Astronomy&Astrophysicsmanuscriptno.aa (cid:13)c ESO2015 February3,2015 Intranight optical variability of radio-quiet BL Lacertae objects YuanLiu1,JinZhang2,andShuangNanZhang1,2 1 KeyLaboratoryofParticleAstrophysics,InstituteofHighEnergyPhysics,ChineseAcademyofSciences,P.O.Box918-3,Beijing 100049,China e-mail:[email protected]; [email protected] 2 NationalAstronomicalObservatories,ChineseAcademyofSciences,Beijing100012,China e-mail:[email protected] 5 ... 1 0 ABSTRACT 2 Aims.Intranightvariation(ormicrovariation)isacommonphenomenonofradio-loudBLLacobjects.However,itisnotclearwhether b therecentlyfoundradio-quietBLLacobjectshavethesameproperties.Theoccurrencerateofintranightvariationishelpfulindis- e tinguishingthemechanismofthecontinuumofradio-quietBLLacobjects. F Methods.Weconducted aphotometric monitoringof8radio-quiet BLLacobjectsbytheXinglong2.16mandLijiang2.4mtele- 2 scopes. The differential light curves are calculated between each target and two comparison stars. To quantify the variation, the significanceofvariationisexaminedbyascaledF-test. ] Results.Nosignificantvariationisfoundinthe11sessionsoflightcurvesof8radio-quietBLLacobjects(onegalacticsourceis A excluded).Thelackofmicrovariationinradio-quietBLLacobjectsisconsistentwiththedetectionrateofmicrovariationinnormal G radio-quietAGNs,butmuchlowerthanforradio-loudAGNs.Thisresultindicatesthatthecontinuaoftheradio-quietBLLacobjects arenotdominatedbyjetsthatwillinducefrequentmicrovariations. . h Keywords.Galaxies:active–Radiationmechanisms:general–BLLacertaeobjects:general p - o r 1. Introduction Lac objects. However,owing to the small size of their sample, t s thisisnotveryconclusive. a Active galactic nuclei(AGNs) are characterizedby their broad Thevariationinshorttimescale(intranight)isanotherchar- [ band continua, strong emission lines, and fast variability. acteristic of classical BL Lac objects. During a very short pe- 2 However,a handfulofabnormalAGNshaverecentlybeendis- riod,e.g. severalhours,the flux of classical BL Lac objectcan v coveredintheSDSSdata,i.e.,weaklinequasarsandradio-quiet changebyseveraltenthsofamagnitude(Wagner&Witzel1995; 2 BLLacobjects(Diamond-Stanicetal.2009;Plotkinetal.2010). Heidt & Wagner 1996; Bai et al. 1998; de Diego et al. 1998). 5 The UV/optical emission lines are absent in their UV/optical However, it is still unclear whether the intranight variation is 6 spectroscopies, though the shape and luminosity of their con- frequentintheradio-quietBLLacobjets,whichwillbehelpful 6 tinuaarecomparabletothenormalAGNs.Thefractionofthese in distinguishingthe originof their continua.Gopal-Krishnaet 0 . special AGNs is small (∼1/1000) in the SDSS DR 7 sample; al.(2013,hereafterGJC2013)andChandetal.(2014,hereafter 1 however,itcouldbeanimportantstageintheevolvingsequence CKG2014) claim that they have detected a considerable frac- 0 of AGNs (Hryniewicz et al. 2010; Liu & Zhang 2011). In the tionofintranightvariationinasampleofweak-lineAGNs(duty 5 early stage of an active cycle of AGNs, the radiative feedback cycle∼5%),andthisfractioncanbehigherifthesignal-to-noise 1 canexpelthegasfrombroadlineregionsandresultinweakor ratioofthelightcurveisfurtherincreased.However,aswedis- : v eveninthedisappearanceofbroademissionlines(Liu&Zhang cussinthispaper,somegalacticsourcesmaycontaminatetheir Xi 2011). Other models, such as of a cold accretion disk, an ex- sample. tremelyhighaccretionrate,shieldinggas,orabnormalBL Lac In this paper, we report the result of our monitoring cam- r a objects,havealsobeenproposed(Shemmeretal.2010;Plotkin paignof radio-quietBL Lac objects.Observationsand data re- etal.2010;Laor& Davis2011;Wuetal. 2012).Theoriginof ductionaredescribedin Section2,andthenthe significanceof suchweak-lineAGNsisstillnotclear.Boththethermal(accre- theintranightvariationisshowninSection3.Section4discusses tiondisk)ornon-thermal(jet)componentmayexplaintheweak theimplicationofourresultsandpresentstheconclusions. linefeature. The optical continua of classical BL Lac objects are dom- 2. Observationsanddatareduction inated by the synchrotron emission from the relativistic jets, therefore high polarization and fast variability are important Theamplitudeofintranightvariationisnormallyseveraltenths characteristics of classical BL Lac objects. Heidt & Nilsson of a magnitude, so the desired error of our observed mag- (2011)foundthepolarizationdegreesoftheradio-quietBLLac nitude is .0.05 mag with the exposure time not longer than candidates are low. Plotkin et al. (2010) investigated the long- 10 min. The corresponding magnitude threshold is R ∼ 18.5 term variability of radio-quiet BL Lac objects using the data for the 2 m class telescopes we used. Seven radio-quiet BL from SDSS stripe 82 and find that the variation amplitude of Lac objects were selected from Plotkin et al. (2010), and radio-quietBLLacobjectsissmallerthanthatofradio-loudBL SDSS J094533.99+100950.1was selected from Hryniewicz et 1 YuanLiuetal.:Intranightopticalvariabilityofradio-quietBLLacertaeobjects Table1.Informationonobservations. Object(SDSS) RA DEC R Redshift Date Telescopea Filter Duration(h) Nb J081250.80+522530.8 123.212 52.425 17.85 1.152 2011.2.12 L R 4.4 27 2012.1.27 L R 5.7 32 J085025.60+342750.9 132.607 34.464 18.51 1.389 2011.2.13 L R 3.3 20 J090107.64+384658.8 135.282 38.783 17.87 unknown 2012.1.29 L R 6.9 32 J094533.99+100950.1 146.392 10.164 17.45 1.662 2011.2.10 L V 6.1 25 2012.1.28 L R 6.0 34 J125219.48+264053.9 193.081 26.682 17.51 1.292 2011.4.23 X R 6.3 38 2013.4.14 X R 8.4 47 J132809.59+545452.8 202.040 54.915 17.59 2.096 2011.4.24 X R 4.6 48 J134601.29+585820.1 206.505 58.972 17.46 1.667 2011.4.25 X R 4.1 18 J142943.64+385932.2 217.432 38.992 17.26 0.930 2013.4.13 X R 6.2 33 (a)L−Lijiang2.4m,X−Xinglong2.16m (b)Numberofexposures al. (2010). The above selection criteria are similar to those in 3. Results GJC2013 and CKG2014. Actually, five sources in our sample To detect the underlying variation of the target, we first cal- are shared with GJC2013 and CKG2014 who based their se- culated the differential light curves (DLCs) between the target lection primarily on classification by Plotkin et al. (2010) as a and comparison stars. Two nearby comparison stars (noted as ‘high-confidenceBL Lac candidate’. We additionally included Star1andStar2hereafter)withsimilarmagnitudestothetarget in our sample some low-confidence BL Lac candidates. The wereselectedandtheDLCsofAGN−Star1,AGN−Star2,and sourcesareclassifiedaslow-confidenceonlybecausethecontin- Star1−Star2areshowninFigure1.Thepositionandg−rcolor uumneartheemissionlineishardtodefine,andtheequivalent oftargetsandcomparisonstarsareshowninTable2.Sincethe width of emission lines will be larger or smaller than 5 Å de- colordifferencebetweentargetandstarpairsissmallerthan1.5, pendingonthecontinuumassumptions,whichismainlydueto thevariationinairmassduringtheobservationhaslittleeffecton thenoisyspectraaroundsomeemissionlines.Wethereforethink DLCs(Carinietal.1992;Stalinetal. 2004).We also trieddif- thereshouldbenosystematicdifferencebetweenhigh-andlow- ferent companion stars, and the final significance of intranight confidencesourcesandwillinvestigatethevariationpropertyof variationisquiterobust.Someexposureswithbadweatherwere subsamplesinfutureworks. excludedfromtheDLCs,whichledtosomegapsintheDLCs. Toquantifythesignificanceofthevariationoflightcurves, The observations were carried out by BFOSC (BAO Faint we performed a scaled F-test, which is more powerful and re- ObjectSpectrographandCamera)ontheXinglong(China)2.16 liablethanthetraditionalC-test(deDiego2010).Thescaled F m telescope and YFOSC (Yunnan Faint Object Spectrograph value(Howelletal.1988)isdefinedas and Camera ) on the Lijiang (China) 2.4 m telescope. All ob- servationswereperformedinJohnsonRband,exceptforSDSS s2 J094533.99+100950.1 in Johnson V band. The exposure time F = AGN−Star1 , (1) was 300 s or 600 s depending on the weather conditions. The Γ2s2 Star1−Star2 detailedinformationaboutthesampleandobservationsisshown inTable1.Intotal,thereare11sessionsoflightcurvesofthese wheres2 = 1 N (X −X¯)2,andxcanstandforAGN-Star1or eightsources. x N−1i=1 i Star1-Star2. P The photometric data were reduced with the standard rou- ThedefinitionofΓ2is tinesintheImageReductionandAnalysisFacility(IRAF)soft- ware. The bias frames were extracted from no fewer than ten Γ2 = NStar2 2 NS2tar1(NAGN+P)+NA2GN(NStar1+P) , (2) frames,andthe flat framesdid nothavefewerthan fiveframes N ! N2 (N +P)+N2 (N +P) inonebandforonenightofobservation.ThedarkoftheCCDis AGN  Star2 Star1 Star1 Star2  negligible(comparedwiththereadoutnoiseandtheflatfluctua- which is the scaled factor to account for the different accura- tion)andthereforenotconsidered.Theflatframesforthesame ciesbetweenthephotometriesofthetargetandcomparisonstars band were combined by average, and then the normalized flat (Howelletal.1988).ThevariablesNAGN,NStar1,andNStar2 are framewasgenerated;thenormalizedbiasframewas generated thetotalcounts(sky-subtracted)oftarget,Star1,andStar2,re- bymediancombination.Thenthesourceframeswerecorrected spectively.ThevariablePisdefinedasP=np(NS +Nr2),where bythenormalizedbiasframeandflatframe. npisthenumberofpixelsintheappliedmeasuringaperture,the variable N is the sky photonsper pixel, and N is the readout S r With the corrected source images, we used the package noise (e−/pixel). The value of Γ2 can be calculated frame-by- APPHOT to perform aperture photometry. The values of en- frame. However,the variation in Γ2 of our observationsduring closed,moffat,anddirectforthecomparisonstarsandthetarget one nightis no more than 10%owing to the small variationof sourcewere used to estimate the mean fullwidth at half maxi- ourtargets.Therefore,wehavetakenthemedianvalueofΓ2for mum(FWHM).Theaperturesof thephotometryforindividual theexposuresinonenight.Ourfinalresultisnotsensitivetothis framewerecarriedwith2.5∼3timesofFWHM.Ifthevalueof choice. FWHM significantlychangedduringonenight,we tookdiffer- ThesignificanceofthevariationisdeterminedbytheF dis- entvaluesofFWHMevenforthesamesource. tribution with N − 1 and N − 1 degrees of AGN−Star1 Star1−Star2 2 YuanLiuetal.:Intranightopticalvariabilityofradio-quietBLLacertaeobjects SDSS J081250.80+522530.8 (2011.2.12) SDSS J081250.80+522530.8 (2012.1.27) 0.85 10.8 1 ar ar 0.8 st st N−0.7 N−0.75 G G A A 0.6 0.7 0.3 ar20.2 ar2 0.4 N−st0.1 N−st0.35 G G A 0 A 0.3 −0.35 −0.5 2 2 ar ar −0.4 −st−0.6 −st ar1 ar1−0.45 st−0.7 st −0.5 12.5 13 13.5 14 14.5 15 15.5 16 16.5 16 17 18 19 20 21 UT (h) UT (h) SDSS J085025.60+342750.9 (2011.2.13) SDSS J090107.64+384658.8 (2012.1.29) −0.2 0.3 star1−0.4 star10.25 − − 0.2 N N AG−0.6 AG0.15 0.1 0.4 0.95 ar20.3 ar2 −st0.2 −st 0.9 N N G0.1 G0.85 A A 0 0.8 0.75 0.7 2 2 ar ar 0.7 st0.6 st − − 1 1 ar0.5 ar0.65 st st 0.4 0.6 15 15.5 16 16.5 17 17.5 18 14 15 16 17 18 19 20 21 UT (h) UT (h) SDSS J094533.99+100950.1 (2011.2.10) SDSS J094533.99+100950.1 (2012.1.28) 0.8 0.7 1 1 ar0.75 ar −st 0.7 −st0.65 N N G0.65 G A A 0.6 0.6 −0.4 −0.35 2 2 ar ar −st−0.5 −st −0.4 N N G G−0.45 A−0.6 A −0.5 −1.1 −1 2 2 ar ar −st−1.2 −st−1.05 1 1 star−1.3 star −1.1 −1.15 16 17 18 19 20 21 22 16 17 18 19 20 21 UT (h) UT (h) 3 Fig.1.DifferentiallightcurvesofAGN-Star1,AGN-Star2,andStar1-Star2. YuanLiuetal.:Intranightopticalvariabilityofradio-quietBLLacertaeobjects SDSS J125219.48+264053.9 (2011.4.23) SDSS J125219.48+264053.9 (2013.4.14) 1.85 0.5 1 1 star0.4 star 1.8 N− N−1.75 G0.3 G A A 1.7 0.2 0.7 1.35 2 2 ar ar 1.3 −st0.6 −st N N1.25 G G A0.5 A 1.2 0.4 −1.01.54 0.4 2 2 ar0.3 ar−0.45 st st − − ar10.2 ar1 −0.5 st0.1 st −0.55 14 15 16 17 18 19 20 12 13 14 15 16 17 18 19 20 UT (h) UT (h) SDSS J132809.59+545452.8 (2011.4.24) SDSS J134601.29+585820.1 (2011.4.25) 0.5 0 1 1 ar0.4 ar st st−0.1 − − N N AG0.3 AG−0.2 0.2 −0.3 −0.1 −0.4 2 2 −star−0.2 −star−−00..65 N N G G−0.7 A−0.3 A −0.8 2−0.5 2−0.3 ar ar−0.4 st st 1−−0.6 1−−0.5 star−0.7 star−0.6 −0.7 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 15 15.5 16 16.5 17 17.5 18 18.5 19 UT (h) UT (h) SDSS J142943.64+385932.2 (2013.4.13) 0.3 1 ar0.25 st − N 0.2 G A 0.15 ar21.15 st N− 1.1 G A 1.05 0.95 2 ar st 0.9 − 1 ar st0.85 15 16 17 18 19 20 UT (h) 4 Fig.1.continued YuanLiuetal.:Intranightopticalvariabilityofradio-quietBLLacertaeobjects Table2.Informationontargetsandtheircompanionstars. Object(SDSS) Date RA(J2000) DEC(J2000) r(SDSS) g−r(SDSS) J081250.80+522530.8 2011.2.12 081250.80 +522530.8 18.05 0.3 Star1 081251.29 +522646.4 17.28 1.4 Star2 081249.52 +522626.2 17.89 1.3 J081250.80+522530.8 2012.1.27 081250.80 +522530.8 18.05 0.3 Star1 081251.29 +522646.4 17.28 1.4 Star2 081249.52 +522626.2 17.89 1.3 J085025.60+342750.9 2011.2.13 085025.60 +342750.9 18.66 0.4 Star1 085026.96 +342635.9 19.22 1.0 Star2 085017.77 +342650.5 18.53 0.7 J090107.64+384658.8 2012.1.29 090107.64 +384658.8 18.12 0.1 Star1 090106.48 +384708.7 18.14 1.4 Star2 090105.15 +384824.5 17.36 1.3 J094533.99+100950.1 2011.2.10 094533.99 +100950.1 17.66 0.4 Star1 094527.96 +100847.8 16.89 0.4 Star2 094537.93 +100808.9 18.01 0.7 J094533.99+100950.1 2012.1.28 094533.99 +100950.1 17.66 0.4 Star1 094527.96 +100847.8 16.89 0.4 Star2 094537.93 +100808.9 18.01 0.7 J125219.48+264053.9 2011.4.23 125219.48 +264053.9 17.70 0.2 Star1 125227.12 +263849.7 17.44 1.1 Star2 125214.26 +263911.5 17.15 1.3 J125219.48+264053.9 2013.4.14 125219.48 +264053.9 17.70 0.2 Star1 125223.02 +263842.9 15.82 0.6 Star2 125223.82 +264142.6 16.43 0.3 J132809.59+545452.8 2011.4.24 132809.59 +545452.8 17.84 0.1 Star1 132758.21 +545400.2 17.54 0.8 Star2 132822.83 +545554.7 18.14 0.5 J134601.29+585820.1 2011.4.25 134601.29 +585820.1 17.74 0.2 Star1 134606.60 +585808.2 18.01 1.5 Star2 134555.76 +585734.8 18.46 1.3 J142943.64+385932.2 2013.4.13 142943.64 +385932.2 17.55 0.0 Star1 142939.99 +390219.6 17.21 0.9 Star2 142930.47 +390008.7 16.33 0.6 freedom, where N and N are the number of weakblazarcomponentinradio-quietAGNsisanalternativeto AGN−Star1 Star1−Star2 observationsintheAGN-Star1andStar1-Star2DLCs,respec- microvariation(Czerny et al. 2008). Though the occurrence of tively. microvariations is not a smoking gun of jets, the fraction and The results of the significance are listed in Table amplitude of the microvariations in radio-quiet and radio-loud 3. Since we exchanged the position of Star 1 and onesarequitedifferent. Star 2 in equations (1) and (2), there are two values Gupta & Joshi (2005) compiled the microvariationsof dif- of significance for (AGN−Star1)/(Star1−Star2) and ferentclassesofAGNsandfoundthedetectionfractionsofmi- (AGN−Star2)/(Star2−Star1). crovariation in radio-quiet and radio-loud (non-blazars) AGNs As indicated by the results of F-test, we only detect a sig- are∼10%and∼35-40%,respectively.Forblazars,thefractions nificant variation (∼3σ level) in SDSS J090107.64+384658.8. are∼60-65%and∼80-85%fortheobservationsthatarelessthan However,due tothe largepropermotionof thissource(62±11 andmorethan6h,respectively.Inaddition,theyalsoclaimthat mas/yr from Monet et al. 2003), its extragalactic nature is theamplitudeofthemicrovariationofradio-loudonesislarger doubtable. Therefore, we would like to exclude it from the fi- thanthatofradio-quietones. nalsampleofradio-quietBLLacobjects.Asaresult,thereisno significantvariationdetectedin ourobservationsof radio-quiet Carinietal. (2007)established a sample of 117radio-quiet BLLacobjects. AGNsthathavebeeninvestigatedformicrovariationsandfound a detection rate of microvariations for the entire sample of 21.4%.Ifthecriteriafor‘radio-quiet’arestrengthenedtoR< 1 4. Discussionsandconclusions (Ristheratiooftheradio[5GHz]fluxtooptical[4400Å]flux), thedetectionrateofmicrovariationsisonly15.9%. Radio-loud AGNs and blazars can exhibit microvariation with a largeamplitudeupto ∼100%.However,somemicrovariation Goyal et al. (2013) analyzed 262 sessions of light curves eventsarealsoobservedinradio-quietAGNswithhighsignifi- of 77 AGNs from their uniform AGN monitoring data and cance(Stalinetal.2004;Gupta&Joshi2005).Themechanism found the duty cycles of intranight variation of radio-quiet of the microvariationin radio-loudAGNs is believed to be the quasars, radio-intermediate quasars, lobe-dominated quasars, fluctuation caused by the shocks in jets. However,the instabil- low optical-polarization core-dominated quasars, high optical- ityorflaresintheaccretiondiskcanalsoinducemicrovariation polarizationcore-dominatedquasars,andTeVblazarsare10%, even for the radio-quiet AGNs (Mangalam & Wiita 1993). A 18%,5%,17%,43%,and45%,respectively. 5 YuanLiuetal.:Intranightopticalvariabilityofradio-quietBLLacertaeobjects Table3.Resultsofthesignificanceofvariations. Object(SDSS) Date Γ Γ F F Significance Significance 1 2 1 2 1 2 J081250.80+522530.8 2011.2.12 1.16 1.66 0.70 1.34 18.4% 76.9% J081250.80+522530.8 2012.1.27 1.50 1.85 0.51 0.72 3.44% 18.7% J085025.60+342750.9 2011.2.13 1.09 0.59 1.20 1.41 64.9% 77.0% J090107.64+384658.8 2012.1.29 1.84 1.34 1.75 2.68 93.8% 99.6% J094533.99+100950.1 2011.2.10 0.45 1.25 1.15 0.93 63.5% 43.0% J094533.99+100950.1 2012.1.28 0.51 1.26 0.74 1.16 20.1% 66.2% J125219.48+264053.9 2011.4.23 1.80 1.58 0.39 0.36 0.25% 0.13% J125219.48+264053.9 2013.4.14 6.76 7.15 0.49 0.46 0.91% 0.50% J132809.59+545452.8 2011.4.24 0.79 1.29 1.05 0.91 56.6% 37.3% J134601.29+585820.1 2011.4.25 0.60 0.96 0.67 1.18 20.9% 63.1% J142943.64+385932.2 2013.4.13 2.05 1.40 1.51 2.23 87.4% 98.7% Notes. Thesubscripts‘1’and‘2’ofvariablesstandfortheresultsof(AGN−Star1)/(Star1−Star2) and(AGN−Star2)/(Star2−Star1),respectively. Nosignificantmicrovariationisdetectedinourfinalsample, SDSS J121929.45+471522.8as a DC white dwarf in their cat- intensessionsoflightcurves.The1σupperlimitofthefraction alog. SDSS J090107.64+384658.8is classified as an uncertain ofmicrovariationis15%usingthemethodofCameron(2011)1. DCwhitedwarfbyEisensteinetal.(2006)andasanuncertain Inderivingthisupperlimit,wetreatedthesourcesequally.The DC+M binarysystembyKleinmanetal.(2013).Thevariation weights of sources should not be the same owing to different observedislikelyduetotheoscillationoraccretionofthewhite exposuretimes,signal-to-noiseratios,andobservationnumbers; dwarfs(Winget&Kepler2008;Fontaine&Brassard2008). however,thispotentialminorcorrectionwill notchangeourfi- The DC white dwarfs are generally difficult to positively nal conclusion. This low fraction in our sample of radio-quiet identifydueto their featurelessspectra andthe discrepancyin- BL Lac objectsis consistentwith thatof the radio-quietAGNs deed exists in different catalogs. However,we think these pos- but much lower than for the radio-loud ones and blazars. This sible galactic sources should be excluded from the sample of indicatesthatthecontinuumofradio-quietBLLacobjectsisnot AGNsto besafe. We detectedthe intranightvariationin SDSS dominated by the jet component. Actually, the SED of radio- J090107.64+384658.8 but excluded it from the sample. After quietBLLacobjectsissimilartothenormalradio-quietAGNs the possible white dwarfs are removed from the sample of (Laneetal.2011),whichfurthersupportstheaccretiondiskori- GJC2013 and CKG2014, only one significant variation (SDSS gin of the continuum.Accurate black holemass measurements J090843.25+285229.8) is detected in the remaining 22 light candeterminetheaccretionstate ofradio-quietBLLacobjects curves, which is also the only significant event in all 32 light andfurtherdistinguishthedifferentmodelsrelatedtotheaccre- curves(including10additionalonesofthispaper)ofweak-line tiondiskoriginoftheircontinua,whichwillbeexploredinour AGNsuptonow.Thislowoccurrencerateisconsistentwiththe futureworks. rateforthenormalradio-quietAGNs. GJC2013 and CKG2014 detected significant variations Our present sample of radio-quiet BL Lac objects is still (confidence level >99% for two comparison stars) of SDSS toosmalltoconstrainthedutycycleofthemicrovariationwell. J090843.25+285229.8andSDSSJ121929.45+471522.8intheir We will enlargeoursample, especiallyforthe monitoringtime 29 light curves. Based on these two events, they derived a longerthansix hours,andimprovethe accuracyof photometry duty cycle ∼5% of intranight variation from their sample on to∼0.01magtodetectsmallervariations. weak-lineAGNs. However,two sourcesintheirsample(SDSS Observationsinmorebandswillhelptoinvestigatethecolor- J090107.64+384658.8 and SDSS J121929.45+471522.8) are behaviorandfurtherconstrainthemechanismofthecontinuum likely to be galactic sources owing to the large proper mo- oftheradio-quietBLLacobjects. tion. The proper motions of SDSS J090107.64+384658.8 and SDSS J121929.45+471522.8from USNO-B are 62±11 mas/yr Acknowledgements. The authors thank the referee for useful comments that and112±4mas/yr,respectively(Monetetal. 2003).Thesetwo improved the paper. This work is supported by 973 Program of China under bright sources (V∼18) are well above the completeness limit grant 2014CB845802, by the National Natural Science Foundation of China V=21 of the USNO-B catalog (Monet et al. 2003). The posi- under grant Nos. 11103019, 11133002, and 11103022, and 11373036, and bytheStrategic PriorityResearch Program“TheEmergenceofCosmological tionalerrorofoneepochis∼200mas.Thus,thepropermotion Structures”oftheChineseAcademyofSciences,GrantNo.XDB09000000.We of ∼100 mas/yr can be accurately detected with the epoch dif- acknowledge the support of the staff of the Xinglong 2.16m and the Lijiang ference of ∼40 years. The systematic error of the proper mo- 2.4mtelescope. FundingfortheLijiang2.4mtelescope hasbeenprovidedby tioniscomparabletothestatisticalerror.Munnetal.(2004)and CAS and the People’s Government of Yunnan Province. This work was par- tiallysupportedbytheOpenProjectProgramoftheKeyLaboratoryofOptical Roeser et al. (2010) have further calibrated the USNO-B cata- Astronomy,NAOC,CAS. logwithSDSSand2MASSastrometry,andtheresultingproper motionsofthesetwosourcesareconsistentwiththosefromthe USNO-B catalog. No radio or X-ray counterpartis found near References their positions. Actually, Rebassa-Mansergas et al. (2010) lists Bai,J.M.,Xie,G.Z.,Li,K.H.,Zhang,X.,&Liu,W.W.1998,A&AS,132,83 1 Giventhesamplesizenandobservedsuccesscountsk,theupper Cameron,E.2011,PASA,28,128 limit p isdefinedby 1 (a+b−1)! pa−1qb−1dp = (1−c)/2,wherea = Carini,M.T.,Miller,H.R.,Noble,J.C.,&Goodrich,B.D.1992,AJ,104,15 u pu (a−1)!(b−1)! Carini,M.T.,Noble,J.C.,Taylor,R.,&Culler,R.2007,AJ,133,303 k+1,b=n−k+1,qR=1−p,andcistheconfidencelevel. 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