find that the present EGRET data can limit many supersymmetric
models if the density of the Galactic dark matter halo is cuspy or spiked
toward the Galactic center. We also discuss the ability of GLAST to test these
models.Comment: 4 pages, 3 figure
Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe
The Extragalactic Background Light (EBL) includes photons with wavelengths
from ultraviolet to infrared, which are effective at attenuating gamma rays
with energy above ~10 GeV during propagation from sources at cosmological
distances. This results in a redshift- and energy-dependent attenuation of the
gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts
(GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray
blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using
photons above 10 GeV collected by Fermi over more than one year of observations
for these sources, we investigate the effect of gamma-ray flux attenuation by
the EBL. We place upper limits on the gamma-ray opacity of the Universe at
various energies and redshifts, and compare this with predictions from
well-known EBL models. We find that an EBL intensity in the optical-ultraviolet
wavelengths as great as predicted by the "baseline" model of Stecker et al.
(2006) can be ruled out with high confidence.Comment: 42 pages, 12 figures, accepted version (24 Aug.2010) for publication
in ApJ; Contact authors: A. Bouvier, A. Chen, S. Raino, S. Razzaque, A.
Reimer, L.C. Reye