Einstein Slew Survey Research Articles

Gamma-Ray and Multiwavelength Emission from Blazars

Blazars are now well understood as approaching relativistic jets aligned with the line of sight. The long-time uncertainty about the demographics of blazars is starting to become clearer: since the Fermi blazar sample includes a larger fraction of high-frequency peaked blazars (like the typical X-ray-selected blazars in, say, the Einstein Slew Survey sample) than did the higher-flux-limit EGRET blazar sample, these low-luminosity sources must be more common than their higher luminosity, low-frequency-peaked cousins. Blazar spectral energy distributions have a characteristic two-component form, with synchrotron radiation at radio through optical (UV, X-ray) frequencies and gamma-rays from X-ray through GeV (TeV) energies. Multiwavelength monitoring has suggested that gamma-ray flares can result from acceleration of electrons at shocks in the jet, and there appears to be an association between the creation of outflowing superluminal radio components in VLBI maps and the gamma-ray flares. In many cases, the gamma-ray emission is produced by inverse Compton upscattering of ambient optical-UV photons, although the contribution from energetic hadrons cannot be ruled out. The next few years of coordinated gamma-ray, X-ray, UV, optical, infrared and radio monitoring of blazars will be important for characterizing jet content, structure, and total power.

Observational evidences of multiple shock waves in X-ray selected BL Lacerate objects

The results of optical R-band photometry of three X-ray selected BL Lacertae objects 1ES 0502+675, 1ES 0806+524 and 1ES 1959+650 from the Einstein Slew Survey are presented. The observations are performed during 1997–2006 with the 70-cm meniscus type telescope of Abastumani Astrophysical Observatory. The objects show clear long-term variability with time scales of 1–3 years without evident periodicity. The main results consist in the discovery of multiple-peak structures on historical light curves as predicted theoretically on the basis of the assumption that long-term variability of blazars are triggered by ultrarelativistic shock waves propagating through their jets. They are direct evidence of the existence of reverse shocks besides the main one arising by a single disturbance. Two-peak maxima are found for 1ES 0502+675 and 1ES 0806+524. The more complicated structure shows 1ES 199+650 - a maximum with four consequent peaks. The relative strengths of the main and reverse shocks are mainly equal according to the shape of the respective peaks. The brightness dip between them is on average 60% less than in the case of consequent main shocks. Optical maximum epochs, covered well by the observations, show that the main shocks are either not always accompanied by reverse ones or the later are not always strong enough to be discovered by the observations.

Swift XRT and UVOT deep observations of the high-energy peaked BL Lacertae object PKS 0548–322 close to its brightest state

Aims. We observed the high-energy peaked BL Lac object PKS 0548-322 (BZB J0550-3216) with Swift to study the temporal and spectral properties of its synchrotron emission simultaneously in the optical, ultraviolet, and X-ray energy bands. Methods. We carried out a spectral analysis of 5 Swift XRT and UVOT observations of PKS 0548-322 taken over the period April-June 2005. Results. The X-ray flux of this BL Lac source was found to be approximately constant at a level of F (2-10 keV) ≃ 4 × 10 -11 erg cm -2 s -1 which is a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power-law spectral distribution. A log-parabolic model describes the X-ray data well and gives a best-fit curvature parameter of 0.18 and a peak energy in the spectral energy distribution of about 2 keV. The UV spectral data from Swift UVOT join closely with a power-law extrapolation of the soft X-ray data points, suggesting that the same component is responsible for the observed emission in the two bands. The combination of synchrotron peak in the X-ray band and the high-intensity state confirms PKS 0548-322 as a prime target for TeV observations. X-ray monitoring and coordinated TeV campaigns are highly advisable.

Intrinsic Curvature in the X‐Ray Spectra of BL Lacertae Objects

We report results from XMM-Newton observations of 13 X-ray bright BL Lacertae objects, selected from the Einstein Slew Survey sample (SSS). The survey was designed to look for evidence of departures of the X-ray spectra from a simple power-law shape (i.e., curvature and/or line features) and to find objects worthy of deeper study. Our data are generally well fit by power-law models, with three cases having hard (Γ < 2; dN/dE ∝ E-Γ) spectra that indicate synchrotron peaks at E ≳ 5 keV. Previous data had suggested a presence of absorption features in the X-ray spectra of some BL Lac objects. In contrast, none of these spectra show convincing examples of line features in either absorption or emission, suggesting that such features are rare among BL Lac objects, or, more likely, are artifacts caused by instrumental effects. We find significant evidence for intrinsic curvature [steepening by dΓ/d(log E) = 0.4 ± 0.15] in 14 of the 17 X-ray spectra. This cannot be explained satisfactorily via excess absorption, since the curvature is essentially constant from 0.5-6 keV, an observation that is inconsistent with the modest amounts of absorption that would be required. We use the XMM-Newton Optical Monitor data with concurrent radio monitoring to derive broadband spectral energy distributions and peak frequency estimates. From these, we examine models of synchrotron emission and model the spectral curvature we see as the result of episodic particle acceleration.

R Band Photometry of X-Ray Selected BL Lacertae Objects: 1ES 0229+20, 1ES 0323+02, 1ES 0502+67, 1ES 0647+25, 1ES 0806+52, 1ES 0927+50, 1ES 1028+51, 1ES 1959+65 and 1ES 2344+51

We present optical R band photometry for nine X-ray Einstein Slew Survey Sample BL Lacertae objects: 1ES 0323+022, 1ES 0502+675, 1ES 0647+250, 1ES 0806+524, 1ES 1028+511, 1ES 1959+650 and 1ES 2344+514. Two blazars 1ES 0229+200 and 1ES 0927+500 didn't show any detectable variability. They have all been observed with the ST-6 CCD camera attached to the Newtonian focus of the 70-cm meniscus telescope. These data provide optical information on sources that have been rarely observed in the optical band. Variability on long time scales within one magnitude in R band was detected for all of the observed objects except two. The largest variation was observed for 1ES 0502+675 and equals to ΔR=1 m . 07.

BeppoSAX spectral survey of BL Lacs – New spectra and results

We present BeppoSAX LECS, MECS, and PDS spectra of eleven X-ray selected BL Lacertae objects. Combining these sources with the ones presented elsewhere we have a sample of 21 BL Lacs from the Einstein Medium Sensitivity and Einstein Slew Survey. The sample shows strong correlations of several physical parameters with the peak frequency of the synchrotron branch of the spectral energy distribution. In particular the peak frequency is correlated to the X-ray spectral shape: objects with the peak near to the X-ray band show harder and straighter X-ray spectra than those of the low energy peaked sources. This work shows that the recently proposed unification scenario for different types of blazars can hold also within the class of high frequency peaked BL Lac objects.

ITAL]ROSAT[/ITAL] High Resolution Imager Identifications of Suspected Stellar Sources from the [ITAL]Einstein[/ITAL] 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.

Obscuration in the Host Galaxies of Soft X‐Ray–selected Seyfert Nuclei

We define a new sample of 96 low-redshift (z < 0.1), soft X-ray-selected Seyfert galaxies from the catalog of the Einstein Slew Survey. We probe the geometry and column depth of obscuring material in the host-galaxy disks using galactic axial ratios determined mainly from the Digitized Sky Survey. The distribution of host-galaxy axial ratios clearly shows a bias against edge-on spirals, which confirms the existence of a geometrically thick layer of obscuring material in the host-galaxy planes. Soft X-ray selection recovers some of the edge-on objects missed in UV and visible surveys but still results in 30% incompleteness for type 1 nuclei. We speculate that thick rings of obscuring material such as the ones we infer for these Seyfert galaxies might be commonly present in early-type spirals, sitting at the inner Lindblad resonances of the nonaxisymmetric potentials of the host galaxies.

Unifying models for X-ray-selected and radio-selected BL Lac objects

We discuss alternative interpretations of the differences in the Spectral Energy Distributions (SEDs) of BL Lacs found in complete Radio or X-ray surveys. A large body of observations in different bands suggests that the SEDs of BL Lac objects appearing in X-ray surveys differ from those appearing in radio surveys mainly in having a (synchrotron) spectral cut-off (or break) at much higher frequency. In order to explain the different properties of radio and X-ray selected BL Lacs Giommi and Padovani proposed a model based on a common radio luminosity function. At each radio luminosity, objects with high frequency spectral cut-offs are assumed to be a minority. Nevertheless they dominate the X-ray selected population due to the larger X-ray-to-radio-flux ratio. An alternative model explored here (reminiscent of the orientation models previously proposed) is that the X-ray luminosity function is "primary" and that at each X-ray luminosity a minority of objects has larger radio-to-X-ray flux ratio. The predictions of the two scenarios, computed via a Montecarlo technique, are compared with the observed properties of BL Lacs in the two samples extracted respectively from the 1 Jy radio survey and the Einstein Slew Survey. We show that both models can explain a number but not all the observed features. We then propose a completely new approach, based on the idea that the physical parameter which governs the shape of the SEDs, is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity we show that the observational properties of the two surveys can be reproduced at least with the same accuracy as the two previous models.

Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew Survey

Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew Survey

Active galaxies observed during the Extreme Ultraviolet Explorer all-sky survey

We present observations of active galactic nuclei (AGNs) obtained with the Extreme Ultraviolet Explorer (EUVE) during the all-sky survey. A total of 13 sources were detected at a significance of 2.5 sigma or better: seven Seyfert galaxies, five BL Lac objects, and one quasar. The fraction of BL Lac objects is higher in our sample than in hard X-ray surveys but is consistent with the soft X-ray Einstein Slew Survey, indicating that the main reason for the large number of BL Lac objects in the extreme ulktraviolet (EUV) and soft X-ray bands is their steeper X-ray spectra. We show that the number of AGNs observed in both the EUVE and ROSAT Wide Field Camera surveys can readily be explained by modelling the EUV spectra with a simple power law in the case of BL Lac objects and with an additional EUV excess in the case of Seyferts and quasars. Allowing for cold matter absorption in Seyfert galaxy hosts drive up the inferred average continuum slope to 2.0 +/- 0.5 (at 90% confidence), compared to a slope of 1.0 usually found from soft X-ray data. If Seyfert galaxies without EUV excesses form a significant fraction of the population, then the average spectrum of those with bumps should be even steeper. We place a conservative limit on neutral gas in BL Lac objects: N(sub H) less than 10(exp 20)/sq cm.

1ES 1113+432: Luminous, soft X-ray outburst from a nearby cataclysmic variable (AR Ursae Majoris)

A remarkable X-ray transient from the Einstein Slew Survey, 1 ES 1113+432, is identified with a nearby, short-period cataclysmic variable. Wenzel (1993) has confirmed that the optical counterpart is the variable star, AR UMa (cataloged as 'semiregular'), erroneously reported 5.7 min southeast of the true position. One of the Einstein slew observations recorded a flux of 43 IPC counts/s, which is an order of magnitude above the flux observed from the brightest cataclysmic variables in other X-ray surveys. The outburst spectrum is extremely 'soft,' with an implied blackbody temperature of approximately 22 eV. The optical counterpart (V = 16.5) exhibits a strong UV component, TiO bands from an M star, and broadened Balmer emission lines. Optical states as bright as V approx. 13 were found on photographs from the Harvard Plate Library, confirming outburst behavior in the optical counterpart. The historical photographic record suggests that 1ES 1113+432 remains in a low-accretion state most of the time. Both of the soft X-ray spectrum and the transitions between high and low-accretion states are suggestive of the AM Her (magnetic) subclass. Photometric observations in the I band show 0.18 mag modulations at a period of 0.966 hr. These are interpreted as ellipsiodal variations in the secondary star for a binary period of 1.932 hr, which is near the lower boundary of the 'period gap' in the histogram, of orbital periods of accreting white dwarfs. Thus 1ES 1113+432 provides the rare opportunity to study a secondary star in a cataclysmic binary that has evolved through the period gap. The optical spectral features from the secondary imply a spectral type of approximately M6 and a distance of approximately 88 pc. The peak luminosity in the soft X-ray component (unabsorbed) is then estimated to be 3 X 10(exp 33) ergs/s, assuming emission from a blackbody slab with a temperature of 22 eV. While this luminosity is higher than previous measures of the soft X-ray component, it does not exceed the amount of radiation that could be emitted from the accretion-heated surface of a white dwarf.

Einstein X ray observations of the core of the Shapley Supercluster in northern Centaurus

We present Einstein x ray observations of the core of the Shapley Supercluster, one of the richest and densest known mass concentrations in the local (z less than 0.1) universe. We used Imaging Proportional Counter (IPC) observations supplemented with data from the Einstein Slew Survey to determine the locations and structure of mass concentrations in the region. An x ray map composed of IPC observations of the central (10 deg x 10 deg) region of the Shapley Supercluster is presented. We present evidence that the X-ray clusters observed within 5 deg of the core of the supercluster are on average brighter than those of corresponding richness class distributed throughout the sky. However, we measure no significant difference in the galaxy formation efficiency of these cluster of galaxies compared to other, more isolated clusters. We also find one previously uncataloged cluster-sized mass concentration in the core of the Shapley Supercluster. This new cluster, 'SC 1327-312', is relatively x ray bright (F(sub x) = 1.1 + or - 0.2 x 10(exp -11) erg sec(exp -1) cm(exp -2)) and L(sub x) = 1.1 + or - 0.2 x 10(exp 44) erg sec(exp -1) within 10 minutes, assuming z = 0.0477, H(sub 0) = 50, q(sub 0) = 0). As SC 1327-312 lies well within an Abell radius of the richness R = 4 cluster Shapley 8 (A3558), we suggest it may contribute to an artificially high galaxy count and richness classification for shapley 8. From slew data, we estimate an x ray luminosity for Shapley 8 which is just half the mean luminosity of the four other R = 4 clusters observed by the IPC, further suggesting the richness classification to be an overestimate.