5,046 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
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
Sample positioning in neutron diffraction experiments using a multi-material fiducial marker
An alternative sample positioning method is reported for use in conjunction with sample positioning and experiment planning software systems deployed on some neutron diffraction strain scanners. In this approach, the spherical fiducial markers and location trackers used with optical metrology hardware are replaced with a specifically designed multi-material fiducial marker that requires one diffraction measurement. In a blind setting, the marker position can be determined within an accuracy of ±164 µm with respect to the instrument gauge volume. The scheme is based on a pre-determined relationship that links the diffracted peak intensity to the absolute positioning of the fiducial marker with respect to the instrument gauge volume. Two methods for establishing the linking relationship are presented, respectively based on fitting multi-dimensional quadratic functions and a cross-correlation artificial neural network
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