K. T. S. Brazier

SNR G320.4–01.2 and PSR B1509–58: new radio observations of a complex interacting system

We describe radio continuum and spectral-line observations of the source G320.4–01.2 (MSH 15–52) and the coincident young pulsar B1509–58 made with the Australia Telescope Compact Array. Based on a comparison between X-ray and radio observations, we argue that the two main radio components of G320.4–01.2 are a single supernova remnant (SNR), which H I absorption indicates is at a distance of 5.2 ± 1.4 kpc. A high-resolution correspondence between radio and X-rays argues that the pulsar is interacting with the SNR via an opposed pair of collimated outflows. The outflow itself is seen as an elongated X-ray feature surrounded by a highly polarized radio sheath, while the interaction with the SNR manifests itself as a ring of radio/X-ray knots within the optical nebula RCW 89. We reject the hypothesis that the pulsar outflow powers the entire RCW 89 region. n n n nSNR G320.4–01.2 and PSR B1509–58 agree in distance and in rotation measure, and appear to be interacting. We thus conclude that the two objects are associated and have an age of 1700 yr. We propose that the SNR resulted from a high-energy or low-mass supernova that occurred near the edge of an elongated cavity. Such a model can account for the bilateral appearance of the SNR, its large apparent age, the significant offset of the pulsar from the centre of the SNR and the faintness of the pulsar-powered nebula at radio wavelengths.

The implications of radio-quiet neutron stars

We collate the evidence for rotation-powered neutron stars that are visible as X-ray sources and not as radio pulsars. To date, 10 objects have been proposed, and one, Geminga, has been confirmed as a pulsar by the detection of 4.2-Hz pulsations. Several indicators have been used to support the proposition that the X-ray sources are isolated neutron stars, including high X-ray to optical/radio flux ratios, a constant X-ray flux and coincidence with a γ-ray source. Seven of the published neutron star candidates are located near the centres of supernova remnants, two of them within plerions, suggesting that these are young objects (τ < 20 000 yr). The remaining candidate neutron stars have no associated supernova remnant and may be older systems, powered either by their rotation, like Geminga, or possibly by accretion from the interstellar medium. n n n nQuantitative upper limits exist for the radio fluxes of eight of the 10 objects, and reveal a population at least an order of magnitude less luminous at radio wavelengths than known radio pulsars of similar power or age. A simple explanation within the context of existing models is that these objects are pulsars in which the radio beams are directed away from Earth. They are still visible as X-ray sources because the thermal surface emission, which dominates the soft X-ray emission in most young to middle-aged radio pulsars, is radiated in all directions. In the cases where hard X-ray or γ-ray fluxes are seen, the beaming explanation implies different emission sites for the non-thermal high-energy radiation and the unseen radio beams. From the numbers of candidate neutron stars and radio pulsars younger than 20 000 yr and within 3.5 kpc, the radio beaming fraction of young pulsars is estimated to be roughly 50 per cent and certainly much less than 100 per cent. We find the local neutron star birth rate to be at least 13 Myr−1 kpc−2. This extrapolates to a Galactic rate of one neutron star born every ∼90 yr. We conclude that probably all neutron stars are born as radio pulsars, and that most young, nearby pulsars have already been discovered.

A candidate gamma-ray pulsar in the supernova remnant CTA 1

ABSTRA C T We present a detailed analysis of the high-energy gamma-ray source 2EG J0008+7307. The source has a steady flux and a hard spectrum, softening above 2 GeV. The properties of the gamma-ray source are suggestive of emission from a young pulsar in the spatially coincident CTA 1 supernova remnant, which has recently been found to have a non-thermal X-ray plerion. Our 95 per cent uncertainty contour around the> 1 GeV source position includes the point-like X-ray source at the centre of the plerion. We propose that this object is a young pulsar and is the most likely counterpart of 2EG J0008+7307.

Multiwavelength studies of the peculiar gamma-ray source 3EG J1835+5918

The source 3EGu2009J1835+5918 was discovered early in the CGRO mission by EGRET as a bright unidentified γ-ray source outside the galactic plane. Especially remarkable, it has not been possible to identify this object with any known counterpart in any other wavelengths band since then. Analyzing our recent ROSAT HRI observation, for the first time we are able to suggest X-ray counterparts of 3EGu2009J1835+5918. The discovered X-ray sources were subject of deep optical investigations in order to reveal their nature and conclude on the possibility of being counterparts for this peculiar γ-ray source.

Optical identification of EGRET source counterparts

As part of a program for searching for Geminga-like candidates through the combination of γ-ray, X-ray and optical data, we have engaged ourselves in optical observations of selected ROSAT point sources. The strategy followed has been to survey the error boxes of the best ROSAT candidates in search of optical counterparts. If an optical source is present, as in the case of 2EG J2020+4026, an optical spectrum is acquired in order to ascribe the likelihood of the star been the X-ray and γ-ray source. More interestingly, in the case of the EGRET source 2EG J0008+73, inside the CTA 1 SNR, no optical counterpart was found and the limits imposed restrict the nature of the γ-ray source. In both cases, the most likely interpretation is that the γ-ray source is a Geminga-type object.

A candidate γ-ray pulsar in CTA 1

2EG J0008+7307 is a prominent EGRET source 10.5 degrees from the Galactic Plane. We report the results of our study of this object, made as part of our on-going programme to identify individual EGRET point sources. We find that 2EG J0008+7307 has a constant flux and is located within the CTA 1 supernova remnant. It has recently been discovered that the centrally-concentrated X-ray emission from CTA 1 is non-thermal [14]. The 95% confidence contour for the EGRET source location includes only one point-like X-ray source, which is at the center of the non-thermal X-ray emission. We propose that this object is a unknown γ-ray pulsar.

Optical studies of potential counterparts for unidentified EGRET sources

We present optical observations of candidate X-ray counterparts for the EGRET sources 3EGu2009J2020+4017, 3EGu2009J0010+7309 and 3EGu2009J1835+5918. Preliminary spectroscopic observations of the late-type star coincident with the X-ray counterpart of 3EGu2009J2020+4017, the γ-Cygni source, show no evidence of binariety. In the case of 3EGu2009J0010+7309, the CTA-1 source, we performed deep optical imaging, finding a red V∼22 magnitude object inside the ROSAT contour. Finally, a detailed analysis of optical observations of several X-ray sources within the EGRET error box of 3EGu2009J1835+5918 is presented.