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Helfand, D. J.

Publications and source records attributed to Helfand, D. J..

At least 55 records · Page 3
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NASA NTRS1984

X-ray surveys of the Magellanic Clouds

The proximity, well-determined distance, and low foregound obscuration of the Magellanic Clouds makes them an ideal target for the study of galactic X-ray source populations. Steady progress since the initial detection of X-rays from the LMC in 1968 culminated in the recent Einstein Observatory surveys from which over three dozen supernova remnants and about 10 compact binaries have been identified. This review records the 15-year history of Magellanic Cloud X-ray research, summarizes current knowledge, and offers a brief prospectus of what the next 15 years may hold.

Helfand, D. J.↗
NASA NTRS1983

X-ray emission and spin-up evolution of the 6.1-ms pulsar

An upper bound is calculated for the X ray luminosity of the binary pulsar PSR1953+29 and discussed in terms of effects on theretical models for the pulsar's formation and spin-up. The upper limit was obtained by summing the detected counts measured by the Einstein Observatory within a circle of radius 150 arcsec and subtracting local background fluxes. A flux upper bound of less than 4 x 10 to the 32 erg/sec, a neutron star radius of 15 km, a temperature less than 1,000,000 K, and a pulsar age of at least 3000 year are obtained. The neutron star could be much older than 10,000,000 yr, the spin-down rate is up to 10 to the minus 17 str/sec, and formation occurred by accretion when the companion was in a red giant phase. The pulsar has a mass of about 0.3 solar mass, travels an orbit of about 10 to the 13 cm, and evolved from a white dwarf near its Chandrasekhar limit. It is suggested that the magnetic surface field of the pulsar has stabilized near 1 billion G.

Helfand, D. J.↗
NASA NTRS1983

X-rays from radio pulsars - The detection of PSR 1055-52

The short-period pulsar PSR 1055-52 has been detected as a soft X-ray source in the course of an Einstein Observatory survey of radio pulsars. Its X-ray to radio luminosity ratio is about 10,000, although the X-rays are not modulated at the neutron star's rotation frequency. High spatial resolution observations suggest that a significant fraction of the emission comes from an extended region surrounding the pulsar. Several possible scenarios for the origin of both point and extended X-ray emission from isolated neutron stars are investigated: radiation from the hot stellar surface, from hot polar caps, and from an optically thick atmosphere, as well as from a circumstellar nebula emitting thermal bremsstrahlung or synchrotron radiation. It is concluded that the spatial, spectral, and temporal characteristics of this source are most consistent with a model in which relativistic particles generated by the pulsar are radiating synchrotron X-rays in the surrounding magnetic field; i.e., that PSR 1055 is embedded in a mini-Crab nebula. Observational tests of this hypothesis are suggested, and the implications of this result for pulsar evolution are briefly discussed.

Cheng, A. F.↗
NASA NTRS1983

Constraints on gamma-ray bursters from soft X-ray transients

The Columbia Astrophysics Laboratory's Einstein imaging proportional counter database has been searched for possible X-ray counterparts to gamma ray bursts. The limiting sensitivity was about 10 to the 10th erg/sq cm with a total exposure of about 3 x 10 to the 6th s divided among about 1000 1 deg x 1 deg fields. Four events with a pointlike spatial distribution were detected but their extremely soft spectra makes them unlikely candidates for gamma ray burst counterparts. The results are used to place limits on the X-ray and gamma ray luminosity ratio and log(number)N-log(flux)S relation for gamma ray bursts. The constraints placed on models for their origin are commented on.

Helfand, D. J.↗
NASA NTRS1983

G29.7-0.3: another supernova remnant with an identity crisis

New radio and X-ray observations of the galactic supernova remnant G29.7-0.3 show that it is composed of two spectrally distinct components: a steep-spectrum, incomplete shell 3 arcmin in extent enclosing a flat-spectrum, X-ray emitting region 30 arcsec across. Thus, G29.7-0.3 joins the ranks of supernova remnants which exhibit a combination of Crab-like and shell remnant attributes. The Crab-like core has the highest ratio of X-ray radio luminosity of all the Crab-like remnants observed to date, suggesting that it is an extremely young object.

Becker, R. H.↗
NASA NTRS1983

High-resolution X-ray and radio maps of the millisecond pulsar

The 1.5 millisecond pulsar (PSR1937+214) discovered in the radio source 4C21.53 is more closely located and characterized. High-resolution radio maps of the region were obtained with the Very Large Array, and no evidence was found in the morphology of 4C21.53 to connect it with the pulsar, which was determined to be at a different distance. A high-resolution imager X-ray map of the region was made using the Einstein Observatory, and no detectable X-ray emission could be associated with either 4C21.53 or the pulsar. From the stringent upper limit on X-ray emissions, substantial differences in the magnetospheric structure of the pulsar compared to that of other radio pulsars are inferred.

Becker, R. H.↗
NASA NTRS1983

Supernova remnants in the Magellanic Clouds

The present updated catalog of SNRs in the Large and Small Magellanic Clouds (LMC and SMC) incorporates remnants recently discovered as a result of coordinated X-ray, optical, and radio surveys, and contains 25 confirmed SNRs in the LMC and six in the SMC. Optical images are included for all of the new SNRs, together with X-ray isophotes for 24 of the SNRs in the LMC which have been obtained with the high resolution imager and imaging proportional counter of the Einstein Observatory. The cumulative number-diameter relation for Type II SNRs with D smaller than 50 pc in the LMC is N(D)= 0.36 D exp 1.0 + or 0.2, suggesting that the SNRs have evolved much faster than expected from Sedov theory and that the free expansion phase dominates their evolution up to diameters of 30-40 pc. SNR production frequencies in the LMC and SMC are calculated to be 1/275 and 1/800 years, respectively, or nearly equal to the galaxies' luminosity ratios.

Mathewson, D. S.↗
NASA NTRS1983

X-rays from radio pulsars - The portable supernova remnants

Neutron stars are the longest-lived remnants of supernova explosions. As a reservoir of thermal energy remaining from the explosion and generated by frictional coupling between core and crust, as a storehouse of magnetic and rotational kinetic energy which allows the star to act as a high energy particle accelerator, and as the source of a deep gravitational potential which can generate heat from infalling matter, neutron stars remain capable of producing high energy radiation for a Hubble time. The results of an extensive survey of supernova remnants and radio pulsars performed with the imaging instruments on board the Einstein Observatory are reviewed and the implications of these results for pulsar physics and for the origin and evolution of galactic neutron stars are discussed.

Helfand, D. J.↗
NASA NTRS1982

Optical identification of Balmer-dominated supernova remnants in the Large Magellanic Cloud

Optical emission-line images are presented for four SNR's that are luminous X-ray sources and that were recently discovered during an X-ray survey of the LMC. The images reveal distinct filamentary shells that are prominent in the Balmer lines of H but are either absent or very weak in forbidden O III and forbidden S II emission. One remnant exhibits clear evidence for velocity-broadened H-alpha emission, and all four are expanding into regions with a relatively low H I density. It is shown that both the broad H-alpha emission and the Balmer-dominated spectra can be understood in terms of a very high-velocity nonradiative shock encountering gas that is partially neutral. The optical, X-ray, and radio properties of the LMC remnants are compared with those of relatively young galactic SNR's. It is concluded that all four LMC remnants resulted from Type I supernovae.

Tuohy, I. R.↗
NASA NTRS1982

Two X-ray selected BL Lacertae candidates

Two serendipitous Einstein Observatory X-ray sources with faint (V approximately 19) optical counterparts having the featureless spectra characteristic of BL Lacertae objects are identified. Observations with the VLA at 6 cm establish that these sources have radio to optical luminosity ratios which are lower than the mean for BL Lac objects by one to two orders of magnitude but which are not extreme when compared with those of the known X-ray bright BL Lac objects. It is therefore suggested that these two objects are among the first examples of radio-quiet BL Lac objects.

Chanan, G. A.↗
NASA NTRS1982

An X-ray study of two Crablike supernova remnants - 3C 58 and CTB 80

The instruments on the Einstein Observatory have been used to study the X-ray properties of two Crablike SNRs: 3C 58 and CTB 80. Images of the objects are similar, showing an extended plateau of emission surrounding a pronounced central maximum consistent with the presence of a point source. The X-ray spectrum of 3C 58 is consistent with a power law of spectral index 0.5 + 0.5 or -0.2 modified by absorption in a hydrogen column density N sub H of (2 + or - 0.5) x 10 to the 21st/sq cm. These data suggest an analogy with the Crab Nebula, where the extended X-ray emission is nonthermal and the pointlike emission emanates from a central neutron star. Quantitative comparisons of the X-ray and radio properties for six Crablike remnants indicate significant differences which may reflect an evolutionary sequence.

Becker, R. H.↗
NASA NTRS1982

An unbiased survey of field star X-ray emission

To determine the X-ray luminosity function of normal stars, 1700 stars brighter than 10th magnitude were surveyed by the imaging proportional counter aboard the Einstein Observatory. Seventy star positions were found to contain excess X-ray counting rates. The number of stars per square degree in a number of magnitude intervals was calculated as a function of spectral type and luminosity class, and the total number of stars for each spectral type brighter than magnitude 9.5 derived in this manner was compared with the 1700-star sample. The agreement is good, as is that between the surface density of soft X-ray sources and the number of stellar emitters predicted from the field star survey. It is concluded that stars probably do not contribute significantly to the diffuse soft X-ray background. The findings are consistent with the notion that stellar age and/or rotation velocity are important determinants of stellar X-ray emission level.

Helfand, D. J.↗
NASA NTRS1982

An X-ray survey of supernova remnants in the Large Magellanic Cloud

Preliminary observation results for SNRs in the LMC are presented, and the current status of programs investigating these observational data is assessed. High Resolution Imager maps indicate that the remnant diameters used in early rate calculations were systematically underestimated. A preliminary SN rate of 1/110-350 years is derived upon reconsideration of these data. It is noted that most of the remnants detected in the LMC have very soft spectra, and would be undetectable as X-ray sources at distances of 5-10 kpc in the galactic plane.

Helfand, D. J.↗
NASA NTRS1981

The correlation of X-ray emission with strong millimeter activity in extragalactic sources

A sample of 25 strong extragalactic millimeter sources have been observed at about 1 keV with the Imaging Proportional Counter on the Einstein X-Ray Observatory and at 90 GHz with the NRAO 11 m telescope. A strong correlation between the millimeter and X-ray flux densities is found. This result is consistent with a synchronous self-Compton model in which the X-ray emission arises from the Compton scattering of millimeter radio photons off the relativistic electrons which created them.

Owen, F. N.↗
NASA NTRS1981

Spectroscopy from 2 to 200 keV

The astrophysical processes responsible for line and continuum emission in the spectra range 2 keV to 200 keV are examined from the viewpoint of designing a spectrometer which would operate in this regime. Phenomena considered include fluorescent line radiation in X-ray binaries, magnetically shifted iron lines and cyclotron emission from neutron star surfaces, line emission from cosmically abundant elements in thermal plasmas, and nuclear deexcitation lines in fresh nucleosynthetically produced matter. An instrument consisting of a approximately 10 sq cm array of planar germanium detectors surrounded by a large sodium-iodide anticoincidence shield is described and projected background rates and sensitivities are considered. A sample observing program for a two-day shuttle-based mission is included as an example of the wide range of scientific questions which could be addressed by such an instrument.

Helfand, D. J.↗
NASA NTRS1981

A soft X-ray study of the Large Magellanic Cloud

It is estimated on statistical grounds that of the 97 sources detected in a survey of the Large Magellanic Cloud (LMC) by means of the Einstein Laboratory's imaging X-ray telescope, less than 25 are likely to be either foreground stars or background quasars. The largest class of identified sources discovered is that of supernova remnants (SNRs). It is confirmed by means of high resolution maps that six more sources are also SNRs, and an additional 20 to 25 unidentified sources are suspected, in light of the available temporal, spatial and spectral information, to be SNRs. Of the remaining approximately 40 sources, about half are expected to be either background active galactic nuclei or foreground stars. The unexpectedly large number of SNRs found in the survey suggests that present knowledge of the galactic remnant population may be incomplete.

Long, K. S.↗
NASA NTRS1981

Unpulsed X-rays from pulsars

Preliminary results of several programs to detect thermal X-ray emission from isolated neutron stars are presented. Results of Einstein pulsar surveys indicate that either the majority of supernovas which leave remnants do not produce neutron stars, or the cooling calculations are in need of substantial revision. When appropriate relativistic thermodynamics and updated high energy nuclear physics are included, the new calculations predict significantly lower temperatures for standard neutron star equations of state. X-ray results give strong evidence that five of the seven historical remnants and a large majority of the other remnants of less than 1000 yr do not contain radio pulsars. A survey of known radio pulsars is also presented, which is designed to test the heating mechanisms required by various theories of pulsar emission and neutron star structure, and consists of a survey of all known pulsars within 300 pc.

Helfand, D. J.↗