The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac objects spanning the intermediate range of spectral energy distributions between “classical” X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evolution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available

Bignall, Hayley

Landt, H.

Padovani, P.

Perlman, E.

Reynolds, C.

English

espace@Curtin

PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL LacobjectsHayley Bignall∗Joint Institute for VLBI in EuropeE-mail: bignall@jive.nlHermine LandtHarvard-Smithsonian Center for AstrophysicsE-mail: hlandt@head.cfa.harvard.eduCormac ReynoldsJoint Institute for VLBI in EuropeE-mail: reynolds@jive.nlPaolo PadovaniEuropean Southern ObservatoryE-mail: Paolo.Padovani@eso.orgEric PerlmanUniversity of MarylandE-mail: perlman@jca.umbc.eduThe recentDeep X-ray Radio Blazar Survey(DXRBS) has identified a sample of BL Lac ob-jects spanning the intermediate range of spectral energy distributions between “classical” X-rayselected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed toanswer some of the currently open questions regarding the nature of BL Lacs and their place ina unified model of AGN. High-resolution radio imaging provides direct information on jet evo-lution and beaming parameters. We present some preliminaryresults from EVN & MERLINobservations of sources in the DXRBS BL Lac sample for which little or no high-resolution radiodata were previously available.8th European VLBI Network SymposiumSeptember 26-29, 2006Toruń, Poland∗Speaker.c© Copyright owned by the author(s) under the terms of the CreativCommons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL Lac objects Hayley Bignall1. IntroductionIt is generally agreed that BL Lacertae objects (BL Lacs) are radio galaxies with their jetsoriented close to our line of sight. Together, BL Lacs (which have weak orbsent optical emissionlines) and flat-spectrum quasars (with strong emission lines) are commonly called blazars. Thereare, however, a number of unresolved questions regarding the parent opulation of BL Lacs, the ori-gin of their different spectral energy distributions (SEDs), and their relationship with flat-spectrumquasars. Radio observations at high angular resolution provide directinformation on the structureand evolution of jets, which is important to help resolve some of the outstanding problems withunified models for AGN.Our current knowledge of BL Lacs is based mainly on studies of two samples: th radio-selected 1-Jy sample[1] and the X-ray selectedEinsteinMedium Sensitivity Survey (EMSS) sample[2].These two surveys, with their widely different selection bands and relativy high flux limits, intro-duced a dichotomy in the class of BL Lacs, with the sources found by each survey having distinctlydifferent properties. The EMSS had no radio selection criteria, and its BLLacs were found to havelow extended radio powers and morphologies typical of Fanaroff-Riley type I (FR I) radio galaxies[3], [4]. For some 1-Jy BL Lacs, however, FR II-like properties have been reported, suggestingthat these sources may be the beamed counterparts of more powerful radio galaxies, and hencemore closely related to flat-spectrum quasars than the X-ray selected BL Lacs [5]–[7]. The EMSSfound only BL Lacs with broad-band emission properties indicative of high-energy (UV/soft X-ray) synchrotron emission peaks, whereas the majority of sources selected by the 1-Jy survey werelow-energy (IR/optical) peaked BL Lacs.It is not yet clear if the observed differences are predominantly due to different viewing anglesand a frequency dependence of relativistic beaming ([8], [9]), or if the jets have a large range ofintrinsic synchrotron emission peak frequencies,νpeak [10]. Finding an answer is relevant for thephysics of jet formation. If orientation plays a role, it would imply that low- andhigh-energysynchrotron emission are produced at different locations in the jet. On theother hand, if BL Lacshave jets with a large range ofνpeak, it would follow that physical properties around black holes inAGN, e.g. maximum electron energy and magnetic field strength, can have a very l rge spread.2. The classification of BL Lacs and the DXRBS BL Lac sampleRecently, the criteria used to classify radio sources as BL Lacs have been qu stioned. Marchãet al. [11] proposed revised criteria for separating blazars (beamed obj cts) from radio galaxies,based on the Ca H&K break (a stellar absorption feature) in the optical spectrum, as well as sep-arating BL Lacs from flat-spectrum quasars based on the equivalent width of emission lines. Theproblem of blazar classification was re-addressed by Landt et al. [12], who argued that all radio-loud AGN can be effectively separated into intrinsically weak- and strong-lined sources based onthe narrow [OIII ] emission line.In order to disentangle the effects of different viewing angles, intrinsic jet power and environ-ment of the AGN host galaxies, all of which influence the observed properties of radio sources, itis important to compare sources which have been selected in a uniform way.Recently, new sam-ples of BL Lacs have been found by cross-correlating existing radio and X-ray source catalogues,2PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL Lac objects Hayley Bignallsearching for compact sources which are then classified based on optical identification. Thesesearches tend to find BL Lacs with properties intermediate between those of thEMSS and 1-Jysamples, providing strong evidence that there is in fact a continuum of BL Lac SEDs, and that theobserved dichotomy is simply a product of selection effects of the earlier surveys. Detailed studiesof intermediate samples are expected to provide a more complete picture of the BLLac class ofobjects and their relation to radio galaxies. One such sample was drawn fromtheDeep X-ray RadioBlazar Survey(DXRBS).The details of the DXRBS sample selection are described in [13] and [14]. Briefly, the samplewas selected by cross-correlating the WGACAT [15] of “serendipitous”X-ray sources detected bythe ROSATPosition Sensitive Proportional Counter (PSPC) with several publicly available radiocatalogues. A radio spectral index cut was applied to reject most steep-sp ctrum sources, sincethe aim of the DXRBS was to find beamed objects, so-called blazars, that aredominated by aflat-spectrum radio core.The majority of DXRBS sources are strong emission-line quasars, but a size ble number – 45sources – are classified as BL Lacs according to the criteria of Marchã et al. [11]. Of the DXRBSBL Lacs, 11 had already been identified in earlier surveys and the rest were newly discovered. TheDXRBS BL Lac sample is comparable in size to the 34 sources of the 1-Jy and 44 sources of theEMSS BL Lac samples. The DXRBS sample contains roughly equal numbers of low- (αrx > 0.78;26 sources) and high-energy peaked BL Lacs (αrx ≤ 0.78; 19 sources).All DXRBS sources classified as BL Lacs have been subsequently observed with the VLA at20 cm in A and C configurations, and with the ATCA for the sources withδ < −40◦ (Landt et al.,2007, in preparation). More sensitive optical spectroscopy is also being undertaken for the sourcesfor which we lack firm redshift measurements.Of the northern DXRBS BL Lacs, 16 sources had little or no VLBI data available in theliterature or archives as of early 2005, so EVN and MERLIN observations were proposed. VLBIobservations are important to investigate relativistic beaming and account for this in comparingBL Lacs with radio galaxies.3. EVN and MERLIN observationsCombined EVN and MERLIN observations were undertaken during 3-5 June 2005. The 16 tar-get sources were observed in a 48-hr period (one additional sourcewas in the target list, but was notscheduled due to its position being close to the Sun in early June). The observations were carriedout at 1.6 GHz, with the VLBI data recorded at 128 Mbps in two 8-MHz subbands. Full polariza-tion information was obtained, but only total intensity data are presented here. Phase-referencingwas used for 11 of the target sources, which either had arcsecond positional uncertainties or thepeak flux density in VLA A-conf. data was substantially lower than 100 mJy. Initial processing ofMERLIN data was done with local software at Jodrell Bank Observatory, and standard proceduresin AIPS were used for subsequent processing of both MERLIN and EVN data.The flux density scale of the MERLIN data is tied to 3C286, which is also used tocalibratethe linear polarization position angle. The flux density of OQ208 in our MERLIN data was foundto be 1.14 Jy, and MERLIN data for all other sources were calibrated based on this value. TheEVN data were calibrated independently based on measured gain curves and system temperatures.3PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL Lac objects Hayley BignallX-ray catalogue name Redshift SpeakVLA StotMER SpeakMER StotEVN SpeakEVN[mJy/beam] [mJy] [mJy/beam] [mJy] [mJy/beam]WGAJ0245+1047 0.070 29 36 35 33 24WGAJ0313+4115 0.029 55 97 44 41 19WGAJ0558+5328 0.036 297 296 95 39 22WGAJ0847+1133 0.199 34 21 11 11RXJ09168+523 0.190 58 60 52 49 29WGAJ0940+2603 ... 372 427 355WGA1012+06 0.727 300 115 97 69WGA1202+44 ... 61 50 34 25WGAJ1204−0710 0.185 140 99 67 48WGAJ1231+2848 1.000 86 94 78 65 32WGAJ1311−0521 0.160 28 18 18WGAJ1320+0140 1.235 497 471 138WGAJ1539−0658 ... 44 63 62 55 48WGAJ1744−0517 0.310 14 11 11EXO1811+3143 0.117 103 117 105 91 80WGAJ1840+5452 0.646 102 114 88 95 82Table 1: The DXRBS BL Lacs observed with EVN and MERLIN at 1.6 GHz. Peakand total flux densitiesfrom preliminary analysis of EVN and MERLIN data are shown, along with source redshifts and peak fluxdensities from VLA A configuration data at 1.4 GHz (where available).No combined calibration or analysis of EVN and MERLIN data has yet been done for the datapresented here.The observed sources (listed here by their associated X-ray cataloguec unterparts), redshifts,and peak flux densities (where available) from 1.4-GHz VLA data in A configuration are listedin Table 1, along with peak and total flux densities from MERLIN and EVN images at 1.6 GHz.The EVN and MERLIN results presented are based on preliminary images made in Difmap, usinguniform weighting, and iterativeCLEAN and self-calibration. All sources are clearly detected. Notethat the VLA data, as well as being at a slightly lower frequency, are not simultaneous with theEVN+MERLIN data and the sources may be variable. Although the EVN and MERLIN results arevery preliminary, the values in Table 1 give an indication of the distribution of emission on differentangular scales. There is clearly a wide variation in radio properties of the different sources withinthis sample. The typical angular resolution for the data from each array is 1–2 arcseconds for theVLA in A configuration, 0.1–0.2 arcseconds for MERLIN, and∼ 3 mas for the EVN. All of thedata now available will allow us to investigate source structure from milliarcsecond to arcminuteangular scales, corresponding to a range from parsec scales out to hundreds of kpc (depending onsource redshift), where often large changes in jet orientation and collimation re observed as thejet evolves and interacts with the surrounding medium.VLA images for one source in the sample, RX J0916.8+5238, are displayedin Figure 1 andpreliminary EVN and MERLIN images are displayed in Figure 2. This object had already been4PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL Lac objects Hayley BignallARC SECARC SEC40 30 20 10 0 -10 -20 -30 -40403020100-10-20-30-40RXJ09168+523Figure 1: RX J0916.8+5238 as observed with the VLA in C configuration (left) and A configuration (right)at 1.4 GHz. The residual rms for each image isσ = 0.06 mJy/beam andσ = 0.02 mJy/beam, respectively,and contours are shown at (-1,1,√2n)×3σ , wheren = 1,2,3... up to the maximum in the image. The peakvalues are 69 mJy/beam for the left-hand image and 58 mJy/beam for the right-hand image.Figure 2: RX J0916.8+5238 as observed with MERLIN (left) and the EVN (right) at 1.6 GHz. The MERLINimage peak is 52 mJy/beam, and contours are shown at(1,2,4,8,16,32,64)× 0.3 mJy/beam. The EVNimage peak is 29 mJy/beam, and contours are shown at(−1,1,2,4,18,16)×1 mJy/beam. The residual rmsfor these preliminary images is∼ 0.2 mJy/beam.identified as a BL Lac at redshiftz= 0.19 from theROSATAll-Sky Survey by Nass et al. [16] priorto the DXRBS. Assuming a WMAP-based cosmology, an angular scale of 1 arcsecond translatesto 3 kpc at the source. The images show the complex extended structure andlarge misalignment ofthe jet in this case between pc and kpc scales.4. SummaryWe have observed 16 BL Lac objects identified in theDeep X-ray Radio Blazar Survey(DXRBS)with the EVN and MERLIN at 18 cm. Combined with 20-cm VLA data and other avail ble VLBIdata for the entire sample, these data will be useful for investigating the radioproperties of differenttypes of BL Lacs selected in a uniform way and spanning the intermediate rang between high- andlow-energy peaked BL Lacs.5PoS(8thEVN)026EVN & MERLIN studies of a new sample of BL Lac objects Hayley BignallAcknowledgmentsThe European VLBI Network is a joint facility of European, Chinese, South African andother radio astronomy institutes funded by their national research councils. MERLIN is a NationalFacility operated by the University of Manchester at Jodrell Bank Observatory on behalf of PPARC.The VLA is operated by the National Radio Astronomy Observatory, a facilityof the NationalScience Foundation operated under cooperative agreement by Associated Universities, Inc.References[1] M. Stickel, J.W. Fried, H. Kuehr, P. Padovani and C.M. Urry, The optical identification status of the 1Jy radio source catalogue, A&AS374(1991) 211.[2] S.L. Morris et al.,The luminosity function and cosmological evolution of X-ray selected BL Lacertaeobjects, ApJ380(1991) 49.[3] E.S. Perlman and J.T. Stocke,The radio structure source of X-ray-selected BL Lacertae obj cts, ApJ406(1993) 430.[4] T.A. Rector, J.T. Stocke, E.S. Perlman, S.L. Morris and I.M. Gioia,The properties of theX-ray-selected EMSS sample of BL Lacertae objects, AJ 120(2000) 1626.[5] R.T. Kollgaard, J.F.C. Wardle, D.H. Roberts and D.C. 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Perlman and P. Giommi,A physical classification scheme for blazars,MNRAS351(2004) 83.[13] E.S. Perlman et al.,The Deep X-ray Radio Blazar Survey. I. Methods and first results, AJ 115(1998)1253.[14] H. Landt, P. Padovani, E.S. Perlman, P. Giommi, H. Bignall and A. Tzioumis,The Deep X-ray RadioBlazar Survey (DXRBS) - II. New identifications, MNRAS323(2001) 757.[15] N.E. White, P. Giommi and L. Angelini,The WGA catalog of ROSAT point sources, BAAS(1994)1372 [http://wgacat.gsfc.nasa.gov/wgacat/wgacat.html][16] P. Nass, N. Bade, R.I. Kollgaard, S.A. Laurent-Muehleisen, D. Reimers and W. Voges,BL Lacertaeobjects in the ROSAT All-Sky Survey: new objects and comparison of different search techniques.,A&A 309(1996) 419.6

EVN & MERLIN studies of a new sample of BL Lac objects

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EVN & MERLIN studies of a new sample of BL Lac objects

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