4,233 research outputs found
Predictions of Gamma-ray Emission from Globular Cluster Millisecond Pulsars Above 100 MeV
The recent Fermi detection of the globular cluster (GC) 47 Tucanae
highlighted the importance of modeling collective gamma-ray emission of
millisecond pulsars (MSPs) in GCs. Steady flux from such populations is also
expected in the very high energy (VHE) domain covered by ground-based Cherenkov
telescopes. We present pulsed curvature radiation (CR) as well as unpulsed
inverse Compton (IC) calculations for an ensemble of MSPs in the GCs 47 Tucanae
and Terzan 5. We demonstrate that the CR from these GCs should be easily
detectable for Fermi, while constraints on the total number of MSPs and the
nebular B-field may be derived using the IC flux components.Comment: 12 pages, 2 figures, accepted for publication in ApJ
Constraining A General-Relativistic Frame-Dragging Model for Pulsed Radiation from a Population of Millisecond Pulsars in 47 Tucanae using GLAST/LAT
Although only 22 millisecond pulsars (MSPs) are currently known to exist in
the globular cluster (GC) 47 Tucanae, this cluster may harbor 30-60 MSPs, or
even up to ~200. In this Letter, we model the pulsed curvature radiation (CR)
gamma-ray flux expected from a population of MSPs in 47 Tucanae. These MSPs
produce gamma-rays in their magnetospheres via accelerated electron primaries
which are moving along curved magnetic field lines. A GC like 47 Tucanae
containing a large number of MSPs provides the opportunity to study a
randomized set of pulsar geometries. Geometry-averaged spectra make the testing
of the underlying pulsar model more reliable, since in this case the relative
flux uncertainty is reduced by one order of magnitude relative to the variation
expected for individual pulsars (if the number of visible pulsars N=100). Our
predicted spectra violate the EGRET upper limit at 1 GeV, constraining the
product of the number of visible pulsars N and the average integral flux above
1 GeV per pulsar. GLAST/LAT should place even more stringent constraints on
this product, and may also limit the maximum average accelerating potential by
probing the CR spectral tail. For N=22-200, a GLAST/LAT non-detection will lead
to the constraints that the average integral flux per pulsar should be lower by
factors 0.03-0.003 than current model predictions.Comment: 10 pages, 2 figures, to appear in the Astrophysical Journal Letter
Probing millisecond pulsar emission geometry using light curves from the Fermi Large Area Telescope
An interesting new high-energy pulsar sub-population is emerging following
early discoveries of gamma-ray millisecond pulsars (MSPs) by the Fermi Large
Area Telescope (LAT). We present results from 3D emission modeling, including
the Special Relativistic effects of aberration and time-of-flight delays and
also rotational sweepback of B-field lines, in the geometric context of polar
cap (PC), outer gap (OG), and two-pole caustic (TPC) pulsar models. In contrast
to the general belief that these very old, rapidly-rotating neutron stars (NSs)
should have largely pair-starved magnetospheres due to the absence of
significant pair production, we find that most of the light curves are best fit
by TPC and OG models, which indicates the presence of narrow accelerating gaps
limited by robust pair production -- even in these pulsars with very low
spin-down luminosities. The gamma-ray pulse shapes and relative phase lags with
respect to the radio pulses point to high-altitude emission being dominant for
all geometries. We also find exclusive differentiation of the current gamma-ray
MSP population into two MSP sub-classes: light curve shapes and lags across
wavebands impose either pair-starved PC (PSPC) or TPC / OG-type geometries. In
the first case, the radio pulse has a small lag with respect to the single
gamma-ray pulse, while the (first) gamma-ray peak usually trails the radio by a
large phase offset in the latter case. Finally, we find that the flux
correction factor as a function of magnetic inclination and observer angles is
typically of order unity for all models. Our calculation of light curves and
flux correction factor for the case of MSPs is therefore complementary to the
"ATLAS paper" of Watters et al. for younger pulsars.Comment: 51 pages, 23 figures, 3 tables; low-resolution figures; accepted for
publication by Ap
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