arXiv subjects

Roger W. Romani

Publications and source records attributed to Roger W. Romani.

180 records · Page 10Linked to original sources

arXiv2002

A search for giant pulses in Vela-like pulsars

We have carried out a survey for `giant pulses' in 6 young, Vela-like pulsars. In no cases did we find single pulses with flux densities more than 10 times the mean flux density. However, in PSR B1706-44 we have detected giant micro-pulses very similar to those seen in the Vela pulsar. In PSR B1706-44 these giant micro-pulses appear on the trailing edge of the profile and have an intrinsic width of ~1 ms. The cumulative probability distribution of their intensities is best described by a power-law. If the power-law continues to higher intensities, then 3.7x10^6 rotations are required to obtain a pulse with 20 times the mean pulse flux. This number is similar to the giant pulse rate in PSR B1937+21 and PSR B1821-24 but significantly higher than that for the Crab.

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arXiv2002

Chandra Imaging of the Gamma-Ray Source GeV J1809-2327

We report on Chandra imaging observations of the Galactic Unidentified $γ$-ray source GEV J1809-2327, comparing the X-ray images with new VLA 1.46 GHz and 4.86 GHz maps. The X-ray images reveal a point source connected to a non-thermal X-ray/radio nebula, supporting a pulsar/wind model for the $γ$-ray emitter. We also detect numerous X-ray sources from the young stellar association in the adjacent HII region S32.

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arXiv2001

Pulsar Wind Nebulae as High Energy Gamma-Ray Emitters

There is growing evidence that some variable high-energy gamma-ray sources are associated with pulsar wind nebulae. We review the current status of X-ray and radio studies of the sources most likely to be significant emitters above 100 MeV. The assumption of the gamma-radiation arising from synchrotron processes puts a lower limit on the magnetic field and suggests there may be significant doppler boosting causing the asymmetries seen in the morphology of these nebulae. However, these sources are also in close proximity to molecular clouds, suggesting that variable gamma-ray emission may arise from a ram pressure confined pulsar wind nebula in a dense medium.

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arXiv2001

Multiwavelength Studies of PSR J1420-6048, a Young Pulsar in the Kookaburra

We present X-ray, radio, and infrared observations of the 68 ms pulsar PSR J1420-6048 and its surrounding nebula, a possible counterpart of the gamma-ray source GeV J1417-6100/3EG J1420-6038. Pulsed X-ray emission at the radio period is marginally detected by ASCA from a source embedded in the hard spectrum X-ray nebula AX J1420.1-6049. At radio wavelengths, the pulsar is found to be strongly linearly and circularly polarized, and the polarization sweep is measured. A comparison of high resolution ATCA radio imaging of the Kookaburra's upper wing (G313.6+0.3), which contains the pulsar and the X-ray nebula, with infrared images suggests the radio emission is partly non-thermal.

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arXiv2001

Phase-resolved Crab Studies with a Cryogenic TES Spectrophotometer

We are developing time- and energy-resolved near-IR/optical/UV photon detectors based on sharp superconducting-normal transition edges in thin films. We report observations of the Crab pulsar made during prototype testing at the McDonald 2.7m telescope with a fiber-coupled transition-edge sensor (TES) system. These data show substantial (d[alpha]~0.3), rapid variations in the spectral index through the pulse profile, with a strong phase-varying IR break across our energy band. These variations correlate with X-ray spectral variations, but no single synchrotron population can account for the full Spectral Energy Distribution (SED). We also describe test spectrophotopolarimetry observations probing the energy dependence of the polarization sweep; this may provide a new key to understanding the radiating particle population.

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arXiv2001

Giant Pulses from the Millisecond Pulsar B1821-24

We have carried out a survey for `giant pulses' in 5 millisecond pulsars. We detect individual pulses from the high Edot pulsar PSR B1821-24 with energies exceeding 50x the mean pulse energy. These giant pulses are concentrated in a narrow phase window coincident with the power-law non-thermal pulse seen in hard X-rays. This is the third example of the giant pulse phenomenon. It supports the idea that large B fields in the outer magnetosphere are critical to the formation of such pulses and further suggests a direct connection between giant pulses and high energy emission.

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arXiv2001

X-Ray and Radio Observations of Bright GeV Sources

We present X-ray and radio studies of sources which are brightabove 1 GeV (F_{>1GeV} > 4e-8 ph/cm^2/s. Only 11 out of ~30 of these gamma-ray sources have been identified with lower energy counterparts: 5 blazars and 6 pulsars. Three of these pulsars are surrounded by radio pulsar wind nebulae (PWN), two of which are also seen as bright, extended X-ray synchrotron nebulae. The ASCA X-ray telescope has observed 28 of the bright GeV sources, revealing an excess of F_{2-10keV} > 10e-12 ergs/cm^2/s sources within the {\it EGRET} error contours of the unidentified sources. Although several supernova remnants are positionally coincident with these sources, we find no X-ray evidence of high energy particle production in SNR shell shocks consistent with the GeV positions. We also present initial results from follow on radio imaging studies of several fields containing unidentified sources. We have discovered new X-ray/radio nebulae in three of these fields which are strong candidates for PWN. These sources, along with a similar nebula in CTA 1 and the PWN around PSR B1853+01 in W44, are all positionally coincident with variable EGRET sources. This suggests a class of variable gamma-ray sources associated with synchrotron emitting regions powered by the winds of young pulsars.

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arXiv2000

The ASCA Catalog of Potential X-Ray Counterparts of GeV Sources

We present a catalog of 2-10 keV ASCA Gas Imaging Spectrometer images of fields containing bright sources of GeV emission. The images cover $\sim 85%$ of the 95% confidence position contour for 28 of the 30 sources with $> 1$ GeV fluxes above $4\times 10^{-8}$ ph ${\rm cm}^{-2} {\rm s}^{-1}$. We find an excess of hard X-ray sources with $F_{2-10 keV} \ga 10^{-12}$ ergs ${\rm cm}^{-2} {\rm s}^{-1}$ positionally coincident with unidentified sources of GeV emission. We comment on radio-loud and radio-quiet pulsar candidates, as well as several SNR and massive binaries as possible sources of the GeV emission. We also find evidence for a class of variable $γ-$ray sources associated with extended regions of hard X-ray emission, and propose that for these sources, a significant percentage of the $γ-$ray emission is generated in synchrotron nebulae surrounding fast, young pulsars.

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arXiv2000

Light Curves of Rapidly Rotating Neutron Stars

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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arXiv2000

Magnetospheric Scattering and Emission in Millisecond Pulsars

We model the formation of magnetospheric components of millisecond pulsar light curves, deriving an approximate model for the curved space, `swept-back' dipole field and following photon emission, propagation, and scattering. Magnetospheric pulse components are strongly affected by rapid rotation and Schwarzschild effects.

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arXiv1998

The `Rabbit': A Potential Radio Counterpart of GeV J1417-6100

We have mapped the radio emission in the error ellipse of GeV J1417-6100 (2EGS J1418-6049) at 13cm and 20cm using the Australia Telescope Compact Array. We find a large shell with extended wings, at the edge of which is a non-thermal, polarized structure with a center filled morphology (the `Rabbit'), coincident with an extended, hard X-ray source. We discuss the various sources seen within the ellipse as potential counterparts of the gamma-ray source. We conclude that the most likely scenario is that the Rabbit is a wind nebula surrounding a radio-quiet gamma-ray pulsar.

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arXiv1997

X-ray Emission from the Guitar Nebula

We have detected weak soft X-ray emission from the Pulsar Wind Nebula trailing the high velocity star PSR 2224+65 (the `Guitar Nebula'). This X-ray flux gives evidence of γ~10^7 eV particles in the pulsar wind and constrains the properties of the post-shock flow. The X-ray emission is most easily understood if the shocked pulsar wind is partly confined in the nebula and if magnetic fields in this zone can grow to near equipartition values.

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arXiv1996

Gamma-Ray Pulsars and Massive Stars in the Solar Neighborhood

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.

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arXiv1996

Magnetized Iron Atmospheres for Neutron Stars

Using a Hartree-Fock formalism, we estimate energy levels and photon cross sections for atomic iron in magnetic fields B ~ 10^13 G. Computing ionization equilibrium and normal mode opacities with these data, we construct LTE neutron star model atmospheres at 5.5 < Log(T_eff) < 6.5 and compute emergent spectra. We examine the dependence of the emergent spectra on T_eff and B. We also show the spectral variation with the angle between the magnetic field and the atmosphere normal and describe the significant limb darkening in the X-ray band. These results are compared with recent detailed computations of neutron star H model atmospheres in high fields and with low field Fe and H model atmospheres constructed from detailed opacities. The large spectral differences for different surface compositions may be discernible with present X-ray data; we also note improvements needed to allow comparison of Fe models with high quality spectra.

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arXiv1995

Ultra-Low Frequency Gravitational Radiation from Massive Black Hole Binaries

For massive black hole binaries produced in galactic mergers, we examine the possibility of inspiral induced by interaction with field stars. We model the evolution of such binaries for a range of galaxy core and binary parameters, using numerical results from the literature to compute the binary's energy and angular momentum loss rates due to stellar encounters and including the effect of back-action on the field stars. We find that only a small fraction of binary systems can merge within a Hubble time via unassisted stellar dynamics. External perturbations may, however, cause efficient inspiral. Averaging over a population of central black holes and galaxy mergers, we compute the expected background of gravitational radiation with periods Pw ~ 1-10 yr. Comparison with sensitivities from millisecond pulsar timing suggests that the strongest sources may be detectable with modest improvements to present experiments.

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arXiv1994

Gamma-Ray Pulsars: Beaming Evolution, Stats and Unident. EGRET Sources

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 .

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arXiv1994

$γ$-Ray Pulsars: Emission Zones and Viewing Geometries, A Computer Animation

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 $α$ of the dipole and the viewing angle $ζ$ are chosen to match the Crab parameters; $α$ = 70, $ζ$ = 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; $α$ = 45, $ζ$ = 65. The final scene is a possibility for Geminga; $α$ = 20, $ζ$ = 75. These angles are poorly constrained as there is no radio emission.

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arXiv1994

$γ$-Ray Pulsars: Emission Zones and Viewing Geometries

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 $γ$-ray production by charge depleted gaps in the outer magnetosphere. This model accounts for the radio to $γ$-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 $γ$-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 $γ-$ray emission.

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