Intranight optical variability of radio-quiet BL Lacertae objects

0.11 MB·English

by  Yuan Liu

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
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