Photopolarimetric Monitoring of Blazars in the Optical and Near-Infrared Bands with the Kanata Telescope. I. Correlations between Flux, Color, and Polarization

摘要

Wereportonthecorrelationbetweentheflux,color,andpolarizationvariationsontimescalesofdaysa€“monthsinblazars,anddiscusstheiruniversalaspects.Weperformedmonitoringof42blazarsintheopticalandnear-infraredbandsfrom2008to2010usingTRISPECattachedtothe``Kanata''1.5-mtelescope.Wefoundthat28blazarsexhibited``bluer-when-brighter''trendsintheirwholeorapartoftime-seriesdatasets.Thiscorrespondsto88%ofobjectsthatwereobservedfor$$gt$$10days.Thus,ourobservationunambiguouslyconfirmedthatthe``bluer-when-brighter''trendiscommonintheemissionfromblazarjets.Thistrendwasapparentlygeneratedbyavariationcomponentwithaconstantandrelativelybluecolorandanunderlyingredcomponent.Prominentshort-termflaresontimescalesofdaysa€“weekstendedtoexhibitaspectralhysteresis;theirrisingphaseswerebluerthantheirdecayphasesaroundtheflaremaxima.Incontrasttothestrongfluxa€“colorcorrelation,thecorrelationofthefluxandpolarizationdegreewasrelativelyweak;only10objectsshowedsignificantpositivecorrelations.Rotationsofpolarizationweredetectedonlyinthreeobjects:PKS1510$$-$$089,3C454.3,andPKS1749$$+$$096,andpossiblyinS50716$$+$$714.Wealsoinvestigatedthedependenceofthedegreeofvariabilityontheluminosityandthesynchrotronpeakfrequency,$$ u_{ mpeak}$$.Asaresult,wefoundthatlowerluminosityandhigher$$ u_{ mpeak}$$objectshadsmallervariationsintheiramplitudesbothintheflux,color,andpolarizationdegree.Ourobservationsuggeststhepresenceofseveraldistinctemittingsources,whichhavedifferentvariationtime-scales,colors,andpolarizations.Weproposethattheenergyinjectionby,forexample,internalshocksinrelativisticshellsisamajorfactorforblazarvariationsontimescalesofbothdaysandmonths.