239 research outputs found
Evidence for Gamma-ray Jets in the Milky Way
Although accretion onto supermassive black holes in other galaxies is seen to
produce powerful jets in X-ray and radio, no convincing detection has ever been
made of a kpc-scale jet in the Milky Way. The recently discovered pair of 10
kpc tall gamma-ray bubbles in our Galaxy may be a sign of earlier jet activity
from the central black hole. In this paper, we identify a gamma-ray cocoon
feature in the southern bubble, a jet-like feature along the cocoon's axis of
symmetry, and another directly opposite the Galactic center in the north. Both
the cocoon and jet-like feature have a hard spectrum with spectral index ~ -2
from 1 to 100 GeV, with a cocoon luminosity of (5.5 +/- 0.45) x 10^35 erg/s and
luminosity of the jet-like feature of (1.8 +/- 0.35) x 10^35 erg/s at 1 to 100
GeV. If confirmed, these jets are the first resolved gamma-ray jets ever seen.Comment: 14 pages, 11 figures, accepted by Ap
Is the 130 GeV Line Real? A Search for Systematics in the Fermi-LAT Data
Our recent claims of a Galactic center feature in Fermi-LAT data at
approximately 130 GeV have prompted an avalanche of papers proposing
explanations ranging from dark matter annihilation to exotic pulsar winds.
Because of the importance of such interpretations for physics and astrophysics,
a discovery will require not only additional data, but a thorough investigation
of possible LAT systematics. While we do not have access to the details of each
event reconstruction, we do have information about each event from the public
event lists and spacecraft parameter files. These data allow us to search for
suspicious trends that could indicate a spurious signal. We consider several
hypotheses that might make an instrumental artifact more apparent at the
Galactic center, and find them implausible. We also search for an instrumental
signature in the Earth limb photons, which provide a smooth reference spectrum
for null tests. We find no significant 130 GeV feature in the Earth limb
sample. However, we do find a marginally significant 130 GeV feature in Earth
limb photons with a limited range of detector incidence angles. This raises
concerns about the 130 GeV Galactic center feature, even though we can think of
no plausible model of instrumental behavior that connects the two. A modest
amount of additional limb data would tell us if the limb feature is a
statistical fluke. If the limb feature persists, it would raise doubts about
the Pass 7 processing of E > 100 GeV events. At present we find no instrumental
systematics that could plausibly explain the excess Galactic center emission at
130 GeV.Comment: 16 pages, 22 figure
Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50
degrees above and below the Galactic center, with a width of about 40 degrees
in longitude. The gamma-ray emission associated with these bubbles has a
significantly harder spectrum (dN/dE ~ E^-2) than the IC emission from
electrons in the Galactic disk, or the gamma-rays produced by decay of pions
from proton-ISM collisions. There is no significant spatial variation in the
spectrum or gamma-ray intensity within the bubbles, or between the north and
south bubbles. The bubbles are spatially correlated with the hard-spectrum
microwave excess known as the WMAP haze; the edges of the bubbles also line up
with features in the ROSAT X-ray maps at 1.5-2 keV. We argue that these
Galactic gamma-ray bubbles were most likely created by some large episode of
energy injection in the Galactic center, such as past accretion events onto the
central massive black hole, or a nuclear starburst in the last ~10 Myr. Dark
matter annihilation/decay seems unlikely to generate all the features of the
bubbles and the associated signals in WMAP and ROSAT; the bubbles must be
understood in order to use measurements of the diffuse gamma-ray emission in
the inner Galaxy as a probe of dark matter physics. Study of the origin and
evolution of the bubbles also has the potential to improve our understanding of
recent energetic events in the inner Galaxy and the high-latitude cosmic ray
population.Comment: 46 pages, 28 figures, accepted by Ap
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