Jonathan F. Schachter

The Identification of EGRET Sources with Flat-Spectrum Radio Sources

We present a method to assess the reliability of the identification of EGRET sources with extragalactic radio sources. We verify that EGRET is detecting the blazar class of active galactic nuclei (AGNs). However, many published identifications are found to be questionable. We provide a table of 42 blazars that we expect to be robust identifications of EGRET sources. This includes one previously unidentified EGRET source, the lensed AGN PKS 1830-210, near the direction of the Galactic center. We provide the best available positions for 16 more radio sources that are also potential identifications for previously unidentified EGRET sources. All high Galactic latitude EGRET sources (|b| > 3?) that demonstrate significant variability can be identified with flat-spectrum radio sources. This suggests that EGRET is not detecting any type of AGN other than blazars. This identification method has been used to establish with 99.998% confidence that the peak ?-ray flux of a blazar is correlated with its average 5 GHz radio flux. An even better correlation is seen between ?-ray flux and the 2.29 GHz flux density measured with VLBI at the base of the radio jet. Also, using high-confidence identifications, we find that the radio sources identified with EGRET sources have greater correlated VLBI flux densities than the parent population of flat radio spectrum sources.

The Einstein Slew Survey

A catalog of 819 sources detected in the Einstein IPC Slew Survey of the X-ray sky is presented; 313 of the sources were not previously known as X-ray sources. Typical count rates are 0.1 IPC count/s, roughly equivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources have positional uncertainties of 1.2 arcmin (90 percent confidence) radius, based on a subset of 452 sources identified with previously known pointlike X-ray sources (i.e., extent less than 3 arcmin). Identifications based on a number of existing catalogs of X-ray and optical objects are proposed for 637 of the sources, 78 percent of the survey (within a 3-arcmin error radius) including 133 identifications of new X-ray sources. A public identification data base for the Slew Survey sources will be maintained at CfA, and contributions to this data base are invited.

An Intense Gamma-Ray Flare of PKS 1622–297

We report the observation by the Compton Gamma Ray Observatory of a spectacular flare of radio source PKS 1622-297. A peak flux of (17 ± 3) × 10-6 cm-2 s-1 (E > 100 MeV) was observed. The corresponding isotropic luminosity is 2.9 × 1049 ergs s-1. We find that PKS 1622-297 exhibits γ-ray intraday variability. A flux increase by a factor of at least 3.6 was observed to occur in less than 7.1 hr (with 99% confidence). Assuming an exponential rise, the corresponding doubling time is less than 3.8 hr. A significant flux decrease by a factor of ~2 in 9.7 hr was also observed. Without beaming, the rapid flux change and large isotropic luminosity are inconsistent with the Elliot-Shapiro condition (assuming that gas accretion is the immediate source of power for the γ-rays). This inconsistency suggests that the γ-ray emission is beamed. A minimum Doppler factor of 8.1 is implied by the observed lack of pair-production opacity (assuming X-rays are emitted cospatially with the γ-rays). Simultaneous observation by EGRET and OSSE finds a spectrum adequately fitted by a power law with photon index of -1.9. Although the significance is not sufficient to establish this beyond doubt, the high-energy γ-ray spectrum appears to evolve from hard to soft as a flare progresses.

The new BL Lacertae objects discovered by an efficient X-ray/radio/optical technique

The discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew Survey by means of x ray/radio vs. x ray/optical color-color diagrams and confirmation by optical spectroscopy are reported. These 13 BL Lacs were discovered using a technique which exploits the characteristic broad band spectra of BL Lacs. New VLA detections provide accurate fluxes (f(6 cm) is approximately 0.5 mJy) and 2 in. positions, facilitating the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless optical spectra. Nine of these lie within the range of colors of known x ray selected BL Lacs. Of the remaining four, one is apparently x ray louder (by a factor of 1.5) or optically quieter (by 0.8 mags); and three are optically louder (by 1-1.3 mags) than x ray selected BL Lacs. Approximately 50 new BL Lacs in total are expected from VLA work and upcoming Australia Telescope observations, yielding a complete Slew Survey sample of approximately 90 BL Lacs.

Bowen fluorescence in Scorpius X-1

The first measurements of O III Bowen fluorescence lines in the near-ultraviolet spectra of the X-ray binary system Scorpius X-1 are presented. The flux-calibrated spectra from the atmospheric limit to 1 micron also reveal the nitrogen Bowen lines near-4640 A, the Balmer and Paschen series of hydrogen, the Paschen and Brackett series of He II, and numerous lines from Fe II and He I, among others. All of these lines probably arise in emission-line regions photoionized by X-rays produced near the neutron star surface. The observed line intensities are used to estimate explicitly the roles of the O1 and O3 channels of the Bowen cascades, as well as the possible importance of charge transfer between H I asnd O IV. 83 refs.

Bowen fluorescence in AM Herculis stars

The study relates the observations of the Bowen fluorescence lines in AM Her and EF Eri, two AM Her systems. The implications of source variability and the checks performed in order to verify the accuracy of the data are reviewed. The line ratios for H I and He II are shown to deviate considerably from those expected; in this light, the H I and He II emission in AM Her is analyzed in detail. This approach makes it possible to put limits on line and continuum optical depths as well as temperatures. These limits along with the observed Bowen emission are used to construct a picture of the emission-line regions in the two sources, producing the values for the size, density, and temperature of a region. The derived value of the size is found to be significantly less than the size of the Roche lobe of the white dwarf, while the large range of densities is attributed to the exponential temperature dependence in the expression used for density. 45 refs.

Bowen fluorescence in a sample of Seyfert nuclei

New observations of the O III and N III Bowen fluorescence lines in a sample of two Seyfert 2 nuclei and six Seyfert 1 nuclei with relatively narrow permitted lines are discussed. Included are new measurements of O III 3133, 3444 A, which are the two strongest cascades produced in the Bowen process, and possible detections of O III 3341A, a secondary cascade expected to be produced following emission of a 3133 A photon. Measurements of the forbidden S II and forbidden O III lines are used to estimate densities for the program objects. The flux-calibrated spectra and the Bowen yields, together with the derived electron densities, are used to construct a model spherical Bowen-emitting region. 40 refs.

Radio Polarization in EGRET Blazars

We demonstrate a new way to search for blazars by correlating the positions of gamma-ray sources from the Second EGRET Catalog, and polarized radio sources from the National Radio Astronomy Observatory Very Large Array Sky Survey. Using this survey, we first investigated the radio polarization properties of Angel & Stockman blazars and concluded that blazars have a characteristic radio polarization at 1.4 GHz, P1.4 > 1%. Using this information and the NASA/IPAC Extragalactic Database, we were successful in reconfirming 14 of 18 known EGRET blazars. In addition, 12 potential identifications, eight of which are unique to this paper, were made for unidentified or low-confidence identification EGRET objects. We suggest four new plausible associations for gamma-ray sources previously identified with AGNs in the Second EGRET Catalog. Finally, one low-confidence AGN suggested by Thompson and coworkers is found to be an unlikely identification.

Gamma-ray bright quasars and BL Lacertae objects: EGRET data

The Compton Gamma-Ray Observatory (CGRO) Instruments, in particular, EGRET, have recently discovered new, gamma-ray bright active galactic nuclei (AGNs)(fluxes approximately 0.5-3 times as bright as 3C 273). Yet current sample sizes are modest (for example, EGRET as of 1993 January has detected 17 quasars and five BL Lac objects), and the sources probe only a small range in flux. Only by using a large, unbiased sample of AGNs can we answer key questions about source evolution, contributions to the gamma-ray background, and the different gamma-ray properties of quasars and BL Lac objects. Therefore, we need gamma-ray data on a much larger number of AGNs. We are currently pursuing two approaches toward remedying this situation: first, new (phase 2) OSSE, COMPTEL, and EGRET observations of X-ray selected AGNs predicted to be gamma-ray bright on simple assumptions; even an upper limit can set constraints on continuum models (e.g., pair production, synchrotron self-Compton); second, searches for radio-selected AGNs in wide-field EGRET data obtained during the pointings. For these AGN, we expect a large number of faint sources (i.e., mainly low signal-to-noise ratio detections and upper limits), a continuous progression down to the limiting EGRET sensitivity. Although special techniques must be employed to deal carefully with low-significance sources, the end product will be a much larger sample than currently available on which to base deductions about the gamma-ray properties of AGNs. Here we report the first results of this program. We find an upper limit for 3C 382 and demonstrate, using the 3C 382 field, how serendipitous blazars can systematically searched for in existing EGRET public (Phase 1) data. Preliminary results indicate that synchrotron self-Compton processes dominate.

ROSAT High Resolution Imager Identifications of Suspected Stellar Sources from the Einstein Slew Survey

We present analysis of a series of ROSAT High Resolution Imager (HRI) observations of 16 X-ray detections from the Einstein Slew Survey (ESS). The data were taken to resolve ambiguous ESS identifications based on optical spectroscopy. For 10 of the 16 detections, the previous identification has been confirmed—six with solar-type stars (spectral types F–K), one each with a cataclysmic variable, a B2 V, a dMe star, and one with a BL Lac object. For another detection, two potential counterparts are unresolvable even with the HRI. The HRI does resolve two other ESS detections into two distinct X-ray sources, but no HRI source is found for the remaining three X-ray detections. HRI-derived fluxes are consistent with those measured by the ROSAT All-Sky Survey for their 12 common detections, and improved positions are obtained for these sources. We calculate values of fX/fV and LX for these stars where possible and find values consistent with those expected for corresponding spectral types.