Engineering topics

Helfand, D. J.

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

At least 19 records

NASA NTRS2001

A Possible X-Ray and Radio Counterpart of the High-Energy Gamma-Ray Source 3EG J2227+6122

The identity of the persistent EGRET sources in the Galactic plane is largely a mystery. For one of these, 3EG J2227+6122, our complete census of X-ray and radio sources in its error circle reveals a remarkable superposition of an incomplete radio shell with a flat radio spectrum, and a compact, power-law X-ray source with photon index Gamma = 1.5 and with no obvious optical counterpart. The radio shell is polarized at a level of approx. = 25%. The anomalous properties of the radio source prevent us from deriving a completely satisfactory theory as to its nature. Nevertheless, using data from ROSAT, ASCA, the VLA, and optical imaging and spectroscopy, we argue that the X-ray source may be a young pulsar with an associated wind-blown bubble or bow shock nebula, and an example of the class of radio-quiet pulsars which are hypothesized to comprise the majority of EGRET sources in the Galaxy. The distance to this source can be estimated from its X-ray absorption as 3 kpc. At this distance, the X-ray and gamma-ray luminosities would be approx. = 1.7 x 10(exp 33) and approx. = 3.7 x 10(exp 35) erg/s, respectively, which would require an energetic pulsar to power them. If, on the contrary, this X-ray source is not the counterpart of 3EG J2227+6122, then by process of elimination the X-ray luminosity of the latter must be less than 10(exp -4) of its gamma-ray luminosity, a condition not satisfied by any established class of gamma-ray source counterpart. This would require the existence of at least a quantitatively new type of EGRET source, as has been suggested in studies of other EGRET fields.

Halpern, Jules P.↗

NASA NTRS1996

Isolating the foreground of the X-ray background

The foreground component of the X-ray background can be studied by cross-correlating its intensity with galaxy catalogs. The two-point cross-correlation function between 67 Einstein X-ray fields and the APM galaxy catalogs was computed. A signal with an amplitude of W(sub xg) = 0.04 +/- 0.01, significant at the 3.5 sigma level, was computed. The reality of this signal was tested. The way in which this signal can be used to constrain models of the X-ray background is discussed.

Refreigier, A.↗

NASA NTRS1994

The detection of pulsed X-ray emission from a nearby radio pulsar

As part of a search for thermal surface radiation from nearby neutron stars, we have carried out a 45,000 s observation of the nearby radio pulsar PSR 1929+10 with the ROSAT PSPC. After background subtraction, a net of 420 +/- 25 photons in the 0.1 to 2.0 keV band were detected at the position of the pulsar, corresponding to a luminosity of 1.2 x 10(exp 30) ergs/sec for a source distance of 250 pc, or approximately 3 x 10(exp -4) of the pulsar's spin-down luminosity. We find coherent pulsations from PSR 1929+10 at the radio period of 0.2265 s. The folded light curve is well fitted by a sinusoidal oscillation with a pulsed fraction of about 30%. The total spectrum is fitted by a blackbody with a temperature T(sub infinity) approximately equals 3.2 x 10(exp 6) K; the implied emitting area has a radius of less than 50 m. The maximum of the X-ray light curve coincides with the radio pulse, suggesting that we are detecting the hot magnetic polar cap of the star. The temperature limit for the remainder of the stellar surface is T(sub infinity) less than 3 x 10(exp 5) K. The spatial distribution of the photons is consistent with the instrument point-spread function. We discuss the implications of these results for the temperature distribution over the surface of the star and use this detection to constrain various heating mechanisms for rotation-powered neutron stars. We also use a simple model of general relativistic light bending near the stellar surface in conjunction with the radio emission geometry and the X-ray pulsed fraction to derive a radius for the neutron star of R = 9 +/- 3 km.

Yancopoulos, S.↗

NASA NTRS1993

Observations of (S III) emission from Galactic radio sources - The detection of distant planetary nebulae and a search for supernova remnant emission

Narrow-band near-infrared imaging observations at wavelengths corresponding to forbidden S III 9069,9532 A have been carried out at the MDM 1.3 m telescope for 23 radio sources near the Galactic plane in an attempt to detect emission associated with nebulae marking the endpoints of stellar evolution. While none of the known remnants or remnant candidates were detected, 10 of the 11 PN candidates from a new radio imaging survey of the Galactic plane were clearly seen in the forbidden S III 9532 A line. We present a calculation of the relative efficacy of searching for PNe in the forbidden O III and forbidden S III lines; for the majority of all PNe, the observed forbidden S III 9532 A line is predicted to be stronger than forbiden O III 5007 A whenever the visual extinction exceeds 3 magnitudes. This makes forbidden S III the superior tracer of PNe at distances exceeding a few kpc. We briefly comment on the significance of this approach to defining the spatial distribution of the PN population of the Galaxy.

Kistiakowsky, V.↗

NASA NTRS1992

A distance determination for the supernova remnant G27.4+0.0 and its central X-ray source

We have determined the distance to the supernova remnant G27.4+0.0, one of the handful of remnants with a bright central X-ray source. H I absorption data obtained with the VLA toward the remnant and adjacent H II regions allow us to constrain the distance to G27.4+0.0 to lie between 6 and 7.5 kpc. We briefly discuss the implications of this result for the nature of the central source.

Sanbonmatsu, K. Y.↗

NASA NTRS1992

Compact radio sources near the Galactic plane

The results of the extension of the 20-cm Galactic-plane survey reported by Zoonematkermani to Galactic latitudes of +/- 1.8 deg over the central region of the Milky Way are reported. A total of 1457 discrete radio sources down to flux densities of less than about 5 mJy, and 95 percent completion is achieved at 20 mJy. A detailed comparison of all radio sources from the survey in this longitude range with the IRAS Point Source Catalog provides classification for 13 percent of the objects, including 159 compact H II regions, and nearly 100 planetary nebulae, over 70 of which are identified. The identity of the remaining radio sources is discussed.

Helfand, D. J.↗

NASA NTRS1992

Where have all the sources gone?

In the continuing quest for an understanding of the cosmic x-ray background, the most difficult technical barrier remains the mismatch between the wavelength band in which the discrete source populations are best characterized (0.1 to 3.5 keV), and the band in which the spectrum of the background is well determined and the bulk of the energy density resides (3.5 to 50 keV). A progress report in an attempt to resolve this conundrum using the Einstein Imaging Proportional Counter (IPC) database and analysis system is presented. The IPC data base is described and how it can be used to search both for missing clusters of galaxies and highly obscured active galactic nuclei is addressed. All 4083 imaging proportional counter observations were examined after flat fielding and editing to optimize sensitivity for diffuse emission on various size scales ranging up to 10 feet. A '2 sigma source' catalog which, when filtered by matching with source catalogs from other wavelength regimes, is shown to reveal new source classes which may be significant contributors to the cosmic x-ray background is being constructed. Optimization techniques are described and the source catalogs which result are presented. Limits on the contributions both classes of sources make to resolving the discrepancy with the higher energy observations are set.

Helfand, D. J.↗

NASA NTRS1992

Ultrasoft x ray bursts: A clue to the origin of gamma ray bursts?

A search was conducted for short timescale (less than or approx. 10 s) faint X ray bursts in the complete 201,000,000 photon Einstein Observatory imaging proportional counter (IPC) data base. Some 73,000,000 events were extracted which were distributed among 11,230 observing intervals with an average observing time of approx. 1400 seconds. To exclude all sources, only those spatial pixels were searched with a count rate less than 6 x 10(exp -4) counts/sec sq arcmin. The mean count rate was much less. All files with less than 400 secs of observing time were excluded. The data was then binned into overlapping cells 4'.3 by 4'.3 by 10 secs in volume, and all the space-time cells were searched to locate cells with more than 5 counts in a 10 sec interval; i.e., a minimum flux enhancement of 50 over the mean rate for that point in space. Some 52 such events were found. An annulus was inspected around the event to see if it coincided with a general rise in the counter's event rate. Ten events did coincide with counter phenomena; these were associated with sunrise, sunset, or the approach of the South Atlantic Anomaly and were excluded. The remaining 42 events were then examined in an attempt to associate them with instrumental or environmental artifacts. The event occurrence times are uniformly distributed over the course of the mission and show no correlation with local solar time; their positions are randomly distributed in geographic, Galactic, and celestial coordinates. Their summed radial distribution is consistent with the point response function of the IPC for a soft source, indicating that the events are focused by the Observatory's mirror system. The integrated spectrum is, indeed, quite soft and is very similar to that of the cataclysmic variable star U Gem in outburst. Most of the events have risetimes of less than or approx. 1 sec and decay times ranging from 1 to 20 sec. Their positions are not associated with any class of catalogued objects. Several possible origins are discussed for the events; if they are associated with gamma ray bursters, they provide interesting constraints on the space density and repetition rate of the underlying source population.

Helfand, D. J.↗

NASA NTRS1991

The intensity and spectrum of the diffuse X-ray background

The paper reports the first measurement of the intensity and spectrum of the diffuse X-ray background in the 0.16-3.5 keV band which is free from contamination by sources with fluxes greater than 4 x 10 to the -14th ergs/sq cm s. These results have been made possible by a number of techniques for reducing cosmic-ray contamination and instrumental artifacts in the data collected by the Einstein Observatory imaging proportional counter.

Wu, X.↗

NASA NTRS1991

Faint X-ray source counts and the origin of the X-ray background

A thorough reanalysis of the Einstein Observatory Deep Survey data is presented in order to determine the contribution of detected, discrete sources of X-ray emission to the cosmic X-ray background. Substantial discrepancies with previously published work on this problem are found. A detailed discussion of data editing and source algorithms buttresses a claim of having constructed a complete, flux-limited sample of the faintest sources detectable with the Einstein imaging proportional counter, the most sensitive X-ray instrument yet flown. A total of 33 sources is found in a survey region of about 3.3 sq deg down to a minimum flux threshold of 4 x 10 to the 14th ergs/sq cm/s in the 0.3-3.5 keV band. Roughly 30 percent of the objects are foreground stars, leading to an extragalactic source surface density of 70,000/sr at this threshold. The integrated contribution from discrete sources to the number of cosmic X-ray background photons measured in this same band with the same instrument is 12 percent + or - 3 percent, substantially below previous estimates. Implications of these results for the origin of the background are discussed.

Hamilton, T. T.↗

NASA NTRS1991

The detection of X-rays from the hot interstellar medium of the Large Magellanic Cloud

A comprehensive reanalysis is presented of the Einstein IPC data base on the LMC, and a number of new algorithms are used to improve the reliability of the point source detection in this crowded region and to produce the first large-scale map of diffuse emission free from the effects of solar scattered X-rays and cosmic-ray particles. Algorithms described in detail include a technique to decontaminate fields containing solar scattered flux, a mechanism for obtaining a spectrum-weighted vignetting function, and a source excision and smoothing algorithm which results in a diffuse map of uniform statistical quality. A catalog of discrete X-rays sources in the direction of the LMC is presented which contains 33 new sources and eliminates a number of spurious and/or marginal directions from previous lists. A possible detection of the long-sought shadowing effect on the cosmic X-ray background produced by cold LMC gas is reported.

Wang, Q.↗

NASA NTRS1991

The detection of X-ray emission from the OB associations of the Large Magellanic Cloud

A systematic study of the X-ray properties of OB associations in the Large Magellanic Cloud has been carried out using data from the Einstein Observatory. An excess of young, X-ray-bright supernova remnants is found in the vicinity of the associations. In addition, diffuse X-ray emission is detected from over two dozen other associations; luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (the detection threshold) to 10 to the 36th ergs/s. For several of the more luminous examples, it is shown that emission from interstellar bubbles created by the OB stellar winds alone is insufficient to explain the emission. It is concluded that transient heating of the bubble cavities by recent supernovae may be required to explain the observed X-rays and that such a scenario is consistent with the number of X-ray-bright associations and the expected supernova rate from the young stars they contain.

Wang, Q.↗

NASA NTRS1991

An X-ray image of the violent interstellar medium in 30 Doradus

A detailed analysis of the X-ray emission from the largest H II region complex in the Local Group, 30 Dor, is presented. Applying a new maximum entropy deconvolution algorithm to the Einstein Observatory data, reveals striking correlations among the X-ray, radio, and optical morphologies of the region, with X-ray-emitting bubbles filling cavities surrounded by H-alpha shells and coextensive diffuse X-ray and radio continuum emission from throughout the region. The total X-ray luminosity in the 0.16-3.5 keV band from an area within 160 pc of the central cluster R136 is about 2 x 10 to the 37th ergs/sec.

Wang, Q.↗

NASA NTRS1989

A search for X-ray counterparts to small-diameter Galactic radio sources

By comparing observations obtained with the Einstein Observatory to the Molonglo catalog of small-diameter radio sources near the Galactic plane, the number of supernova remnants contained in the radio sample has been constrained. The one known remnant among the 47 sources falling in Einstein fields was easily detected and another candidate remnant (G274.7-2.8) was discovered. The latter object has properties consistent with that of either a young shell-like or composite remnant, although an identification as a radio-loud quasar cannot be excluded. Apart from cases in which very large absorbing column densities are appropriate, upper limits to the X-ray emission from the remainder of the sample places most of the objects outside of the ranges of parameters that characterize the Galactic remnant population.

Helfand, D. J.↗

NASA NTRS1988

The dual nature of the supernova remnant G351.2 + 0.1

The Galactic SNR G351.2 + 0.1 has been imaged at frequencies of 1.4, 5, and 15 GHz. The images reveal a source composed of two distinct components: a broken elongated shell measuring 4 x 6 arcmin and a central core about 15 arcsec in diameter. The two components are spectrally distinct. The shell has a nonthermal spectrum (alpha = -0.4) typical for Galactic SNRs, while the central core has an inverted spectrum (alpha = +0.27). The central core may be an analog of the Crab Nebula (a synchrotron nebula powered by a pulsar), but if so it is not typical of such objects.

Becker, R. H.↗

NASA NTRS1987

High-resolution X-ray and radio images of the galactic SNR G39.2-0.3

Images of the galactic supernova remnant G39.2-0.3 at radio, infrared, and X-ray wavelengths are presented. The 6 and 20 cm VLA maps reveal a shell-brightened source with substantial emission in the central regions and a rather high degree of linear polarization throughout; no significant spectral variations over the face of the remnant are apparent. The Einstein X-ray image also shows emission from both the limb and the central region, while the IRAS data show no flux that can be unambiguously associated with the remnant, although an uncataloged small H II region is seen adjacent to the shell. It is concluded that, contrary to previous suggestions, there is no evidence for a Crab-like component in G39.2-0.3, and that the central emission plateau is most likely simply a shell filament which happens to fall across the center of the remnant.

Becker, R. H.↗

NASA NTRS1987

The origin of the diffuse X-ray background

The Einstein Deep Survey fields are used to reexamine the problem of the origin of the extragalactic diffuse X-ray background. An approach based on fluctuation analysis is adopted, and a sophisticated Monte Carlo scheme is used to compare the X-ray images with a variety of models representing point and diffuse source contributions. Results are presented for two classes of models: those which determine the flux cutoff beyond which pointlike sources fail to contribute significantly if the log N-log S slope is fixed at the measured value of -1.5, and those which set limits on the slope of the log N-log S relation below its last measured point at the Deep Survey threshold. In both cases, it is found that the known population of X-ray-emitting quasars and any reasonable extrapolations therefrom do not comprise the entire X-ray background.

Hamilton, T. T.↗

NASA NTRS1987

High-resolution radio observations of the supernova remnant G24.7 + 0.6 and the discovery of an ultracompact H II region

The Crab-like SNR G24.7 + 0.6 with 12 arcmin resolution at 20 cm wavelength has been observed. The image reveals a bright Crab-like core surrounded by a faint shell. An ultracompact H II region lies adjacent to the remnant core and several other compact H II regions lie just outside the remnant shell. Optical, far-infrared, and X-ray observations of the region are also presented. The specific attributes of this remnant and its place in the classification of composite versus Crab-like remnants as well as the general question of the association between Crab-like remnants and H II regions are discussed.

Becker, R. H.↗