arXiv subjects

D. A. Frail

Publications and source records attributed to D. A. Frail.

174 records · Page 10Linked to original sources

arXiv1997

OH Satellite Line Masers in the Nucleus of NGC253

We report the detection of 1720 and 1612 MHz OH ground-state satellite line maser regions in the starburst galaxy NGC253. We find ten compact maser features all of which are located along the narrow ridge of bright radio continuum emission within the central 10 square arcseconds of the nucleus. The compact OH features appear physically associated with regions of dense molecular gas, supernova remnants, and HII regions. The data for several of the features are consistent with collisional inversion from the interaction of supernova remnant shocks with molecular gas, similar to that found for Galactic supernova remnants (Frail et al. 1996). For many other features near the nucleus of NGC253, we find conjugate behavior in the 1720 and 1612 MHz lines, with one line showing stimulated emission and the other showing stimulated absorption. These results suggest the importance of infrared pumping associated with the active star formation in NGC253 and is consistent with large column densities of molecular gas (N[H_2] >~ 10^{22}cm^{-2}).

astro-ph↗

arXiv1997

OH Satellite Line Masers and an AGN Candidate in M82

We report the detection of OH satellite line masers at 1720 MHz and 1612 MHz from the nuclear region of the starburst galaxy M82. The observations were aimed at detecting 1720 MHz maser emission from the known radio emitting SNR in the nuclear region. At 1720 MHz we detect six features above the 5-sigma limit set by noise, four in emission and two in absorption. Three of the emission features appear closely associated with known discrete continuum radio sources, and one of the absorption features is precisely coincident with the discrete continuum source 44.01+59.6. The latter source also exhibits strong 1612 MHz emission at the same velocity. No other 1612 MHz features were detected. All of the 1720 MHz emission features are consistent with masers pumped by collisions with molecular hydrogen at densities between 10^3 cm^{-3} and 10^5 cm^{-3}, and T_k between 50 K and 250 K. The absorption and emission associated with the two satellite lines in 44.01+59.6, together with other evidence, points to the possibility that this source is the AGN in M82.

astro-ph↗

arXiv1997

The Radio Spectral Index of the Crab Nebula

We present the results of a new, comprehensive investigation of the radio spectral index of the Crab Nebula supernova remnant. New data at 74 MHz are combined with data at 327 MHz, 1.5 GHz and 5 GHz. In contrast to previous claims, little spatial variation in the spectral index is seen. In particular, between 327 MHz and 5 GHz we see no evidence of spectral steepening near the edge of the nebula, the ``jet'' or the ionized filaments. The rms limits on any spectral index variations in these regions amount to no more than 0.01. We believe that earlier reports of large steepening were the result of correlator bias and image registration problems. An elongated feature was detected 1\arcmin northwest of the pulsar which may be a continuation of the well-known wisp-like structures seen closer to the center of the nebula. At 74 MHz, we see for the first time evidence of free-free absorption by the thermal material in the Crab Nebula's filaments. Apart from some possible renewed acceleration occurring in the wisps, the dominant accelerator of relativistic electrons in the Crab Nebula is the pulsar itself.

astro-ph↗

arXiv1997

Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC443

(abridged) - We present arcsecond resolution observations from the VLA of the satellite line of the hydroxyl molecule (OH) at 1720.53 MHz toward three Galactic supernova remnants: W28, W44 and IC443. All of our observations are consistent with a model in which the OH(1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a non-dissociative shock. Supernova remnants with OH(1720 MHz) maser emission may be promising candidates to conduct high energy searches for the sites of cosmic ray acceleration.

astro-ph↗

arXiv1997

Radio Monitoring of the January 11, 1997 Gamma-Ray Burst

We report on a comprehensive radio monitoring program of the bright gamma-ray burster GRB970111. These VLA observations were made at a frequency of 1.4 GHz and span a range of post-burst timescales between 28 hours and one month. Despite extensive sampling at sub-milliJansky sensitivities, no radio source was detected above 0.5 mJy in the current best error box (~14 arcmin^2) for GRB970111. A highly unusual radio source, VLA J1528.7+1945, was seen to drop in flux density by a factor of two in our monitoring period but it lies outside the error box and thus it is unlikely to be related to GRB970111. Cosmological fireball models of gamma-ray bursts make predictions of late-time emission occurring at longer wavelengths. The absence of a flaring or fading radio counterpart to GRB970111 provides strong constraints on these models.

astro-ph↗

arXiv1996

The Pulsar Wind Nebula Around PSR B1853+01 in the Supernova Remnant W44

We present radio observations of a region in the vicinity of the young pulsar PSR B1853+01 in the supernova remnant W44. The pulsar is located at the apex of an extended feature with cometary morphology. We argue on the basis of its morphology and its spectral index and polarization properties that this is a synchrotron nebula produced by the spin down energy of the pulsar. The geometry and physical parameters of this pulsar-powered nebula and W44 are used to derive three different measures of the pulsar's transverse velocity. A range of estimates between 315 and 470 km/s are derived, resulting in a typical value of 375 km/s. The observed synchrotron spectrum from radio to X-ray wavelengths is used to put constraints on the energetics of the nebula and to derive the parameters of the pulsar wind.

astro-ph↗

arXiv1995

Does the Crab Have a Shell?

We present deep images of a region around the Crab nebula made with the VLA, utilizing new imaging and deconvolution algorithms in a search for a faint radio shell. The existence of a high-velocity, hydrogen-rich envelope has been predicted to account for the low total mass and kinetic energy of the observed nebula. No radio emission was detected from an extended source outside the Crab nebula. Our limits on surface brightness are sufficiently low to rule out the existence of a shell around the Crab whose brightness is at least two orders of magnitude below SN\thinspace{1006}, the faintest historical shell-type supernova remnant. We consider models for the progenitor star and the pre-supernova environment and conclude that if a fast, outer shock exists then it has a sharply reduced efficiency at accelerating relativistic particles from the kinetic energy of the blast wave. We also looked for a steepening of the spectral index along the boundary of the Crab nebula itself, the signature of an outer shock. However, contrary to previous claims, no such steepening was found. The absence of any evidence at radio wavelengths that either the Crab nebula or a hypothetical shell is interacting with the ambient medium leads to an interpretation that the supernova of 1054 AD was a peculiar low energy event.

astro-ph↗

arXiv1994

Radio Monitoring and High Resolution Observations of the Soft Gamma Ray Repeater 1806-20. by Vasisht et al. Postscript file, 13 pages

We report the results of radio flux-monitoring and high resolution observations at 3.6 cm with the VLA, of the central condensation in G10.0-0.3, the radio nebula surrounding the soft gamma ray repeater (SGR) 1806-20. The quiescent X-ray source AX1805.7-2025 is coincident with the radio core suggesting that G10.0-0.3 is a plerionic supernova remnant. The monitoring experiment was performed in 10 epochs spread over five months, starting just before the latest reactivation of SGR 1806-20 in gamma-rays. There is no apparent increase in the radio flux density from the central region of G10.0-0.3 on timescales of days to months following the gamma-ray bursts. At a resolution of 1 arcsec the peak region of G10.0-0.3 is seen to consist of a compact source with diffuse, one-sided emission, reminiscent of core-jet geometry seen in AGNs and some accreting Galactic binaries. By analogy with these latter sources, we argue that the SGR 1806-20 is coincident with the core component. If so, this is the first arcsecond localization of a high energy transient. The lack of radio variability and the low brightness temperature of the central component distinguish SGR 1806-20 from other accreting binaries. The structure of the high resolution radio image also does not particularly resemble that seen in the vicinity of young pulsars. Thus there is no compelling observational evidence for either of the two models discussed for SGRs, the pulsar model or the accreting binary model.

astro-ph↗

arXiv1994

The Radio Lifetime of Supernova Remnants and the Distribution of Pulsar Velocities at Birth

We have made VLA images of the fields around three young pulsars which have resulted in the discovery of two new supernova remnants and confirmation of a third. We argue that, in at least two cases and perhaps the third, the pulsars are physically associated with these supernova remnants. A review of all known young pulsars shows that the majority are associated with supernova remnants. We show that the typical density of the interstellar medium into which the supernova remnants are evolving has a density of 0.2 cm$^{-3}$ instead of the low value of 0.01 cm$^{-3}$ which had been calculated from other studies, and results in a considerably longer radio lifetime for supernova remnants. Both the morphology of the supernova remnants and the location of the pulsars imply that most of these young pulsars are born with large transverse velocities ($\sim$500 km s$^{-1}$). This high velocity mean in the distribution of pulsar velocities appears to be a general property of the pulsar population at birth, not seen in proper motion studies, due to selection effects. We explore the implications of this result as it relates to the origin of these velocities and the galactic distribution of pulsars. High velocity pulsars can escape their supernova remnant in a very short timescale, comparable to the lifetime of the remnant and may even play a role in extending the observable radio lifetime of the remnant. A significant fraction will be capable of escaping the disk of the Galaxy, producing an extended halo population.

astro-ph↗

arXiv1994

Anisotropic Scattering of OH/IR Stars Towards the Galactic Center

We have made angular broadening measurements in the satellite line of the OH molecule at 1612 MHz of 8 OH/IR stars located near the Galactic Center with the Very Large Array. Early observations with the Very Long Baseline Array had revealed a region of pronounced scattering approximately centered on SgrA*. These new observations show that the compact line emission from these stars is scattered by an amount that is two to three times larger than had been previously seen. Additionally, we measure non-circular scattering disks for these objects, evidence that the electron density variations are distributed anisotropically. The present data suggest that the enhanced turbulence is located near the Galactic Center. However, we cannot completely rule out that the scattering is caused by an unrelated region along the line of sight.

astro-ph↗

arXiv1994

The Radio Nebula of the Soft Gamma Ray Repeater SGR 1806-20

Earlier we had suggested that G10.0-0.3, a non-thermal nebula, most likely an old supernova remnant (SNR), was associated with the soft gamma repeater (SGR) SGR 1806-20. Here we present new radio images obtained at the Very Large Array (VLA) of the non-thermal radio nebula G10.0-0.3. The nebula is a plerion with a hierarchy of nested amorphous components culminating in a peak. The recent dramatic detection of an X-ray point source coincident with the radio peak and a hard X-ray burst localized to G10.0-0.3 confirms the SNR-SGR association. We propose that the SGR is an isolated pulsar with both a steady and impulsive relativistic particle wind and these two together power the nebula. We note that both this SGR and SGR 0526-66 are offset from the centers of their SNRs which requires high space motion of the pulsar. We suggest that some natal mechanism produces high velocity and induces SGR activity.

astro-ph↗

arXiv1993

Shock Excited Maser Emission from the Supernova Remnant W28

We present VLA observations in the hydroxyl line OH(1720 MHz) towards the supernova remnant W28. These high resolution images show a more extensive distribution of 1720 MHz emission than had previously been suspected from earlier single-dish observations. A total of 26 distinct maser spots were detected which lie on the edge of the radio continuum emission from W28 and are unresolved with our 12 arcsec resolution. Significant detection of linear and circular polarization in the maser spots is also made. The location of the maser spots, together with the physical conditions in the SNR and the adjacent molecular cloud, argue for a shock excitation mechanism. The possibilities of using the OH(1720 MHz) as a powerful diagnostic of the interaction of supernova remnants with molecular clouds is discussed.

astro-ph↗