arXiv subjects

W. M. Goss

Publications and source records attributed to W. M. Goss.

At least 127 records · Page 7Linked to original sources

arXiv2003

Pulsar Parallaxes at 5 GHz with the Very Long Baseline Array

We present the first pulsar parallaxes measured with phase-referenced pulsar VLBI observations at 5 GHz. Due to the steep spectra of pulsars, previous astrometric measurements have been at lower frequencies. However, the strongest pulsars can be observed at 5 GHz, offering the benefit of lower combined ionospheric and tropospheric phase errors, which usually limit VLBI astrometric accuracy. The pulsars B0329+54, B0355+54 and B1929+10 were observed for 7 epochs spread evenly over 2 years. For B0329+54, large systematic errors lead to only an upper limit on the parallax (pi < 1.5 mas). A new proper motion and parallax were measured for B0355+54 (pi = 0.91 +- 0.16 mas), implying a distance of 1.04+0.21-0.16 kpc and a transverse velocity of 61+12-9 km/s. The parallax and proper motion for B1929+10 were significantly improved (pi = 2.77 +- 0.07 mas), yielding a distance of 361+10-8 pc and a transverse velocity of 177+4-5 km/s. We demonstrate that the astrometric errors are correlated with the angular separation between the phase reference calibrator and the target source, with significantly lower errors at 5 GHz compared to 1.6 GHz. Finally, based on our new distance determinations for B1929+10 and B0355+54, we derive or constrain the luminosities of each pulsar at high energies. We show that, for thermal emission models, the emitting area for X-rays from PSR B1929+10 is roughly consistent with the canonical size for a heated polar cap, and that the conversion of spin-down power to gamma-ray luminosity in B0355+54 must be low. The new proper motion for B1929+10 also implies that its progenitor is unlikely to have been the binary companion of the runaway O-star zeta-Ophiuchi.

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arXiv2003

Broad Recombination Line Objects in W49N on 600 AU Scales

High resolution 7 mm observations of the W49N massive star forming region have detected recombination line emission from the individual ultracompact (UC) HII regions on 50 milliarcsecond (600 AU) scales. These line observations, combined with multifrequency, high-resolution continuum imaging of the region at 7 mm (VLA) and at 3 mm and 1 mm (BIMA), indicate that five to seven of the eighteen ultracompact sources in W49N are broad recombination line objects (BRLOs) as described by Jaffe & Martin-Pintado (1999). BRLOs have both broad radio recombination lines ($Δ$V$>$60 \kms) and rising spectra (S$_ν\simν^α$), with $α$ values greater than 0.4. The broad line widths of the H52$α$ line are probably related to motions in the ionized gas rather than pressure broadening. A number of models have been proposed to explain the long lifetime of UC HII regions, including the photoevaporated disk model proposed by Hollenbach et al. (1994). This model can also explain the broad lines, rising spectra and bipolar morphologies of some sources. We suggest$-$based on line and continuum observations as well as source morphology$-$that in a subset of the W49N ultracompact sources we may be observing ionized winds that arise from circumstellar disks.

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arXiv2003

Imprints of neutron stars in the interstellar medium

We have carried out an HI survey towards X-ray central compact objects (CCOs) inside supernova remnants (SNRs) which shows that many of them are placed within local HI minima. The nature of these minima is not clear, but the most likely explanation is that the CCOs have evacuated the neighboring gas. This survey also allowed us to detect a weak, diffuse radio nebula inside the SNR Vela Jr, probably created by the winds of its associated CCO.

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arXiv2003

New high-resolution radio observations of the SNR CTB 80

We report new high resolution and high sensitivity radio observations of the extended supernova remnant (SNR) CTB 80 (G69.0+2.7) at 240 MHz, 324 MHz, 618 MHz, and 1380 MHz. The imaging of CTB 80 at 240 MHz and 618 MHz was performed using the Giant Metrewave Radio Telescope (GMRT) in India. The observations at 324 MHz and 1380 MHz were obtained using the Very Large Array (VLA, NRAO) in its C and D configurations. The new radio images reveal faint extensions for the asymmetric arms of CTB 80. The arms are irregular with filaments and clumps of size 1' (or 0.6 pc at a distance of 2 kpc). The radio image at 1380 MHz is compared with IR and optical emission. The correspondence IR/radio is excellent along the N arm of CTB 80. Ionized gas observed in the [SII] line perfectly matches the W and N edges of CTB 80. The central nebula associated with the pulsar PSR B1951+32 was investigated with an angular resolution of 10" x 6". The new radio image obtained at 618 MHz shows with superb detail structures in the 8' x 4' E-W ``plateau'' nebula that hosts the pulsar on its western extreme. A twisted filament, about 6' in extent (~3.5 pc), trails behind the pulsar in an approximate W-E direction. In the bright ``core'' nebula (size \~45"), located to the W of the plateau, the images show a distortion in the morphology towards the W; this feature corresponds to the direction in which the pulsar escapes from the SNR with a velocity of ~240 km/s. Based on the new observations, the energetics of the SNR and of the PWN are investigated.

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arXiv2003

VLA observations of 6-cm excited OH

The VLA was used to determine precise positions for 4765-MHz OH maser emission sources toward star-forming regions which had been observed about seven months earlier with the Effelsberg 100-meter telescope. The observations were successful for K3-50, DR21EX, W75N, and W49A. No line was detected toward S255: this line had decreased to less than 5 per cent of the flux density observed only seven months earlier. The time-variability of the observed features during the past 30 years is summarised. In addition, to compare with the Effelsberg observations, the 4750-MHz and 4660-MHz lines were observed in W49A. These lines were found to originate primarily from an extended region which is distinguished as an exceptional collection of compact continuum components as well as by being the dynamical centre of the very powerful H_2 O outflow.

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arXiv2003

Influence of the neutron star 1E 161348-5055 in RCW 103 on the surrounding medium

We have carried out a study of the neutral hydrogen in the direction of the X-ray source 1E 161348-5055, a compact central object (CCO) located in the interior of the supernova remnant (SNR) RCW 103. The HI 21 cm line observations were carried out using the Australia Telescope Compact Array, complemented with single dish data from the Parkes radio telescope to recover information at all spatial scales. We derive a distance to RCW 103 of 3.3 kpc, in agreement with previous distance measurements. We have also detected a small hole in the HI emission which is positionally and kinematically coincident with the location of the CCO which confirms the association between the SNR and the CCO. This is the third case of a depression in HI emission seemingly associated with CCOs in SNRs. The characteristic parameters of the holes such as their size, eccentricity and evacuated mass are similar in all three cases. We estimate the absorbing HI column density towards 1E 161348-5055 to be ~6 x 10^{21} cm^{-2}, a value compatible with a blackbody solution for the CCO X-ray emission. However, the implied brightness temperature is very high comparedto most neutron stars. Moreover, the strong long-term variability in X-rays favours the hypothesis that 1E 161348-5055 is an accreting binary sourcerather than an isolated, cooling neutron star. An analysis of the continuum image obtained at 1.4 GHz from these observations shows no trace of a pulsar wind nebula around 1E 161348-5055, in spite of it being a young object.

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arXiv2003

Proper-Motion Measurements with the VLA. II. Observations of Twenty-eight Pulsars

Using the Very Large Array, we have measured the proper motions of twenty-eight radio pulsars. On average, the pulsars studied are fainter and more distant than those studied in earlier work, reducing the selection biases inherent in surveys restricted to the Solar neighborhood. The typical measurement precision achieved is a few milliarcseconds per year, corresponding to a few tens of kilometers per second for a pulsar a kiloparsec away. While our results compare well with higher-precision measurements done using very-long baseline interferometry, we find that several earlier proper motion surveys appear to have reported overly optimistic measurement uncertainties, most likely because of a failure to fully account for ionospheric effects. We discuss difficulties inherent in estimating pulsar velocities from proper motions given poorly constrained pulsar distances. Our observations favor a distribution with 20% of pulsars in a low velocity component (sigma_1D = 99 km/s) and 80% in a high velocity component (sigma_1D = 294 km/s). Furthermore, our sample is consistent with a scale height of pulsar birthplaces comparable to the scale height of the massive stars that are their presumed progenitors. No evidence is found in our data for a significant population of young pulsars born far from the plane. We find that estimates of pulsar ages based on kinematics agree well with the canonical spin-down age estimate, but agreement is improved if braking indexes are drawn from a Gaussian distribution centered at n=3 with width 0.8.

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arXiv2003

Phase-Referenced VLBA Observations of OH Masers at 4765 MHz

We report VLBA observations of maser emission from the rotationally excited doublet Pi 1/2, J=1/2 state of OH at 4765 MHz. We made phase-referenced observations of W3(OH) at both 4765 MHz and 1720 MHz and found emission in three fields within a about 2000 AU diameter region and verified that in two of the three fields, 4765 MHz and 1720 MHz emission arise from the same position to within about 4 mas (about 5 AU diameter emission regions along an approximately N-S arc with linear extent about 500 AU. In addition, we carried out phase-referenced observations of 4765 MHz emission from K3-50. We searched for the 4765 MHz line in W49 (without phase referencing) and W75N (phase-referenced to the strongest 4765 MHz maser feature in DR21EX); we were unable to detect these sources with the VLBA. For 2 1/2 years (including the dates of the VLBA observations), we carried out monitoring observations of 4765 MHz emission with the VLA. Constraints on models for maser emission at 1720 MHz and 4765 MHz are derived from the observations. These observations are then briefly compared with existing models.

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arXiv2003

The Formaldehyde Masers in NGC 7538 and G29.96-0.02: VLBA, MERLIN, and VLA Observations

The 6 cm formaldehyde (H2CO) maser sources in the compact HII regions NGC 7538-IRS1 and G29.96-0.02 have been imaged at high resolution (beams < 50 mas). Using the VLBA and MERLIN, we find the angular sizes of the NGC 7538 masers to be ~10 mas (30 AU) corresponding to brightness temperatures ~10^8 K. The angular sizes of the G29.96-0.02 masers are ~20 mas (130 AU) corresponding to brightness temperatures ~10^7 K. Using the VLA, we detect 2 cm formaldehyde absorption from the maser regions. We detect no emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. We find that both NGC 7538 maser components show an increase in intensity on 5-10 year timescales while the G29.96-0.02 masers show no variability over 2 years. A search for polarization provides 3-sigma upper limits of 1% circularly polarized and 10% linearly polarized emission in NGC 7538 and of 15% circularly polarized emission in G29.96-0.02. A pronounced velocity gradient of 28 km/s/arcsecond (1900 km/s/pc) is detected in the NGC 7538 maser gas.

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arXiv2003

Observations of the neutral hydrogen surrounding the radio quiet neutron star RX J0822-4300 in Puppis A

We have observed the HI distribution in an area 40' x 40' around the neutron star candidate RX J0822-4300, which is located in the supernova remnant Puppis A. The observations of the 21 cm line were obtained with the Australia Telescope Compact Array (ATCA) and were combined with single dish data from the Southern Galactic Plane Survey. The spatial resolution is 90", and the velocity resolution, 1 km/s . A sensitivity of ~ 0.7 K was achieved. The results revealed a double lobed feature of reduced emission at +16 km/s, centered on the central compact object (CCO), and aligned with an HI hole blueshifted by 13 km/s. The HI depressions have probably been created by the sweeping up of ~ 2 solar masses. The alignement between the lobes and the optical expansion centre of Puppis A suggests that the CCO could be ejecting two opposite jets. The velocity at which the two lobes are best defined allowed us to confirm that the distance to Puppis A is 2.2 kpc, based on a systemic velocity of +16 km/s. The hydrogen column density computed using this systemic velocity is consistent with estimates from models for X-ray spectra, thus reinforcing our conclusion that the kinematic distance is 2.2 k pc.

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arXiv2003

Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the `Knee' in the Primary Cosmic Ray Spectrum

The Monogem ring is a bright, diffuse, 25-degree-diameter supernova remnant easily visible in soft X-ray images of the sky. Projected within the ring is a young radio pulsar, PSR B0656+14. An association between the remnant and pulsar has been considered, but was seemingly ruled out by the direction and magnitude of the pulsar proper motion and by a distance estimate that placed the pulsar twice as far from Earth as the remnant. Here we show that in fact the pulsar was born very close to the center of the expanding remnant, both in distance and projection. The inferred pulsar and remnant ages are in good agreement. The conclusion that the pulsar and remnant were born in the same supernova explosion is nearly inescapable. The remnant distance and age are in remarkable concordance with the predictions of a model for the primary cosmic ray energy spectrum in which the `knee' feature is produced by a single dominant source.

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arXiv2003

The distance and radius of the neutron star PSR B0656+14

We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is pi = 3.47 +- 0.36 mas, yielding a distance of 288 +33 -27 pc. This independent distance estimate has been used to constrain existing models of thermal x-ray emission from the neutron star's photosphere. Simple blackbody fits to the x-ray data formally yield a neutron star radius R_inf ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed.

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arXiv2003

The Discovery of Sgr A*

The compact radio source Sgr A* is associated with a 3.6 million black hole at the center of the Milky Way. The radio source was discovered in February 1974 by Bruce Balick and Robert L.Brown. The National Radio Astronomy Observatory's Green Bank 35 km radio link interferometer was used. We discuss other observations in the years 1965-1985 as well as early VLBI observations. The name Sgr A* was used for the first time in 1982 by Robert L.Brown and has become the accepted name in the intervening years.

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arXiv2003

Variability of Sagittarius A* - Flares at 1 Millimeter

We report the results from recent observations of Sgr A* at short-/sub-millimeter wavelengths made with the partially finished Sub-Millimeter Array (SMA) on Mauna Kea. A total of 25 epochs of observations were carried out over the past 15 months in 2001 March to 2002 May. Noticeable variations in flux density at 1.3 mm were observed showing three ``flares''. The SMA observations suggest that Sgr A* highly increases towards submillimeter wavelengths during a flare suggesting the presence of a break wavelength in spectral index around 3 mm. A cross-correlation of the SMA data at 1 mm with the VLA data at 1 cm show a global delay of $t_{delay} > 3d$, suggesting that sub-millimeter wavelengths tend to peak first. Only marginal day-to-day variations in flux density (2-3 $σ$) have been detected at 1.3 mm. No significant flares on a short time scale ($\sim1$ hr) have been observed at 1.3 mm. We also failed to detect significant periodic signals at a level of 5% (3$σ$) from Sgr A* in a periodic searching window ranging from 10 min to 2.5 hr. The flares observed at the wavelengths between short-centimeter and sub-millimeter might be a result of collective mass ejections associated with X-ray flares that originate from the inner region of the accretion disk near the supermassive black hole.

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arXiv2002

PKS B1400-33: an unusual radio relic in a poor cluster

We present new arcminute resolution radio images of the low surface brightness radio source PKS B1400-33 that is located in the poor cluster Abell S753. The observations consist of 330 MHz VLA, 843 MHz MOST and 1398 and 2378 MHz ATCA data. These new images, with higher surface brightness sensitivity than previous observations, reveal that the large scale structure consists of extended filamentary emission bounded by edge-brightened rims. The source is offset on one side of symmetrically distributed X-ray emission that is centered on the dominant cluster galaxy NGC 5419. PKS B1400-33 is a rare example of a relic in a poor cluster with radio properties unlike those of most relics and halos observed in cluster environments. The diffuse source appears to have had an unusual origin and we discuss possible mechanisms. We examine whether the source could be re-energized relic radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC 5419. The more exciting prospect is that the source is relic plasma preserved in the cluster gaseous environment following the chance injection of a radio lobe into the ICM as a result of activity in a galaxy at the periphery of the cluster.

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arXiv2002

VLA Observations of the "Eye of the Tornado"- the High Velocity \HII Region G357.63-0.06

The unusual supernova remnant candidate G357.7-0.1 and the compact source G357.63-0.06 have been observed with the Very Large Array at 1.4 and 8.3 GHz. The H92$α$ line (8.3 GHz) was detected from the compact source with a surprising velocity of about -210 km/s indicating that this source is an \HII region, is most likely located at the Galactic center, and is unrelated to the SNR. The \HI absorption line (1.4 GHz) data toward these sources supports this picture and suggests that G357.7-0.1 lies farther away than the Galactic center.

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arXiv2002

The Sizes of OH (1720 MHz) Supernova Remnant Masers: MERLIN and VLBA Observations of IC443

MERLIN and VLBA observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant IC443 are presented. Based on MERLIN data with a resolution of 160 mas, the deconvolved sizes of the maser sources are in the range 90 to 180 mas (135 to 270 AU). The 12 mas resolution VLBA images show compact cores with sizes in the range 15 to 55 mas. The maser brightness temperatures are (2-34)x10^6 K for the MERLIN sources and (5-19)x10^8 K for the VLBA cores, in agreement with theory. Unlike the Zeeman Stokes V profiles observed in other OH (1720 MHz) SNR masers, single-handed circular polarization line profiles are observed in IC443 on all angular scales from 1000 to 10 mas resolution. For one line component, the observed line width is 0.24+/-0.07 km/s, compared to an estimated Doppler width of 0.49 km/s. This discrepancy in line widths can be accounted for if the maser emission arises from an elongated ellipsoidal region of masing gas.

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arXiv2002

The Molecular Counterpart of the Galactic Center Arched Filament HII Complex: OVRO Observations of the CS(2-1) Line

The Owens Valley Radio Observatory (OVRO) millimeter array was used to make observations of the CS(2-1) line (at 97.981 GHz) arising from the G0.07+0.04 region of the ``-30 km/s'' molecular cloud near the Galactic center with a spatial resolution of ~8". The ionized edges of this cloud forms the Arched Filament HII regions which are ionized by the adjacent Arches stellar cluster. The OVRO data were combined with single-dish data obtained at the 30-m IRAM telescope by Serabyn & Guesten (1987). A comparison of this CS(2-1) data and the H92alpha recombination line data of Lang, Goss & Morris (2001) reveals that the ionized and molecular gas are physically related, but that their velocities in this region differ by up to 35 km/s. This difference in velocity can be understood if the gas that gave rise to the G0.07+0.04 HII region has been fully ionized. An overall comparison of the molecular and ionized gas across the entire -30 km/s cloud based on the single dish CS(2-1) data and the H92alpha line data illustrates that such differences in velocity between the ionized and molecular gas are common and that the geometrical arrangement of these components is complicated. Much of the ionized gas resides on the near side (to the observer) of the molecular cloud; however, in several regions, some molecular material must lie in front of the HII region. The Arches stellar cluster therefore appears to be located in the midst of the molecular clouds such that some of the near-side cloud surfaces along our line of sight have not been exposed to the ionizing radiation.

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