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Schwartz, D. A.

Publications and source records attributed to Schwartz, D. A..

At least 73 records · Page 4
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NASA NTRS1979

Observations of Type 1 Seyfert galaxies with the HEAO-1 scanning modulation collimator

Precise positions of four X-ray sources associated with Type 1 Seyfert galaxies are reported which were measured with the HEAO 1 scanning modulation collimator. The Seyferts are confirmed as the X-ray emitters in each case. The sources (and corresponding galaxies) studied include 2A 0551+466 (NGC 4151), 2A 1207+397 (MCG 8-11-11), 2A 1914-589 (ESO 141-G55), and A 1021+198 (NGC 3227). It is found that the X-ray source is located in the nucleus or the nuclear region for NGC 4151 and that the galaxy is unresolved by the present instrument in the other cases.

Griffiths, R. E.↗
NASA NTRS1979

X-ray locations and sizes of clusters of galaxies observed with the HEAO 1 scanning modulation collimator

Observations with the HEAO 1 scanning modulation collimator (MC) have yielded improved locations for the centroids of X-ray sources in clusters of galaxies dominated by a giant central galaxy. For A85, A478, A1795, A2029, A2199, A2319 A, and SC 0316-444, these positions are consistent with those of the clusters' dominant cD galaxies. By comparing the HEAO 1 MC flux with that obtained with the HEAO 1 A-2 experiment, estimates of the X-ray sizes for these clusters have been obtained. The correlation of these sizes with other cluster properties is discussed.

Schwarz, J.↗
NASA NTRS1979

HEAO 1 scanning modulation collimator discovery of an extended X-ray source at Cygnus A

The HEAO 1 modulation-collimator data associate the X-ray emission from the Cygnus A region (4U 1957 + 40) with a small area including the radio galaxy. The source appears to be extended, with a size of about 2 arcmin; it may have a complex geometry. Any point-source component must have a flux no greater than 1.3 UFU. A lower limit of 1 microgauss is placed on the magnetic field in the radio lobes, based on the upper limit on the X-ray flux due to inverse Compton scattering of radio electrons on the 2.7-K microwave background radiation. The X-ray emission is more likely due to driven accretion of the intercluster gas onto the giant cD radio galaxy. Therefore this source appears to be similar to the Virgo and Perseus clusters. The density derived for the X-ray-emitting gas is too small for its thermal pressure to confine the radio source.

Fabbiano, G.↗
NASA NTRS1979

3C 273 - X-ray variability, spectrum, and position

Observations of the X-ray source associated with 3C 273 have been carried out from 1.3 to 13.3 keV with the scanning modulation collimator on HEAO 1, yielding several precise positions (0.42 sq arcmin), one of which includes 3C 273. The data show an increase in flux by a factor of at least 3 in 6 months. A substantial low-energy (1.3-2.6 keV) flux during the higher state yields a hydrogen column density of less than 2 x 10 to the 22nd per sq cm and removes a possible X-ray distinction between quasars and other extragalactic nuclei, viz., Seyfert type 1 and BL Lacertae objects. The 90-GHz radio flux decreased during this period (1978.0-1978.5), and the optical flux exhibited increased variability after several years of relative quiescence.

Bradt, H. V.↗
NASA NTRS1979

Location of the recurrent /LMC/ X-ray transient A0538-66 with the HEAO 1 scanning modulation collimator

Two brief (about 12 hr) X-ray flares from the direction of the Large Magellanic Cloud have been observed with the HEAO 1 scanning modulation collimator. It is argued that both originated from the recurrent fast transient A0538-66, first observed with the Ariel 5 satellite. A precise position for the source is obtained, and a B2 Iab star in the Large Magellanic Cloud is suggested as the optical counterpart. Evidence for a 16.66-day periodicity in the outburst times is presented and discussed.

Johnston, M. D.↗
NASA NTRS1979

X-ray observations of high-excitation emission-line galaxies with the HEAO 1 scanning modulation collimator

Data from the scanning modulation collimator on HEAO 1 are presented which firmly establish high-excitation emission-line galaxies as a class of X-ray source. For NGC 5506 and M82, the X-ray emission (1-13 keV) is shown to originate in the nuclear region of the galaxy; NGC 2992 and NGC 526a are also identified as X-ray emitters, but are not resolved. The latter is a new identification for 2A 0120-353, previously associated with the NGC 526/527 group. In these four particular cases, the observed X-ray-emitting galaxies have a close companion, and the X-ray emission may have been triggered by a close encounter

Griffiths, R. E.↗
NASA NTRS1979

X-ray emitting BL Lacertae objects located by the scanning modulation collimator experiment on HEAO 1

It is established that BL Lacertae objects constitute a class of X-ray emitter by precisely locating X-ray sources identified with Mrk 421, PKS 0548-322, PKS 2155-304, and 2A 1219+305. Irregular variations of the order of 1 mag, characteristic of BL Lacertae objects, are reported for the last two objects. The measurement of an extremely steep X-ray spectrum from Mrk 421, in contrast to previous flat X-ray spectra above 3 keV, suggests that different structural states of an accretion disk are being observed.

Schwartz, D. A.↗
NASA NTRS1978

Positions and identifications for galactic X-ray sources 2A1822-371 and 2S1254-690

Precise positions and optical identifications for the galactic X-ray sources 2A1822-371 (4U 1822-37) and 2S 1254-690 (4U 1254-69) were obtained by used of a described scanning modulation collimator on HEAO 1 and are reported. Neither is thought to be substantially reddened since both show strong ultraviolet excesses and are located at a relatively high galactic latitude. The distance of 4U 1254-69 is estimated to be approximately 10 kpc.

Griffiths, R. E.↗
NASA NTRS1978

SS Cygni as a hard X-ray source identified with the scanning modulation collimator on HEAO 1

Hard X-ray emission from a 15 min by 1.5 deg area containing SS Cygni had previously been reported and identified with SS Cygni. This paper reports a 0.4 sq arc min measurement of the position of the X-ray source with the HEAO 1 modulation collimator experiment which leads to an unequivocal identification of SS Cygni as an X-ray emitter in the 2-10 keV energy range. The historical light curve in hard X-rays was also investigated by analyzing relevant Uhuru data, and two instances of possible source detection in the 2-6 keV band were noted. From Ariel 5 observations it is calculated that the X-ray luminosity of SS Cygni in the 2-10 keV energy range is of the order of 10 to the 33rd erg/sec in the optical low state and probably at least an order of magnitude less in the high state. By contrast, the soft X-ray luminosity increases by an order of magnitude between the optical low and high states. A coronal model or accretion onto the white dwarf can account for the inverse correlation of the soft and hard X-rays.

Fabbiano, G.↗
NASA NTRS1978

LMC X-1, X-2, and X-3 - Precise positions from the HEAO 1 modulation collimator

Precise (to about 20 arcsec) positions are reported for the Large Magellanic Cloud X-ray sources LMC X-1, X-2, and X-3, determined with the HEAO 1 scanning modulation collimator. The error regions for LMC X-1 and X-3 contain, respectively, the B5 supergiant R148 and a possibly variable B III-IV star. Spectra taken of the latter confirm the spectral type and show that it is a member of the LMC. A search for a previously reported extended component of LMC X-1 yields upper limits which exclude it.

Johnston, M. D.↗
NASA NTRS1978

Location of unidentified weak X-ray sources by the HEAO-1 scanning modulation collimator

Four weak sources for which precise positions have been obtained from the HEAO-1 Scanning Modulation Collimator (MC) experiment are discussed. One source (4U 0446+44) with the center of the cluster of galaxies containing 3C129 and 3C129.1 has been identified, and its angular size has been measured. One source with a galactic star is identified, although it is at galactic latitude b = 51 deg. The other two sources, 2A 0526-328 and 2A 0922-317, remain unidentified.

Schwartz, D. A.↗
NASA NTRS1978

Location of the Norma transient with the HEAO 1 scanning modulation collimator

A precise position has been obtained for an X-ray transient source in Norma. The location uncertainty includes a variable star previously suggested to be the optical counterpart. This transient is associated with the steady X-ray source MX 1608-52 and probably with an X-ray burst source. A binary system containing a low-mass primary and a neutron-star or black-hole secondary of a few solar masses is consistent with the observations.

Fabbiano, G.↗
NASA NTRS1978

Nova Ophiuchi 1977 - An X-ray nova

This letter reports the observation by the HEAO 1 satellite experiments of the X-ray nova in Ophiuchus discovered by Kaluzienski and Holt (1977), which is designated H1705-25. The SAO/MIT scanning modulation collimator has been used in conjunction with the NRL large-area sky survey to give two possible positions for the X-ray nova, each having errors of 0.3 by 1.8 arcmin. The discovery of a 16.5-magnitude optical nova (Nova Ophiuchi 1977) in one of the error boxes is reported, and the X-ray source is identified with the optical nova. The object is classified as being of the A0620-00 and A1524-61 type; i.e., a low-mass binary system.

Griffiths, R. E.↗
NASA NTRS1978

The X-ray luminosity function of Abell clusters

X-ray sources identified with Abell clusters of galaxies in distance classes less than or equal to 3 give a quantitative estimate of the volume luminosity function for those clusters emitting between 10 to the 44th and 10 to the 45th power ergs/s in X-rays. At higher luminosities, tentative identifications with more distant clusters can be interpreted at least as an upper limit. This limit allows a smooth extension of the luminosity function, but with a fairly steep decrease in the range from 10 to the 45th to 10 to the 46th power ergs/s. No direct information is available for luminosities less than 10 to the 44th power ergs/s because current X-ray surveys are limited to nearby distances with very few Abell clusters; however, constraints at the low-luminosity end can be set by considering the total volume density of Abell clusters and the upper limit to the numbers of unidentified X-ray sources. The luminosity function must flatten or turn over below 10 to the 44th power ergs/s; therefore, counts of distant X-ray clusters to such a faint intrinsic luminosity can yield cosmological information.

Schwartz, D. A.↗
NASA NTRS1978

The scanning modulation collimator experiment on the HEAO-1

The purpose of this experiment is to localize celestial sources of X-rays to a precision of 5 to 20 arcsec, so that they may be identified with optical or radio counterparts. We also study their angular size and structure, to elucidate the X-ray emission mechanism. Four grids, wound with nickel alloy wires, are aligned to produce a modulated X-ray signal when the spacecraft rotation carries the collimator field of view across an X-ray source. An image dissector star camera and rate integrating gyroscopes are used to deduce the celestial position from which a peak X-ray signal arises. On-orbit performance and thermal stability have been very good. Based on the first six weeks of quick look data, we have the following results: the location of an X-ray burst source within a globular cluster; the location of 4U1608-52, a unique object which shows bursting, transient, and steady X-ray behavior and is also an optical source; the location of the X-ray transient H1705-25, which provided Anglo-Australian astronomers the basis for an optical identification with a nova-like object; the location of the X-ray transient H1743-32 in the galactic center; and a precise measurement of the angular extent of the Crab Nebula.

Schwartz, D. A.↗
NASA NTRS1974

Geomagnetic background events observed by Uhuru

Problems due to sporadic background events observed by Uhuru (Small Astronomy Satellite - A) do not compromise the study of discrete X-ray sources. Nevertheless, direct particle effects and auroral type X-ray events in the atmosphere may occasionally occur. Therefore, even on the geomagnetic equator, an experiment must be prepared to recognize and eliminate such events when the ultimate level of sensitivity is desired. The test equipment contained in the satellite and the orbital mechanics of Small Astronomy Satellite - A are described. The sporadic events which were observed by the satellite are analyzed.

Schwartz, D. A.↗