Optical follow-up of BL Lac candidates in the 2BL sample

0.23 MB·English

by  R. Nesci

Astronomy&Astrophysicsmanuscriptno.NN2blfinref February2,2008
(DOI:willbeinsertedbyhandlater)
Optical follow-up of BL Lac candidates in the 2BL sample
R.Nesci,S.SclaviandE.Massaro
DipartimentodiFisica,Universita´LaSapienza,P.leAldoMoro2,I-00185Roma
e-mail:[email protected]
5 ReceivedNovember2004;accepted......
0
0
Abstract. WeinvestigatethenatureoftheBLLaccandidatesinthenorthernpartofthe2BLsample(Londishetal.(2002))
2
searchingforopticalvariabilitybymeansofR bandphotometrywiththeAsiago1.8andLoiano1.5metertelescopesduring
C
n years2002,2003and2004.Wealsomadeaperturephotometryofthecandidatesontheplatesavailableonthewebfromthe
a PalomarandUKSchmidttelescopes.Nosignificantvariabilitywasdetectedforthemajorityoftheobjectswithoutdetected
J radio and/or X-ray emissions, while clear variability was found for the few sources of the sample with strong radio and/or

3 X-rayemission,whichisaconstantfeatureofthe”classical”BLLacs.Someconsiderationsonthenatureofthe2BLsources
2 aremade,takingintoaccounttheirmostrecentdataavailableinpropermotionscatalogues(SuperCOSMOS,USNO-B1)and

intheSloanDigitalSkySurvey.
1
v
Keywords.galaxies:active–galaxies:BLLacertaeobjects:general
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1 1. Introduction fined as “featureless” if the equivalentwidthsof the emission
0 lines were less than 5 Å and/or the CaII break was less than
5 The history of the BL Lacertae objects started in 1968
0.25 (Stockeetal.1991). This definition was later relaxed by
0 (Schmitt1968) with the discovery of the radio emission
Marcha˜etal.(1996)whoincludedintheBLLacclassalsoob-
/
h from the prototype of the class, the variable “star” BL Lac. jects with strongerfeatures(e.g.CaII break≤ 0.40).A recent
p A few years later, after that some other similar objects
discussionofthesecriteriawasmadebyLandtetal.2004,who
- have been found (OJ 287, W Com, ON 325 and AP Lib),
o pointedoutthattherelativeintensityoftheAGNandofthehost
r Strittmatteretal.1972 made the first definition of the class. galaxyisakeyfactorfortheopticallineequivalentwidth.
t SubsequentobservationswithX-raysatellitesshowedthatBL
s
a Lac objects were also remarkableX-ray sources. The first at- Recently Londishetal.(2002) defined the first “optically
:
v temptstoderiveasampleofBLLacsstartingfromcatalogues selected” sample of BL Lac candidates, the 2BL sample, as
i ofopticallyvariabilesourceswereunfruitful,whilethosebased a by-productof the 2QZ sample of QSO (Croometal.2001).
X
onFlatSpectrumRadioSourcesprovidedsoonagooddealof Twostripsofskyweresurveyedbythe2QZproject:onecen-
r
a objects,indicatingthattheradioemissionwasabetterdiscrim- teredon thecelestial equatorandoneat declination−30.The
inant. Severalsamples of BL Lac objectshave been collected selection of the QSO candidates was made on UK Schmidt
up to now,using differentselection criteria (e.g. 1 Jy sample, photographic plates scanned with the APM, searching for
Stickeletal.1991; Extended Medium Sensitivity Survey, point-likesourceswithphotographic(u′−b)and(b−r)Quasar-
Morrisetal.1991, Wolteretal.1994; Rosat-Greenbank like colours. Spectra for these candidates were then obtained
survey, Laurent-Muehleisenetal.1999; Sedentary Survey, with the 2dF fiber spectrograph mounted on the 3.9 m AAT
Giommietal.1999, DXRBS, Landtetal.2001; HRX-BL telescope.BLLaccandidateswereselectedfromthesespectra,
Lac survey, Beckmannetal.2003). In all these samples, the lookingforobjectsinthemagnituderange(19.97≥ b ≥18.25)
presence of a non thermal radio or/and X-ray emission was and appearing featureless. High proper motion sources were
required. thenrejectedasprobableDCwhitedwarfs.
Nowadays,BL Lacobjectsare considereda subsampleof
the population of the Active Galactic Nuclei, and are gener- Theremarkablefeatureofthissampleisthatthelargema-
allydefinedassourceswithfeaturelessopticalspectra,strong jority of the sources (47 out of 56) have no radio emission
andfastvariabilityintheradio,opticalandX-raybands,strong at the NVSS (Condonetal.1998) sensitivity level (2.5 mJy)
and variable radio and X-ray emission and often with a high andthatonly5sourceshavebeendetectedontheROSATAll
degree of polarization. The precise definition of the BL Lac SkySurvey(RASS;Vogesetal.1999,Vogesetal.2000),allof
class is still a matter of debate: the optical spectrum was de- thembeingradioloud.OnlythreeoftheseRASSsourceswere
alreadyreportedintheliteratureasBLLacs.ThelackofX-ray
Sendoffprintrequeststo:R.Nesci andRadioemissioninthelargemajorityofthe2BLsourcesis
2 R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample
puzzling,becauseitimpliesanemission modelsomehowdif- Table 1. Observation log with the Asiago and Loiano tele-
ferentfromthatgenerallyacceptedforBLLacs. scopes.
The Spectral Energy Distribution (SED) of BL Lacs is
indeed well represented, in the Log(ν)-Log(νF ) plane, by a
ν
Year Month,Day Observatory
double-bell shape. The first bump may be well described by
2002 February11,18 Asiago
alog-paraboliclaw(Landauetal.1986;Massaroetal.2004a)
2002 March7-10 Loiano
and can be explained by a synchrotron radiation from rela-
2002 March19 Asiago
tivistic electrons moving in a jet aligned along our line of
2002 May13 Asiago
sight; the second one, from X to γ-ray energies, is presently 2003 February24 Asiago
less well constrained in shape by the observations and is 2003 March22-25 Loiano
generally attributed to Inverse Compton scattering of low- 2003 April23 Asiago
energy photons on the same relativistic electrons. According 2004 February14-15 Asiago
to the peak frequency of the first bump, BL Lac objects 2004 February20-24 Loiano
are further subdivided into HBL (high-energy peaked) and 2004 March23-24 Loiano
2004 March29-30 Asiago
LBL(low-energypeaked),followingtheschemeintroducedby
2004 April26-29 Asiago
Giommi&Padovani(1994).
The faintness (actually upper limits) of the radio/optical
andX-ray/opticalratiosofthe2BLobjectsmeansthatthelow-
In October 2003 Londish revised the 2BL sample in her
energy bump of the SED is much narrower than that of the
PhD thesis (Londish(2003)) using, for a number of sources,
“typical” BL Lacs. There is however no doubt about the ex-
updatedproper motionsfrom the SuperCOSMOSsky survey,
tragalactic nature of a numberof 2BL sources: some of them
5 band photometry from the Sloan Digital Sky Survey Early
have indeed a definite redshiftfrom faintemission or absorp-
tion lines. The most notable case is 2QZ J215454.3−305654 Data Release (Stoughtonetal.2002), her own JHK and opti-
cal photometry.These data allowed to recognizea numberof
(Londishetal.2004), a radio- and X-quiet source which ap-
candidatesintheformer(Londishetal.(2002))listasprobable
peared featureless in the 2dF spectrum but showed a redshift
white dwarfs (from proper motion, variability or black-body
z=0.494inahighS/NspectrumobtainedwiththeESO-VLT.
spectralenergydistribution).We presentanywayinthispaper
Athermaloriginforthebulkoftheemission(e.g.fromanac-
our observational results for all the sources monitored by us,
cretiondisk) ratherthanfroma synchrotronprocessmightbe
evenifsomeofthemaremostlikelystellar.
anexplanation,assuggestedbyLondishetal.2004:thisraises
howeverthequestionwhethersuchsourcescanstillbeproperly
classifiedasBLLacs. 2. Observations
To further explore the characteristics of the 2BL candi-
2.1.CCDobservations
dates we performed a search for optical variability, which
is one of the defining features of the class. In the optical The 2BL sample (Londishetal.(2002)) consists of 56 ob-
range the LBL sources show a large variability (3-4 mag- jects. We found that two sources in the equatorial strip, 2QZ
nitudes), while HBL sources show a moderate variability J105534.36−012617 and 2QZ J121834.8−011955, both de-
range (about half magnitude, see e.g. Heidt&Wagner1996; tectedintheNVSS,werealreadyclassifiedasBLLacobjects.
Kurtanidze&Nikolashvili2002; Marchilietal.2004). It is, TheformerwasidentifiedbecausedetectedintheRASSwitha
however,unlikelythatthe2BLcandidatesareoftheHBLtype: fluxof3.3×10−13ergs−1cm−2 (Appenzelleretal.1998)while
actuallytheirplotintheradio-opticalandoptical-Xspectralin- thelatterwasknownastheopticalcounterpartofaradiosource
dexplane(αRO−αOX,Londishetal.(2002)fig.13)showsthat inthePKScatalogue(Condonetal.1976).
all the X-ray undetectedsourceshave αOX ≥ 1.0, while HBL Our follow-up observations were made during the years
sources have αOX ≤ 0.9 (Beckmannetal.2003). We expect 2002, 2003 and 2004 in the RC band with the Asiago (1.8
therefore to detect a variation of the order of at least several m)andLoiano(1.5m)telescopes,respectivelyequippedwith
tenths of magnitude if the 2BL candidates behave like “nor- theAFOSCandBFOSCinstruments,coveringafieldofview
mal”LBLs. slightlylargerthan8arcmin.SomeAsiagoobservationswere
In this paper we report the results of our optical observa- madeinservicemode.Weatherconditionsdidnotpermitauni-
tions, covering a time interval of three years (2002-2004),of formtimesamplingforalltheobjects.Table1reportsthedates
the 2BL sources well observable from northern mid-latitudes oftheobservingruns.
(the equatorial 2BL sample). Furthermore we retrieved from Twentynineoutthe31sourcesintheequatorialstripwere
on-line archives (ESO, STScI) a number of the Palomar and observed. The typical integration time for the esposures was
UK-ESOSchmidtsurveyplatesandderivedphotographicred 10 minutes; two exposureswere made for the fainter sources
magnitudestoexplorethehistoricalbehaviourofthesesources. and then averaged. The three “radio detected” candidates
Finally, we considered the Sloan Digital Sky Survey Data of the equatorial strip (i.e. 2QZ J105534.36−012617, 2QZ
Release 2 (SDSS-DR2,http://www.sdss.org/dr2/) photometric J121834.88−011956and2QZ J142526.20−011826)were ini-
andpropermotiondata,recentlyavailablesforallthesources tially not included in the monitoring list so our sampling for
oftheequatorialsample. them is poorer. Two sources, 2QZ J114221.4−014812 (red-
R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample 3
shiftz=1.276)and2QZJ114327.3−005050(redshiftz=1.591;
Londishetal.(2002)),wereneverobserved.
DifferentialphotometryoftheBLLaccandidateswasper-
formedusingasetofabouttenstarsinthefieldofeachsource,
spanningarangeoftwoorthreemagnitudesaroundtheobject
of interest. These were selected so that 5 stars were nearly as
faintasthetarget,toestimatetheactualphotometricaccuracy
achievable(checkstars), while the brighteroneswere usedto
putonacommonscaletheinstrumentalmagnitudesfromdif-
ferentruns(referencestars).Thezeropointofthemagnitudes
for each field has been established adopting the GSC2 mag-
nitude for the brightest reference star. A table of the adopted
reference stars and magnitudes is given in Appendix (Table
A1). For each frame a photometric radius equal to the aver-
ageFWHMofthereferencestarswasused.Aperturephotom-
etrywascarriedoutusingtheDAOPHOTroutineoftheIRAF
package.All the 2BL sourcesshoweda point-likePSF in our
images.
During the consistency check between the instrumental
magnitudes obtained with the two telescopes, an appreciable
systematic differencewas foundfor a few stars: inspection of
their colours using the public SDSS magnitudes showed that
this difference was present only for “red” star (SDSS r −i ≥ Fig.1.Thermsdeviationofthefaintreferencestarsasafunc-
0.7). These stars were then discarded for the magnitude zero tion of instrumental magnitude for the Loiano (squares) and
point definition in the final data reduction. In no field did we Asiago (crosses)data. The continuousline marksthe adopted
findmorethanone“red”star.Thetypicalaverageaccuracyof boundarybetweentruevariabilityandphotometricuncertainty
ourmagnitudesasafunctionofastarluminositycanbederived (seetext).
fromFig.1,wherethermsdeviationoftheinstrumentalmag-
nitudes for each check star of all our fields is plotted against
itsaveragemagnitude,separatelyfortheAsiago(crosses)and tal magnitudes rather than to intrinsic source variability. We
Loiano(squares)datasets.Thegeneraltrendofthisplotisofa applied thereforea systematic correctionof −0.04mag to the
largerdispersionforfainterstars,asexpected.Somestarshow- Asiago(thelessnumerous)dataquotedinTable2beforemerg-
ever show a very large dispersion, so we considered them as ingthetwodatasets.
variablesanddiscardedthemascheckstarsinthe dataanaly- Asecondconsiderationisthatgenerallythermsdeviations
sis. of both datasets are smaller than the borderline derived from
To discriminatebetween true variabilityof a BL Lac can- thecomparisonstars,highlysuggestiveofalimited,ifany,op-
didate and photometric uncertainty we binned the magnitude tical variability, at variance with the expectations for BL Lac
values of the check stars at steps of 0.4 mag and computed objects.
for each bin the average rms value and the dispersion of the The resulting lightcurves are shown in Figs 2 to 6 where
data around this average. Then we assumed as upper limit of weplotforeachobjectourmagnitudesanderrorestimates.The
the”statistical”deviationsforeachbinitsaveragermsplusthe averagevalueisshownasadashedline,whiletherange(±1σ)
dispersion.A smoothline approximatingtheseupperlimitsis of the distribution of the magnitudesof each source is shown
showninFig.1andwasadoptedastheborder-linefordetect- bythedottedlines.
ingvariability. Wefoundlargevariabilityonlyfortwosources:1218−01,
Table 2givesthesourcename,thephotographicredmag- aradioloud(notX-raydetected)source,and1106+00,which
nitude derived from the b and (b − r) values published by hasa hot(32000K) thermalspectrumaccordingto theSDSS
Londishetal.(2002),theaverageR magnitude,rmsdeviation photometry (Londish(2003)). The latter source is very faint
C
andnumberofobservationsfortheLoianoandAsiagodatasets (R≃20.3,byfarthefaintestofoursample),andourphotome-
andtheborder-linevalueofthephotometricuncertainty. tryisratherinaccurateatthisfluxlevel(expectedrms=0.14)
A first consideration from the data of Table 2 is that, for soweareunsureofitsvariability.
the large majority of the sources, the average magnitudes in Acomparisonofourdatawiththephotographic2QZrval-
theAsiagoandLoianodatasetsareverysimilar,buttheLoiano ues in Londishetal.(2002) (column 3 of our Table 2) is not
valuesaresystematicallyslightlybrighter,onaverageby0.04 straightforwardbecauseofthepossiblepresenceofsystematic
mag. Given that the Loiano and Asiago observations were effects in the magnitude scale for each field, and to the lim-
madeeachyeartypicallyatanintervalofaboutonemonth(see itedaccuracyofphotographicplatesatthesemagnitudes.Both
Table 1, it is unlikelythat we observedour sourcesalways in APM and GSC2 magnitudes have a formal error of 0.3 mag,
abrighterstateatonetelescope,soweattributethisdifference whichmaybeevenlargeratr≃19.5.Wehavecheckedforour
mainly to the presence of a colourequation in the instrumen- “bright” (R ≤ 18.5) reference stars the consistency between
4 R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample
Table2.ObservedmeanR magnitudesandrmsdeviationsforourLoianoandAsiagoobservations.Thelastcolumnreportthe
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fiducialborderlinevalue,discussedinthetext,foreachsource.
Sourcename 2QZr Loiano rms N Asiago rms N border-line
2QZJ100253.2−001728 19.03 19.03 0.04 4 19.05 0.03 8 0.05
2QZJ102615.3−000630 19.17 19.04 0.02 6 19.03 0.03 5 0.05
2QZJ103607.4+015658 18.57 18.51 0.03 5 18.51 0.03 5 0.04
2QZJ104519.7+002615 18.43 18.43 0.02 5 18.50 0.02 4 0.04
2QZJ105355.1−005538 19.09 19.23 0.07 7 19.29 0.04 5 0.06
2QZJ105534.3−012617 18.50 18.09 0.01 2 18.12 0.05 4 0.03
2QZJ110644.5+000717 >20.93 20.33 0.25 5 20.39 0.05 6 0.14
2QZJ113413.4+001041 18.25 18.18 0.02 7 18.26 0.05 4 0.03
2QZJ113900.5−020140 18.76 18.72 0.06 5 18.74 0.03 4 0.04
2QZJ114010.5−002936 19.97 19.88 0.09 9 19.96 0.07 6 0.12
2QZJ114137.1−002730 18.71 18.85 0.06 7 18.95 - 1 0.05
2QZJ114521.6−024758 18.53 18.46 0.01 5 18.50 0.01 3 0.04
2QZJ114554.8+001023 19.46 19.45 0.03 6 19.48 0.04 2 0.08
2QZJ115909.6−024534 18.77 18.68 0.04 5 18.74 0.05 3 0.04
2QZJ120015.3+000552 18.63 18.58 0.02 5 18.64 0.02 2 0.04
2QZJ120558.1−004216 18.46 18.62 0.02 5 18.63 0.01 3 0.04
2QZJ120801.8−004219 18.86 18.90 0.02 4 18.96 0.01 2 0.05
2QZJ121834.8−011955 18.25 16.60 0.16 5 16.25 0.05 2 0.01
2QZJ122338.0−015619 18.85 18.96 0.01 5 19.04 0.03 1 0.05
2QZJ123437.6−012953 18.97 18.73 0.05 5 18.76 0.02 1 0.04
2QZJ125435.7−011822 18.81 18.87 0.04 4 0.04
2QZJ130009.9−022601 18.67 18.77 0.03 6 18.78 0.01 3 0.04
2QZJ131635.1−002810 19.53 19.44 0.03 6 0.08
2QZJ132811.5+000227 19.79 19.71 0.09 5 19.77 0.05 3 0.10
2QZJ140021.0+001955 19.30 19.53 0.07 8 19.51 0.03 3 0.08
2QZJ140207.7−013033 20.14 19.73 0.08 8 19.87 0.05 4 0.10
2QZJ140916.3−000012 18.47 18.41 0.02 8 18.45 0.01 2 0.04
2QZJ141040.2−023020 19.27 18.68 0.01 6 18.73 0.03 6 0.04
2QZJ142526.2−011826 19.47 19.24 0.01 2 19.31 0.01 2 0.08
GSC2 and our CCD magnitudes, finding always agreement same reference stars selected for the CCD frames: the photo-
withintheformalaccuracyoftheGSC2.Consistencybetween metric radius was taken equal to the average FWHM of the
ourmagnitudesandthosegivenintheon-lineAPMcatalogue bright reference stars. Conversion from photographic magni-
(http://www.ast.cam.ac.uk/∼apmcat/)wasgood(i.e.withinthe tudestoourCCDR magnitudescalewasmadebuildingacal-
C
formalerror)forstarsbrighterthanR∼19,whilestrongdiffer- ibrationcurveforeachimagebymeansofthereferencestars:
enceswerefoundforthefaintersources.Wedecidedtherefore theCCDmagnitudeofthe2BLobjectwasdeterminedbyinter-
to consider as real only a difference greater than 0.3 mag be- polationbetweenthenearestexperimentalpointsofthiscurve
tweenourvaluesandthosederivedfromLondishetal.(2002). and it had an uncertainty of about 0.2 magnitudes. Only in a
Using the 2QZ r photographic data, the variability of fewplateswastheobjectfoundtobeapparentlylowerthanour
1218−01 is confirmed and two more sources, 1055−01 (also faintestreferencestar:inthiscaseswe extendedourCCD se-
with a NVSS radio detection) and 1410−02(radio quiet), are quencetoencompasstheapparentmagnitudeofthecandidate
found to be varied. The variability of 1234−01, 1400+00, BLLacontheplate.
1402−01 and 1425−01 is also possible, while we cannot get The historical (in our CCD-R scale) magnitudes so de-
C
any firm conclusionregarding1106+00because only a lower rivedarereportedinTable3foreachobjectandepoch:thefirst
limit(r≥20.9)canbederivedfromLondishetal.(2002). columngivesthesourcenameshortenedtothemostsignificant
digitsofRAandDEC(HHMMsDD);subsequentcolumnsgive
ourestimatedR magnitudeforeachyear.Forsomefieldswe
C
foundmorethanoneplateinagivenyear,andtheirmagnitudes
2.2.Historicalphotographicdata
arereportedinsubsequentlines.Whenthesourcewastoofaint
For each object we retrieved all the scanned images avail- ontheplatetobereliablymeasuredwereportalowerlimit.
ableon-linecomingfromthePOSS1andPOSS2Surveysand Given the low accuracy achievable for the magnitudes of
the UK-Schmidt plates in the red (E or F emulsion) band. our sources from the Survey photographic plates, we assume
Differentialaperturephotometryofthe2BLobjectswasmade that variability is believable only if a variation of at least 0.6
with the DAOPHOT routine of the IRAF package, using the mag is detected. Small amplitude variability of our objects is
R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample 5
18.6 18.8
1002-00
18.8 19
19 19.2
19.2 19.4
1026-00
19.4 19.6
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18 18
1036+01 1045+00
18.2 18.2
18.4 18.4
18.6 18.6
18.8 18.8
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.6 17.8
1053-00 1055-01
18.8 18
19 18.2
19.2 18.4
19.4 18.6
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
Fig.2.Lightcurvesof2BL candidates:abscissa isin Julian Date −2450000,ordinateis Rmag.The triangleon theleftofthe
verticalbarindicatesthevaluefromLondishetal.(2002)locatedatarbitraryJD. BlackdotsareourCCD datawith errorbars
derivedfromtheinternalconsistencyofthefaintreferencestarsinthedataset.Thedashedlineistheaveragevalueofthesource
inthedataset;thedottedlinesindicatethe±1σstrip(seetext).
thereforenot testable in this way. Only eightsources seem to 2.3.SDSSandpropermotiondata
bevariable:1036+01,1053−00,1055−01,1141−00,1218−01,
1223−01, 1410−02 and 1425−01. All the three radio loud At the time of writing this paper (October 2004) all the ob-
sourcesarevariableandtwoofthem(1055−01and1218−01) jects in the equatorial strip of the 2BL sample (save the ra-
variedmostsignificantly,asexpectedforclassicalBLLacob- dio loud source 1055−01) have been observed photometri-
jects. cally(andseveralalso spectroscopically)bytheSloanDigital
Sky Survey (SDSS) and publicly available through the Data
Release2(DR2).Thisreleasecoversalargerskyareaandsu-
persedestheEarlyDataRelease(EDR,Stoughtonetal.2002)
6 R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample
20 17.8
1134+00
20.2 18
20.4 18.2
20.6 18.4
1106+00
20.8 18.6
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.6 19.4
1140-00
18.8 19.6
19 19.8
19.2 20
1139-02
19.4 20.2
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.2 18
1141-00 1145-02
18.4 18.2
18.6 18.4
18.8 18.6
19 18.8
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
Fig.3.Lightcurvesof2BLcandidates:abscissaisinJulianDate−2450000,ordinateisRmag.SymbolsasinFig.2.
used by Londish(2003). The DR2 photometric observations Schlegeletal.1998, which anywayis always small (E(B−V) ≤
include 5 bands (u′,g′,r′,i′,z′) covering the optical spectral 0.055),giventhehighgalacticlatitudesofthesources.Tocom-
range from 3500 to 9100 Å, so that they allow us to check putetheχ2ofthefitweassumedanerrorof0.03magforeach
whether the spectral energy distribution is consistent with a photometricband,includinganysystematiceffect.Theresults
blackbody(asexpectedforaDCwhitedwarf)orwithapower came out to be largely independent of the assumed redden-
law(F = Aνα) whichistypicalofa BL Lacsource.Typical ing correctionand of the systematic correctionsstated above:
ν
values for α range between −1 and −2 for LBL sources (e.g. in no case did a “good” black-body fit switch into a power-
Vagnettietal.2003),butmaybeflatterforHBLobjects. law one (or vice/versa) including/excludingany of the above
We performedthistestforoursources,applyinga correc- corrections. There are anyway a few cases with no good fit
tionof−0.04tothepublishedu′ magnitudesand+0.02tothe with either spectral shape, which remained “bad” also tuning
z′ magnitudes,asindicatedintherelevantSDSSwebpage.A theabovecorrections.Thevaluesofspectralindicesrangebe-
correctionforthegalacticextinctionwasalsomade,following tween 0.0 and −1.4, which are reasonablefor AGNs. Our fits
R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample 7
19 18.4
1145+00 1159-02
19.2 18.6
19.4 18.8
19.6 19
19.8 19.2
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.4 18.2
1205-00
18.6 18.4
18.8 18.6
19 18.8
1200+00
19.2 19
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.4 15.8
1208-00 1218-01
18.6 16
18.8 16.2
19 16.4
19.2 16.6
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
Fig.4.Lightcurvesof2BLcandidates:abscissaisinJulianDate−2450000,ordinateisRmag.SymbolsasinFig.2.
ofthespectralshapeswithablackbodydistributiongivetem- α for a power law (PL). Column 6 gives the χ2 of the fit:
peraturevalues in fair agreementwith the previousresults by there are 3 degreesof freedomin the fit, so χ2 valuesgreater
Londish(2003), save 1106+00 which is very hot (∼ 130,000 than 9 means that the fit is acceptable only at a confidence
K). level less than 2.5%. Columns 7, 8, 9 give the source proper
We report in Table 4 several quantities for each source: motion according, respectively, to the SDSS, the USNO-B1
sourceshortenedname(column1),theSDSSrmagnitude(col- (Monetetal.2003)catalogueandtheSuperCOSMOSScience
umn 2), the SDSS spectral classification (Sp. Cl., column 3 Archive(SSA,http://surveys.roe.ac.uk/ssa/):thesemotionsare
: Unclass means that the spectrum is too poor according to indicatedinunitsoftheirrmsdeviationfortheSDSSandSSA
the SDSS team for a classification, while n.a. indicates a not while for the USNO-B1 the proper motion probability is re-
available spectrum). In column 4 and 5 we report our best portedasgivenbytheCDS,with9meaningmorethan90%.
fit respectively for the spectral shape given as a temperature AnapproximateconversionoftheSDSSrmagnitudesinto
(in K) in case of a black body (BB) or as a spectral index ourGSC2-basedR magnitudesmaybemadeassumingR =
C C
8 R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample
18.6 18.2
1223-01 1234-01
18.8 18.4
19 18.6
19.2 18.8
19.4 19
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.4 18.6
1254-01
18.6 18.8
18.8 19
19 19.2
1300-02
19.2 19.4
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
19.2 19.2
1316-00 1328+00
19.4 19.4
19.6 19.6
19.8 19.8
20 20
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
Fig.5.Lightcurvesof2BLcandidates:abscissaisinJulianDate−2450000,ordinateisRmag.SymbolsasinFig.2.
r−0.23(seeFukugitaetal.1996).Giventhatthezeropointsof 3. Variabilityproperties
ourmagnitudescaleforourfieldsmayhaveasystematicoffset
ofabout0.2mag,weconsideredasvariableonlythosesources Flux variability at optical frequenciesis one of the character-
whichhaveamagnitudedifference,withrespecttoourLoiano istics of the BL Lac objects. The most monitoredsources are
values, ≥ 0.3 mag. Three objects appear to be variable from generallyLBLsourceswhichshownodefinitetimescale and
thiscomparison,1141−00,1218−01and1425−01,thelasttwo a range of variability of a few magnitudes. HBL sources are
beingradioloud;allthese3objectsappearvariablealsointhe generallyconsideredtobelessvariableintheoptical;however
historicalsurveyplates. mostofthe wellstudiedHBL’s likethe so-calledTeV blazars
(e.g.Mkn421,Mkn501,1ES2344+514,1ES1959+650)are
relativelynearbysourceswithastronghostgalaxy,sothatthe
apparent luminosity variation in the optical is reduced by the
steady stellar componentof the integratedflux. Astold in the
R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample 9
19 19.4
1400+00 1402-01
19.2 19.6
19.4 19.8
19.6 20
19.8 20.2
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18 18.6
1409-00
18.2 18.8
18.4 19
18.6 19.2
1410-02
18.8 19.4
2000 2500 3000 2000 2500 3000
JD - 2450000 JD - 2450000
18.8
1425-01
19
19.2
19.4
19.6
2000 2500 3000
JD - 2450000
Fig.6.Lightcurvesof2BLcandidates:abscissaisinJulianDate−2450000,ordinateisRmag.SymbolsasinFig.2.
Introduction, however, our sources are not likely to be HBL, Theresultsofourvariabilitysearchmaybesummarizedas
giventheundetectionoftheirX-rayflux. follows:
i) only one source (1218−01, radio-loud) showed large
We collect in Table 5 our evaluations of the variability variability in our CCD monitoring (we will discuss on it in
of each source (column 1) in our data (column 2), with re- moredetailattheendofthesection);
spect to the photographic 2QZ data (column 3), to the older ii)threesources(twoofthemradio-loud)showedvariabil-
historical data (column 4) and to the SDSS photometry (col- ity when the 2QZ photographic data are compared with our
umn 5). In columns 6, 7 we report the radio flux in mJy re- CCDdata;
spectivelyatthe frequenciesof 1.4and8.4 GHz accordingto iii) eight sources (including all the 3 radio-loud) showed
Londishetal.(2002) and Londish(2003). The flag X in the variabilitywhenthepublicdigitizedSkySurveyplates(span-
last column marks objects excluded from the 2BL sample by ninga50yearsinterval)aremeasuredusingourreferenceCCD
Londish(2003). sequences.
10 R.Nesci,S.SclaviandE.Massaro:Opticalfollow-upofBLLaccandidatesinthe2BLsample
Table 3. Photographicmagnitudes. Plates up to 1956 are from the POSS1 survey with 103aE emulsion, later plates are IIIaF
fromthePOSS2orUKSchmidtsurveys.
name 1952 1955 1956 1985 1986 1987 1988 1989 1991 1994 1995 1996 1997 1998
1002-00 19.0 19.0 18.9
1026-00 19.6 19.3 19.1
1036+01 19.9 18.7
18.7
1045+00 18.2 18.6
1053-00 19.0 19.4 19.0 19.8
1055-01 18.8 18.5 17.6
1106+00 ≥20.0 20.1 20.5
1134+00 18.2 18.1 18.4
18.1
1139-02 19.1 19.2
1140-00 ≥19.8 ≥19.8
1141-00 ≥19.5 18.7
1145-02 18.5 18.5
1145+00 19.1 19.2
1159-02 19.0 18.8 18.8
18.8
1200+00 19.0 18.8 18.8
1205-00 18.5 18.5 18.5
1208-00 18.5 18.8 18.8
1218-01 17.6 18.6 18.5
18.1
1223-01 18.6 19.2 19.2
1234-01 18.5 18.6 18.9 18.6
19.1
1254-01 18.9 18.9 19.0
19.0
1300-02 19.2 18.8 18.9
1316-00 ≥19.8 19.9 20.1
1328+00 19.5 19.6 19.5
1400+00 19.9 19.5 19.5
1402-01 ≥19.8 20.0 20.0
1409-00 18.7 18.6 18.5
1410-02 18.5 18.8 18.9 18.7
19.1 18.6
19.0
1425-01 20.0 19.7 19.4
iv) three sources showed variability comparing our CCD d)threehaveapossiblehotblackbodyspectrum(1053−00,
datawiththoseoftheSDSS,twoofthembeingradio-loud(the 1223−01and1410−02)andalsoapossiblepropermotionde-
thirdradio-loudsourceisstillnotobservedbytheSDSS). tection: our power-law fit gives for all these sources χ2 val-
Thisgivesatotalofeightvariablesourcesoutofthe29ob- ues much worse then the black-body.1223−01 was excluded
served,asindicatedinTable5.Notably,noneofthe3sources from the sample by Londish(2003) due to its proper motion;
with a spectroscopically confirmed stellar spectrum showed 1410−02 has a strong proper motion in the SDSS/USNO-B1
variabilityinourstudy. catalogues.
Ofthese8sources: From our combined variability and spectral shape study,
a)threeareradioloud(1055−01,1218−01,1425−01);the therefore,itseemsthatallthe3radioloudsourcesamongthe
first two were also detected in the J, H and K bands by the 2BL candidates in the equatorial strip have a variability be-
2MASS survey, with magnitudes 16.7, 16.0, 15.3 (1055−01) havioursimilarto“classical” BL Lacobjects.We remarkthat
and 15.2, 14.4, 13.5 (1218−01);the last two have also SDSS only4ofthesourcesfoundtobevariablearedefinitelyofex-
5-bandphotometrywhichgiveunambiguouspowerlawSEDs. tragalacticnature.
b) one is a radio-quietQSO (1036+01)with a power law Whataboutthe21non-variablesources(atleastatoursen-
spectrum; sitivitylevel)?Twelveofthemhavealreadybeenexcludedby
c) one has a power law spectrum (1141−00) and a faint the2BLsamplebyLondish(2003);ourreanalysisofthespec-
radiodetection,butstrongpropermotionintheSDSS/USNO- tral energy distribution and proper motions confirm that they
B1; are most likely stellar sources (see Table 5). Of the remain-