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NASA NTRS1978

Optical spectroscopy of BL Lacertae objects and their relatives

Absorption lines from intervening gas along the line of sight to BL Lac objects are examined. The absorption line systems observed in several such objects are discussed, and detailed attention is given to the objects PKS 0735+178 and AO 0235+164. Random observations relevant to the nature of BL Lac objects are reviewed, including observations of steep-spectrum 3C QSOs and the peculiar object PKS 0528-250. The nature of the absorbing material is considered.

Smith, H. E.↗
NASA NTRS1993

Radio morphology and parent population of X-ray selected BL Lacertae objects

High-dynamic range (typically 1700:1) radio maps of 15 X-ray BL Lac (XBL) objects from the HEAO-1 Large Area Sky Survey are presented. Morphological characteristics of these sources are compared with Fanaroff-Riley (FR) class I radio galaxies in the context of unified schemes, with reference to one-sided kiloparsec-scale emission. Evidence that cluster membership of XBLs is significantly higher than previously thought is also presented. It is shown that the extended radio powers, X-ray emission, core-to-lobe ratios, and linear sizes of the radio selected BL Lac (RBL) and XBL populations are consistent with an FR I radio galaxy parent population. A source list and VLA observing log and map parameters are provided.

Laurent-Muehleisen, S. A.↗
NASA NTRS1993

Ultraviolet polarimetry and spectroscopy of the BL Lacertae object PKS 2155-304

HST and Faint Object Spectrograph spectropolarimetry is presented for the BL Lac object PKS 2155-304; attention is given to the wavelength dependence of the UV linear polarization in a BL Lac object. The UV polarimetry yields evidence that the UV polarized flux is generated by the synchrotron mechanism responsible for optical polarization. Both the UV and optical emission are produced in the same region of the source.

Allen, Richard G.↗
NASA NTRS1999

Selecting a Large, Bright, Complete Sample of BL Lacertae Objects

The observations funded by this grant were ROSAT HRI (Roentgen Satellite High Resolution Imager) observations of 21 Einstein Slew Survey X-ray source fields which contain BL Lac (Blazar Lacertae) candidates. The observations took place during A07 and A08; 17 of 21 were completed before ROSAT's demise late last year. The goal of these observations was to clarify the position of the X-ray emitter, enabling us to identify the optical counterpart on sky survey plates. This had not been possible previously for these objects because (1) the radio source within the Slew and RASS (Rosat All Sky Survey) error circles lacked a counterpart on optical sky survey plates; (2) more than one plausble radio optical counterpart existed; or (3) the radio and X-ray positions were not adequate to make a firm identification.

Perlman, Eric↗
NASA NTRS1978

X-ray observations of BL Lacertae objects

Results are reported for OSO 8 and HEAO 1 X-ray observations of the BL Lac objects Mrk 421, Mrk 501, and PKS 0548-322 over the energy range from 2 to 60 keV. It is found that the X-ray spectra of Mrk 421 and 501 are similar, exhibit statistically significant soft X-ray excesses, lack low-energy absorption, and are best fitted by power laws over the 2-30-keV range with respective photon indices of approximately 0.91 and 1.2 and respective hydrogen column densities of less than 8 x 10 to the 21st per sq cm and no more than 1.8 x 10 to the 22nd per sq cm. It is noted that the spectral index for both sources is identical, within errors, to the HF radio spectral index and that Mrk 421 experienced a factor of 6 decrease in the 2-10-keV band over a six-month period. A tentative identification of PKS 0548-322 with the source H0548-32 is suggested.

Mushotzky, R. F.↗
NASA NTRS1985

Optical and radio observations for the BL Lacertae objects 1219 + 28, 0851 + 202, and 1400 + 162

Radio- and visible-wavelength observations for the BL Lac object 1219 + 28 (ON 231, W Com) are reported along with optical observations of 0851 + 202 (OJ 287), and radio data for 1400 + 162 (4C 16.39, OQ 100). The obtained data consist of imaging and surface photometry for both 1219 + 28 and 0851 + 202, as well as spectra for 1219 + 28, and Very Long Baseline Interferometry (VLBI) observations for 1219 + 28 and 1400 + 162. Two emission features have been observed in the spectrum of 1219 + 28. Radio observations of 1219 + 28 in three frequencies reveal the presence of a radio jet with at least three components. The obtained results for 1219 + 28 support previous conclusions that at least some BL Lac objects are located at the center of elliptical galaxies.

Weistrop, D.↗
NASA NTRS1986

1E 1415.6+2557 - An X-ray-selected BL Lacertae object in a luminous galaxy

The identification of a fifth serendipitous Einstein source with a new BL Lac object in a very luminous galaxy is reported. The resolved component is well fitted by an exponential disk with scale length 18 kpc and absolute magnitude of roughly -24.2. A redshift of 0.237 is derived from stellar absorption features. No emission lines are seen in the 3200-9000 A wavelength range. Decomposition of the optical spectrum into a standard galaxy plus a power law yields a spectral index of 0.5 + or - 0.5, significantly flatter than in the average BL Lac object. Linear polarization of the nonstellar component is about 6 percent in the 4500-7000 A wavelength range. The X-ray flux in the 0.3-3.5 keV band is 1.16 x 10 to the -11th ergs/sq cm/s, corresponding to a luminosity of 3.5 x 10 to the 45th ergs/s . The radio flux density is 85.6 mJy at 20 cm and 54.5 mJy at 6 cm.

Halpern, J. P.↗
NASA NTRS1993

The radio structure source of X-ray-selected BL Lacertae objects

Arcsecond-size radio structure for a complete sample of 14 X-ray-selected BL Lac objects (XBLs) from the Einstein Extended Medium-Sensitivity Survey is presented. Most objects possess radio morphologies which are similar to those of nearby Fanaroff-Riley type 1 (FR 1) radio galaxies once the effects of surface brightness dimming and beam dilution as a function of redshift are taken into account. In order to test the beamed FR 1 hypothesis for BL Lac objects, the core and extended power levels are determined for these sources and compared with samples of radio-selected BL Lac objects (RBLs) and FR 1 from galaxies from the literature. RBLs and XBLs are found to possess extended (and thus unbeamed) power levels and the largest angular sizes similar to those of the FR 1 galaxies both supporting the beaming hypothesis and a common parent population for XBLs and RBLs.

Perlman, Eric S.↗
NASA NTRS1993

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

We report the recent discovery of ten BL Lac objects in the Einstein IPC Slew Survey by means of X-ray/radio versus X-ray/optical color-color diagrams and confirming optical spectroscopy. These ten BL Lac objects were discovered using a technique proposed by Stocke et al. (1989) which exploits the characteristic broad-band spectra of BL Lac objects. New VLA detections provide accurate fluxes (limiting f(6 cm) about 0.5 mJy) and about 1 arcsec positions, facilitating the determination of an optical counterpart. All ten new BL Lac objects show essentially featureless optical spectra and lie within the range of radio/X-ray/optical colors of known X-ray-selected BL Lac objects, when about 1 mag optical variability is included. We expect about 50 new BL Lac objects in total, from complete optical follow-up of our now completed VLA work and recent Australia Telescope observations, yielding a complete Slew Survey sample of about 90 BL Lac objects.

Schachter, Jonathan F.↗
NASA NTRS1986

Rapid X-ray and optical variability in the X-ray selected BL Lacertae object IE 1402.3 + 0416

Results from X-ray and optical observations of the X-ray-discovered BL Lac object 1E 1402.3 + 0416 are presented, where the X-ray measurements were carried out with the Channel Multiplier Array (CMA) and Medium Energy experiment (ME) detectors on board Exosat. These measurements revealed an intensity decrease by a factor of two on a time scale of a few hours. At maximum flux, the source was significantly greater than at the time of the Einstein Image Proportional Counter (IPC) discovery observation. The 2-6 keV X-ray spectrum was determined by the ME experiment, and IPC, HRI, and CMA data were subsequently compared. The source varied over the years by a factor of five; its brightest state was within the last three years. It is noted that a redshift greater than 0.2 would require that anisotropic emission mechanisms be invoked.

Giommi, P.↗
NASA NTRS1981

The detection of X-ray emission from the BL Lacertae object Markarian 180

The detection of X-ray emission from Mrk 180, an elliptical galaxy which has a BL Lac object embedded in its core is reported. The X-ray observations were made by the Einstein and HEAO 1 satellites. In addition, new multiaperture UBVR photometric observations are presented which were used to separate the nonthermal optical emission from the galactic emission. Analysis of the Einstein data suggests that the continuous X-ray spectrum of Mrk 180 is a power law with an energy spectral index of -1.3. The deconvolution of the optical magnitudes is consistent with a nonthermal source having a spectral index of -1.2. The continuous spectrum of Mrk 180 is steep from the X-ray to the optical and flattens out in the radio.

Mufson, S. L.↗
NASA NTRS1984

BL Lacertae objects: Multifrequency observations and model applications

Quasi-simultaneous spectra covering radio, IR, optical and UV wave bands of BL Lac and active galactic nuclei were obtained. The X-ray flux densities from other epochs, and information on temporal behavior in the various wave bands, are used in model fitting. The required amount of relativistic beaming in these sources is calculated as 30.

Worrall, D. M.↗
NASA NTRS1991

A ubiquitous absorption feature in the X-ray spectra of BL Lacertae objects

The paper presents the broadband (0.5-20-keV) X-ray spectra of five X-ray bright BL Lac objects observed with the Einstein Observatory Solid State Spectrometer (SSS) and Monitor Proportional Counter (MPC) detectors. The combination of moderate energy resolution and broad spectral coverage makes it possible to confirm the presence of an absorption feature at an energy of 650 eV in the BL Lac object PKS 2155-304, originally reported by Canizares and Kruper (1984) based on higher resolution Einstein Objective Grating Spectrometer (OGS) data.

Madejski, Greg M.↗
NASA NTRS1992

The BL Lacertae object PKS 1413 + 135 - Is it within or behind a spiral galaxy?

New observations of the very red quasar or BL Lac object PKS 1413 + 135 are presented. Optical imaging and spectroscopy confirm the claim that the host galaxy to this BL Lac is a spiral, and H-band imaging shows the point source centered in the galaxy to within 0.1 arcsec. A reanalysis of Einstein X-ray data finds evidence for an extremely large column density in this source. While the astrometric evidence supports the hypothesis that PKS 1413 + 135 is the very unusual case of a BL Lac object in a spiral galaxy, a highly extincted nuclear source would produce other observable features in this galaxy that are not observed: narrow emission-line region and large thermal IR flux. Their absence argues for this source being background to the spiral.

Stocke, John T.↗
NASA NTRS1995

Multiwavelength monitoring of the BL Lacertae object PKS 2155-304. 3: Ground-based observations in 1991 November

We present ground-based observations of the BL Lac object PKS 2155-304 during 1991 November. These data were obtained as part of a large international campaign of observations spanning the electro-magnetic spectrum from the radio waves to the X-rays. The data presented here include radio and UBVRI fluxes, as well as optical polarimetry. The U to I data show the same behavior in all bands and that only upper limits to any lag can be deduced from the cross-correlation of the light curves. The spectral slope in the U-I domain remained constant on all epochs but 2. There is no correlation between changes in the spectral slope and large variations in the total or polarized flux. The radio flux variations did not follow the same pattern of variability as the optical and infrared fluxes. The polarized flux varied by a larger factor than the total flux. The variations of the polarized flux are poorly correlated with those of the total flux in the optical (and hence UV domain; see the accompanying paper by Edelson et al.) nor with those of the soft X-rays. We conclude that the variability of PKS 2155-304 in the optical and near-infrared spectral domains are easier to understand in the context of variable geometry or bulk Lorentz factor than of variable electron acceleration and cooling rates.

Courvoisier, T. J.-L.↗
NASA NTRS1982

PKS 2155-304 - Relativistically beamed synchrotron radiation from a BL Lacertae object

The previously reported variability by a factor of two in less than a day is confirmed for the newly discovered BL Lac object PKS 2155-304, by means of the HEAO 1 A-2 experiment's medium- and high-energy detectors. An interpretation of the overall spectrum from radio through X-rays in terms of a synchrotron self-Compton model yields a good description of the data if relativistic beaming is allowed for. The presence of jets is inferred without their direct observation through consideration of the spectrum, combined with an estimate of the size of the source.

Urry, C. M.↗
NASA NTRS1983

Studies of BL Lacertae objects with the Einstein X-ray observatory - The absolute volume density

It is estimated that the local volume density of BL Lac objects which emit greater than 10 to the 44th ergs/s in the 0.5-4.5 keV band is about 10 to the -7th/cu Mpc. This is based on identifications of bright X-ray sources, and the fact that Einstein X-ray observations detect BL Lac objects over a wide, continuous range of flux up to the thresholds of the all-sky X-ray surveys. Since this density is of the same order of magnitude as the local density of Seyfert galaxies, or local density of optically selected quasars, emitting X-rays at the same luminosity, we argue that the difference between these particular astronomical classifications is not solely due to the orientation of a relativistic beam.

Schwartz, D. A.↗
NASA NTRS1991

Rapid ultraviolet variability in the BL Lacertae object PKS 2155 - 304

This letter reports the first clear evidence of a rapid change in the ultraviolet flux of a BL Lac object. Sophisticated analysis of 57 closely spaced IUE spectra yielded clear evidence of an about 12 percent drop in the ultraviolet flux of PKS 2155 - 304 over a 2 day period in October 1989 and a strong indication of a similar drop in a 5 hr period some three weeks later. These are near the fastest variations which could have been detected with these data, but they are still much slower (or weaker) variations than have been observed in the X-rays. However, they are orders of magnitude fater than Seyfert 1 ultraviolet variations. This suggests that a 'standard' Seyfert 1 accretion disk may not be responsible for the ultraviolet emission from PKS 2155 - 304.

Edelson, R. A.↗