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Search Results: 1 - 10 of 585886 matches for " P. A. Romani "
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UV-IR Science Prospects with TES Imaging Arrays
R. W. Romani,J. Burney,P. Brink,B. Cabrera,P. Castle,T. Kenny,E. Wang,B. Young,A. J. Miller,S. W. Nam
Physics , 2002,
Abstract: We are developing photon-counting cameras employing cryogenic arrays of energy-resolving TES (Transition Edge Sensor) pixels. These are being tested in ground-based instruments, but will have their greatest impact when employed on space platforms, where they can cover the 10micrometer-100nm range with high time- and moderate energy- resolution. Here we summarize briefly existing device performance, current directions in array camera development and anticipated capabilities.
Constraining Pulsar Magnetosphere Geometry with Gamma-Ray Light Curves
Roger W. Romani,Kyle P. Watters
Physics , 2009, DOI: 10.1088/0004-637X/714/1/810
Abstract: We demonstrate a method for quantitatively comparing gamma-ray pulsar light curves with magnetosphere beaming models. With the Fermi LAT providing many pulsar discoveries and high quality pulsar light curves for the brighter objects, such comparison allows greatly improved constraints on the emission zone geometry and the magnetospheric physics. Here we apply the method to Fermi LAT light curves of a set of bright pulsars known since EGRET or before. We test three approximate models for the magnetosphere structure and two popular schemes for the location of the emission zone, the Two Pole Caustic (TPC) model and the Outer Gap (OG) model. We find that OG models and relatively physical B fields approximating force-free dipole magnetospheres are preferred at high statistical significance. An application to the full LAT pulsar sample will allow us to follow the emission zone's evolution with pulsar spindown.
The Galactic Population of Young Gamma-ray Pulsars
Kyle P. Watters,Roger W. Romani
Physics , 2010, DOI: 10.1088/0004-637X/727/2/123
Abstract: We have simulated a Galactic population of young pulsars and compared with the Fermi LAT sample, constraining the birth properties, beaming and evolution of these spin-powered objects. Using quantitative tests of agreement with the distributions of observed spin and pulse properties, we find that short birth periods P_0 ~ 50ms and gamma-ray beams arising in the outer magnetosphere, dominated by a single pole, are strongly preferred. The modeled relative numbers of radio-detected and radio-quiet objects agree well with the data. Although the sample is local, extrapolation to the full Galaxy implies a gamma-ray pulsar birthrate 1/(59 yr). This is shown to be in good agreement with the estimated Galactic core collapse rate and with the local density of OB star progenitors. We give predictions for the numbers of expected young pulsar detections if Fermi LAT observations continue 10 years. In contrast to the potentially significant contribution of unresolved millisecond pulsars, we find that young pulsars should contribute little to the Galactic gamma-ray background.
Gamma-Ray Pulsars and Massive Stars in the Solar Neighborhood
I. -A. Yadigaroglu,Roger W. Romani
Physics , 1996, DOI: 10.1086/303601
Abstract: We revisit the association of unidentified Galactic plane EGRET sources with tracers of recent massive star formation and death. Up-to-date catalogs of OB associations, SNRs, young pulsars, HII regions and young open clusters were used in finding counterparts for a recent list of EGRET sources. It has been argued for some time that EGRET source positions are correlated with SNRs and OB associations as a class; we extend such analyses by finding additional counterparts and assessing the probability of individual source identifications. Among the several scenarios relating EGRET sources to massive stars, we focus on young neutron stars as the origin of the gamma-ray emission. The characteristics of the candidate identifications are compared to the known gamma-ray pulsar sample and to detailed Galactic population syntheses using our outer gap pulsar model of gamma-ray emission. Both the spatial distribution and luminosity function of the candidates are in good agreement with the model predictions; we infer that young pulsars can account for the bulk of the excess low latitude EGRET sources. We show that with this identification, the gamma-ray point sources provide an important new window into the history of recent massive star death in the solar neighborhood.
Gamma-Ray Pulsars: Beaming Evolution, Stats and Unident. EGRET Sources
I. -A. Yadigaroglu,Roger W. Romani
Physics , 1994, DOI: 10.1086/176047
Abstract: We compute the variation of the beaming fraction with the efficiency of high energy gamma-ray production in the outer gap pulsar model of Romani and Yadigaroglu. This allows us to correct the fluxes observed for pulsars in the EGRET band and to derive a simple estimate of the variation of efficiency with age. Integration of this model over the population of young neutron stars gives the expected number of gamma-ray pulsars along with their distributions in age and distance. This model also shows that many of the unidentified EGRET plane sources should be pulsars, and predicts the gamma-ray fluxes of known radio pulsars. The contribution of unresolved pulsars to the background flux in the EGRET band is found to be about 5 %. For an animation of our pulsar model see http://geminga.stanford.edu/users/ion/home.html .
$γ$-Ray Pulsars: Emission Zones and Viewing Geometries, A Computer Animation
I. -A. Yadigaroglu,Roger W. Romani
Physics , 1994,
Abstract: The computer animation illustrates the geometries described in a paper by the same authors. The preprint is available as number 9401045. The opening scene shows dipole field lines emanating from the polar caps of a rotating neutron star. The dipole axis is inclined along the green rods. The field lines shown are defined from the condition that they be tangent to the light cylinder (the cylindrical radius at which the tangential velocity of rotation reaches the speed of light). The static dipole field lines are smoothly morphed into the correct retarted-potential vacuum solutions. A red surface spanning these field lines is painted. In the next scene the blue surfaces represent the outer gaps above the surface of last closed field lines. High energy emission (blue) is produced in these outer gaps, and is beamed tangentially along the field lines. The radio emission (green) originates close to the surface of the star and is beamed along the dipole axes. The inclination angle $\alpha$ of the dipole and the viewing angle $\zeta$ are chosen to match the Crab parameters; $\alpha$ = 70, $\zeta$ = 65. The corresponding light curve is computed and shown for these angles, and the red dot traces rotation phase. The next scene shows the situation for angles appropriate to PSR1706-44; $\alpha$ = 45, $\zeta$ = 65. The final scene is a possibility for Geminga; $\alpha$ = 20, $\zeta$ = 75. These angles are poorly constrained as there is no radio emission.
$γ$-Ray Pulsars: Emission Zones and Viewing Geometries
Roger W. Romani,I. -A. Yadigaroglu
Physics , 1994, DOI: 10.1086/175076
Abstract: There are now a half dozen young pulsars detected in high energy photons by the Compton GRO, showing a variety of emission efficiencies and pulse profiles. We present here a calculation of the pattern of high energy emission on the sky in a model which posits $\gamma$-ray production by charge depleted gaps in the outer magnetosphere. This model accounts for the radio to $\gamma$-ray pulse offsets of the known pulsars, as well as the shape of the high energy pulse profiles. We also show that $\sim 1/3$ of emitting young radio pulsars will not be detected due to beaming effects, while $\sim 2.5 \times$ the number of radio-selected $\gamma$-ray pulsars will be viewed only high energies. Finally we compute the polarization angle variation and find that the previously misunderstood optical polarization sweep of the Crab pulsar arises naturally in this picture. These results strongly support an outer-magnetosphere location for the $\gamma-$ray emission.
Altitude Limits for Rotating Vector Model Fitting of Pulsar Polarization
H. A. Craig,Roger W. Romani
Physics , 2012, DOI: 10.1088/0004-637X/755/2/137
Abstract: Traditional pulsar polarization sweep analysis starts from the point dipole rotating vector model (RVM) approximation. If augmented by a measurement of the sweep phase shift, one obtains an estimate of the emission altitude (Blaskiewicz, Cordes, & Wasserman). However, a more realistic treatment of field line sweepback and finite altitude effects shows that this estimate breaks down at modest altitude ~ 0.1R_{LC}. Such radio emission altitudes turn out to be relevant to the young energetic and millisecond pulsars that dominate the \gamma-ray population. We quantify the breakdown height as a function of viewing geometry and provide simple fitting formulae that allow observers to correct RVM-based height estimates, preserving reasonable accuracy to R ~ 0.3R_{LC}. We discuss briefly other observables that can check and improve height estimates.
Discovery of millisecond pulsars in radio searches of southern Fermi LAT sources
M. J. Keith,S. Johnston,P. S. Ray,E. C. Ferrara,P. M. Saz Parkinson,O. Celik,A. Belfiore,D. Donato,C. C. Cheung,A. A. Abdo,F. Camilo,P. C. C. Freire,L. Guillemot,A. K. Harding,M. Kramer,P. F. Michelson,S. M. Ransom,R. W. Romani,D. A. Smith,D. J. Thompson,P. Weltevrede,K. S. Wood
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.18464.x
Abstract: Using the Parkes radio telescope we have carried out deep observations of eleven unassociated gamma-ray sources. Periodicity searches of these data have discovered two millisecond pulsars, PSR J1103-5403 (1FGL J1103.9-5355) and PSR J2241-5236 (1FGL J2241.9-5236), and a long period pulsar, PSR J1604-44 (1FGL J1604.7-4443). In addition we searched for but did not detect any radio pulsations from six gammaray pulsars discovered by the Fermi satellite to a level of - 0.04 mJy (for pulsars with a 10% duty cycle). Timing of the millisecond pulsar PSR J1103-5403 has shown that its position is 9' from the centroid of the gamma-ray source. Since these observations were carried out, independent evidence has shown that 1FGL J1103.9-5355 is associated with the flat spectrum radio source PKS 1101-536. It appears certain that the pulsar is not associated with the gamma-ray source, despite the seemingly low probability of a chance detection of a radio millisecond pulsar. We consider that PSR J1604-44 is a chance discovery of a weak, long period pulsar and is unlikely to be associated with 1FGL J1604.7-4443. PSR J2241-5236 has a spin period of 2.2 ms and orbits a very low mass companion with a 3.5 hour orbital period. The relatively high flux density and low dispersion measure of PSR J2241-5236 makes it an excellent candidate for high precision timing experiments. The gamma-rays of 1FGL J2241.9-5236 have a spectrum that is well modelled by a power law with exponential cutoff, and phasebinning with the radio ephemeris results in a multi-peaked gamma-ray pulse profile. Observations with Chandra have identified a coincident X-ray source within 0.1" of the position of the pulsar obtained by radio timing
Light Curves of Rapidly Rotating Neutron Stars
Timothy M. Braje,Roger W. Romani,Kevin P. Rauch
Physics , 2000, DOI: 10.1086/308448
Abstract: We consider the effect of rapid rotation on the light curves of neutron stars with hot polar caps. For $P \approx 3$ms spin periods, the pulse fractions can be as much as an order of magnitude larger than with simple slowly-rotating (Schwarzschild) estimates. Doppler boosting, in particular, leads to characteristic distortion and ``soft lags'' in the pulse profiles, which are easily measurable in light curves with moderate energy resolution. With $\sim 10^5$ photons it should also be possible to isolate the more subtle distortions of light travel time variations and frame dragging. Detailed analysis of high quality millisecond pulsar data from upcoming X-ray missions must include these effects.
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