8,226 research outputs found
Modeling the Pulse Profiles of Millisecond Pulsars in the Second LAT Catalog of gamma-ray Pulsars
Significant gamma-ray pulsations have been detected from ~40 millisecond
pulsars (MSPs) using 3 years of sky-survey data from the Fermi LAT and radio
timing solutions from across the globe. We have fit the radio and gamma-ray
pulse profiles of these MSPs using geometric versions of slot gap and outer gap
gamma-ray emission models and radio cone and core models. For MSPs with radio
and gamma-ray peaks aligned in phase we also explore low-altitude slot gap
gamma-ray models and caustic radio models. The best-fit parameters provide
constraints on the viewing geometries and emission sites. While the exact
pulsar magnetospheric geometry is unknown, we can use the increased number of
known gamma-ray MSPs to look for significant trends in the population which
average over these uncertainties.Comment: 4 pages, 2 figures, to appear in the proceedings of the 5th
International Symposium on High-Energy Astronom
Discovery of a Spin-Down State Change in the LMC Pulsar B0540-69
We report the discovery of a large, sudden, and persistent increase in the
spin-down rate of B0540-69, a young pulsar in the Large Magellanic Cloud, using
observations from the Swift and RXTE satellites. The relative increase in the
spin-down rate of 36% is unprecedented for B0540-69. No accompanying change in
the spin rate is seen, and no change is seen in the pulsed X-ray emission from
B0540-69 following the change in the spin-down rate. Such large relative
changes in the spin-down rate are seen in the recently discovered class of
'intermittent pulsars', and we compare the properties of B0540-69 to such
pulsars. We consider possible changes in the magnetosphere of the pulsar that
could cause such a large change in the spin-down rate.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations
We present 1-second cadence observations of M32 (NGC221) with the CHIMERA
instrument at the Hale 200-inch telescope of the Palomar Observatory. Using
field stars as a baseline for relative photometry, we are able to construct a
light curve of the nucleus in the g-prime and r-prime band with 1sigma=36
milli-mag photometric stability. We derive a temporal power spectrum for the
nucleus and find no evidence for a time-variable signal above the noise as
would be expected if the nuclear black hole were accreting gas. Thus, we are
unable to constrain the spin of the black hole although future work will use
this powerful instrument to target more actively accreting black holes. Given
the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics,
the absence of a contribution from a nuclear time-variable signal places an
upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a
factor of two more stringent than past upper limits from HST. The low mass of
the black hole despite the high stellar density suggests that the gas liberated
by stellar interactions was primarily at early cosmic times when the low-mass
black hole had a small Eddington luminosity. This is at least partly driven by
a top-heavy stellar initial mass function at early cosmic times which is an
efficient producer of stellar mass black holes. The implication is that
supermassive black holes likely arise from seeds formed through the coalescence
of 3-100 Msun mass black holes that then accrete gas produced through stellar
interaction processes.Comment: 8 pages, 3 figures, submitted to the Astrophysical Journal, comments
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The Multi-Component Nature of the Vela Pulsar Nonthermal X-ray Spectrum
We report on our analysis of a 274 ks observation of the Vela pulsar with the
Rossi X-Ray Timing Explorer (RXTE). The double-peaked, pulsed emission at 2 -
30 keV, which we had previously detected during a 93 ks observation, is
confirmed with much improved statistics. There is now clear evidence, both in
the spectrum and the light curve, that the emission in the RXTE band is a blend
of two separate non-thermal components. The spectrum of the harder component
connects smoothly with the OSSE, COMPTEL and EGRET spectrum and the peaks in
the light curve are in phase coincidence with those of the high-energy light
curve. The spectrum of the softer component is consistent with an extrapolation
to the pulsed optical flux, and the second RXTE pulse is in phase coincidence
with the second optical peak. In addition, we see a peak in the 2-8 keV RXTE
pulse profile at the radio phase.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical
Journa
Bounds on Cross-sections and Lifetimes for Dark Matter Annihilation and Decay into Charged Leptons from Gamma-ray Observations of Dwarf Galaxies
We provide conservative bounds on the dark matter cross-section and lifetime
from final state radiation produced by annihilation or decay into charged
leptons, either directly or via an intermediate particle . Our analysis
utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf
satellites: HESS observations of Sagittarius and VERITAS observations of Draco,
Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the
integrals over the dark matter distributions, we find that these constraints
are largely unable to rule out dark matter annihilations or decays as an
explanation of the PAMELA and ATIC/PPB-BETS excesses. However, if there is an
additional Sommerfeld enhancement in dwarfs, which have a velocity dispersion
~10 to 20 times lower than that of the local Galactic halo, then the
cross-sections for dark matter annihilating through 's required to
explain the excesses are very close to the cross-section upper bounds from
Willman 1. Dark matter annihilation directly into 's is also marginally
ruled out by Willman 1 as an explanation of the excesses, and the required
cross-section is only a factor of a few below the upper bound from Draco.
Finally, we make predictions for the gamma-ray flux expected from the dwarf
galaxy Segue 1 for the Fermi Gamma-ray Space Telescope. We find that for a
sizeable fraction of the parameter space in which dark matter annihilation into
charged leptons explains the PAMELA excess, Fermi has good prospects for
detecting a gamma-ray signal from Segue 1 after one year of observation.Comment: 11 pages, 4 figures. References added. Final published versio
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