Large X-ray flares on stars detected with MAXI/GSC: A universal correlation between the duration of a flare and its X-ray luminosity

摘要

Twenty-threegiantflaresfromthirteenactivestars(eightRSCVnsystems,oneAlgolsystem,threedMestars,andoneyoungstellarobject)weredetectedduringthefirsttwoyearsofourall-skyX-raymonitoringwiththegaspropotionalcounters(GSC)oftheMonitorofAll-skyX-rayImage(MAXI).TheobservedparametersofalltheseMAXI/GSCflaresarefoundtobeattheupperendsforstellarflareswiththeluminosityof1031a€“34a€‰erga€‰sa?’1inthe2a€“20a€‰keVband,theemissionmeasureof1054a€“57a€‰cma?’3,thee-foldingtimeof1hrto1.5d,andthetotalradiativeenergyreleasedduringtheflareof1034a€“39a€‰erg.Notably,thepeakX-rayluminosityof$$5^{+4}_{-2} imes10^{33}:$$erga€‰sa?’1inthe2a€“20a€‰keVbandwasdetectedinoneoftheflaresonIIPeg,whichisoneofthe,orpotentiallythe,largest-ever-observedinstellarflares.X-rayflaresweredetectedfromGTMus,V841Cen,SZPsc,andTWA-7forthefirsttimeinthissurvey.Whereasmostofourdetectedsourcesaremultiple-starsystems,twoofthemaresinglestars(YZCMiandTWA-7).Amongthestellarsourceswithin100pcdistance,theMAXI/GSCsourceshavelargerrotationvelocitiesthantheothersources.Thissuggeststhattherapidrotationvelocitymayplayakeyroleingeneratinglargeflares.CombiningtheX-rayflaredataofnearbystarsandthesun,takenfromliteratureandourowndata,wediscoveredauniversalcorrelationof$$ auproptoL_{ mX}^{0.2}$$fortheflareduration??andtheintrinsicX-rayluminosityLXinthe0.1a€“100a€‰keVband,whichholdsfor5and12ordersofmagnitudein??andLX,respectively.TheMAXI/GSCsampleislocatedatthehighestendsofthecorrelation.