Table Of ContentA&Amanuscriptno. ASTRONOMY
(willbeinsertedbyhandlater)
AND
Yourthesauruscodesare:
ASTROPHYSICS
01(11.01.2;11.02.2Mrk501;13.25.2)
Markarian 501 in X-ray bright state – RXTE observations
8 G.Lamer1andS.J.Wagner2,3
9
1 Institutfu¨rAstronomieundAstrophysik,Abt.Astronomie,Universita¨tTu¨bingen,Waldha¨userstr.64,D-72076Tu¨bingen,Germany
9
1 e-mail:[email protected]
2 LandessternwarteHeidelberg-Ko¨nigstuhl,D-69117Heidelberg,Germany
n 3 MountStromlo&SidingSpringObservatories,PrivateBag,WestonCreekPO,ACT2611,Australia
a
J
Received;accepted
2
1
v Abstract. Mrk501hasbeeninastateofveryhighfluxinX-
7 raysandVHE γ-raysduring1997.In July 1997near its hith-
0 ertomaximumX-raybrightnessintensemultifrequencyobser-
0
vationsofMrk501havebeenperformedatradio,nearinfrared,
1
optical,X-ray,andVHEγ-rayfrequencies.
0
8 Here we report on Rossi X-ray Timing Explorer (RXTE)
9 observationscarriedoutin1997betweenJuly11andJuly16.
h/ TheX-rayspectrumhasbeenwelldetectedupto100keVand
p is bestdescribedby a brokenpowerlaw with spectralindices
- α1 = 0.70andα2 = 0.94belowandabovethe breakenergy
o
r ofEbreak =5.8keV.
t TheX-rayfluxfromMrk501declinedandflaredby∼30%
s
a withinabout3dayseach,showinganunusualanti-correlation
:
v betweenfluxandspectralhardeningofbothpowerlawcompo-
i nents.Thebreakenergyremainedconstant.
X
Fig.1. RXTE ASM light curve of Mrk 501 during 1996 and
TheobservedbroadbandandX-rayspectraconfirmrecent
r 1997 with the epochs of pointed observations with SAX and
a observations,thatthe synchrotroncomponentin Mrk501can
RXTEmarkedbytheshadedbars.
extend up to energies of 100 keV with the maximum power
beingemittedathardX-rays.Onlongertime-scalesthecut-off
frequencyof the synchrotronspectrum changesby more than
twoordersofmagnitude.
athardX-rays,havenotbeenmeasuredduringearlierobserva-
Key words: Galaxies: active – BL Lacertae objects:individ-
tions(1975-1990,Ciliegietal.1995).The1keVfluxdensities
ual:Mrk501–X-rays:galaxies measuredintheseobservationsvarybyafactorof∼3.
Table1. Observationlog
1. Introduction
Markarian501isthesecondclosest(z=0.034)BLLacertaeob- ObsID start end exposure[sec]
ject.FromitsquiescentstateradiotoX-rayenergydistribution [MJD-50000.0] PCA HEXTEa
(αrx ≈ 0.62, fν ∝ ν−α) it can be described as intermediate 20421-01-01-01 640.9746 640.9916 1456 873
between X-ray bright BL Lacs (XBLs) and radio bright ob-
20421-01-01-00 641.1764 641.1916 1296 1709
jects(RBLs).TheobjectusuallydisplaysarelativelysteepX-
20421-01-02-01 641.9740 641.9918 1520 640
ray spectrum (e.g. αx = 1.77 observed by ROSAT in 1991) 20421-01-02-00 642.1519 642.1919 2688 1771
indicating that the energy loss dominated high energy part of 20421-01-03-01 642.9733 642.9918 1584 971
the synchrotronspectrum is observedin X-rays (Lamer et al. 20421-01-03-00 643.1520 643.1920 2880 1843
1996).X-rayspectrawithα significantlylessthanunity,asre- 20421-01-04-01 643.9526 643.9922 1616 1078
x
quiredforthemaximumofthesynchrotronpoweroutputtobe 20421-01-04-00 644.1526 644.1923 3152 996
20421-01-05-01 644.9530 644.9922 1744 1129
Sendoffprintrequeststo:G.Lamer,DepartmentofPhysicsandAs- 20421-01-05-00 645.1530 645.1922 3376 2135
2 G.Lamer&S.J.Wagner:Markarian501inX-raybrightstate–RXTEobservations
Table2. Resultsofbrokenpowerlawfitstotheindividualspectra
MJD duration α1 ±(1σ) Ebreak ±(1σ) α2 ±(1σ) 1keVnorm ±(1σ) χ2(d.o.f)
[days] (E <Ebreak) [keV] (E >Ebreak) [(cm2 skeV)−1]
50640.9831 0.0170 0.756 0.015 5.519 0.149 1.054 0.008 0.1702 0.0030 1.03(224)
50641.1840 0.0152 0.767 0.013 5.984 0.186 1.028 0.009 0.1666 0.0030 1.04(224)
50641.9829 0.0178 0.664 0.016 5.380 0.148 0.962 0.007 0.1400 0.0030 1.04(224)
50642.1719 0.0400 0.687 0.010 5.856 0.143 0.922 0.006 0.1421 0.0020 0.95(224)
50642.9826 0.0185 0.653 0.014 5.621 0.189 0.889 0.008 0.1306 0.0026 0.96(224)
50643.1720 0.0400 0.669 0.011 5.604 0.152 0.893 0.006 0.1227 0.0020 1.11(224)
50643.9724 0.0396 0.674 0.015 5.546 0.161 0.944 0.007 0.1381 0.0030 0.98(224)
50644.1725 0.0396 0.722 0.009 5.889 0.131 0.956 0.005 0.1555 0.0018 0.98(224)
50644.9726 0.0393 0.746 0.010 5.981 0.140 1.029 0.007 0.1803 0.0032 0.97(224)
50645.1726 0.0393 0.730 0.008 5.724 0.090 1.035 0.005 0.1780 0.0020 1.02(224)
integralspectrum 0.702 0.019 5.576 0.208 0.960 0.005 0.1510 0.0040 0.98a(224)
a1%systematicerrorallowed
In1995Mrk501wasdetectedasasourceofTeVgamma- Afterapplyingtheaboveelevationandoffsetcriteria,HEX-
raysbytheWhippleteam(Quinnetal.1996)forthefirsttime. TEspectraandlightcurveshavebeenbinnedandbackground
Its flux level corresponded to 8% of that of the Crab Neb- subtractedusingtheoff-sourcelookingintervals.
ula.ThedetectionwasconfirmedbytheHEGRAcollaboration Spectral fitting was performed with XSPEC 10.0 using
(Bradburyet al. 1997). A recentcompilationof TeV observa- the latest detector response matrix from 26 August 1997 for
tions of Mrk 501 (Protheroe et al. 1997) shows the source at PCA andtheresponsefilesfrom20March1997forHEXTE.
very high level with flares up to 10 Crab throughout the en- ThePCAspectrainthePHAchannelrange3-60(2.5-27keV)
tire observing season of 1997 well into the July observations andthespectrafrombothHEXTEclustersinthechannelrange
presentedhere. 15-100(15-100keV)werecombinedforeachofthe10point-
AsimilarbrighteningofMrk501during1997hasbeenob- ingsandthenfittedwithsingleandbrokenpowerlawspectra.
served in X-rays by the RXTE All Sky Monitor (ASM), the TheuncertaintyintheabsolutefluxcalibrationoftheHEXTE
peakbrightnessinJune1997being∼40mCrab(Fig.1).The instrument, mainly due to uncorrected dead-time effects (see
TeVandASM(2-10keV)lightcurvesshowsimilarflaringac- Rothschild et al. 1997), was accounted for by allowing for a
tivity during1997,the correlatedX-ray and VHE gamma-ray scaling factor between the PCA and HEXTE spectra. Setting
variabilitywillbediscussedinaforthcomingpaper. PCAto1.0,themeanbestfitvaluefortheHEXTEnormaliza-
tionwas0.65.
AttheonsetofthishighactivityinApril1997observations
The absorbingcolumndensity on the line of sightto Mrk
withtheSAXsatellitewerecarriedout(Pianetal.1997).They
501wasfixedto2.87·1020cm−2 asderivedfromtheROSAT
findMrk501withanunusuallyhardX-rayspectrum,extend-
spectrum (Lamer et al. 1996). This value is larger than the
inguptophotonenergiesof100keVwiththehardestspectrum
galactic value as derived from 21 cm H I measurements
observedon16April,whenthesourcewasbrightest.
(1.73·1020cm−2,Starketal.1992).However,theeffectofthis
difference on the spectral results in the RXTE energy range
is negligible. Note that Pian et al. 1997 used the galactic H I
2. Observationsanddataanalysis column density NH =1.73·1020cm−2 for the spectral fits to
theSAXdataandthattheuncertaintyintheNHvaluetowards
ThebrightX-raystateofMrk501inJune/July1997triggered thesourcemayhaveasignificanteffectonthespectralindices
intensemulti-frequencyobservationsfromradiotoVHEgam- measuredwiththeLECSinstrumentatthesoftestX-rayener-
ma-rays. Here we report on RXTE observations between 11 gies.
Julyand16July1997resultingin21300secondsofgooddata
in10pointings(Tab.1).
3. Spectraandvariability
Wehaveusedftools 4.0forthereductionofthePCA
and HEXTE data. PCA good times have been selected from Single power law models with N fixed to any reasonable
H
thestandard2modedatasetsusingthefollowingcriteria:tar- value do not yield acceptable fits to the data of any pointing,
getelevation> 10◦, pointingoffset< 0.01◦, and all 5 PCUs whereasbrokenpowerlawmodelswithbreakenergiesbetween
switchedon.Fortheresultingintervalsspectraandlightcurves 5.5and6.0keVgiveexcellentfitstotheindividualspectra(see
wereextractedusingallXenonlayers.ThecorrespondingPCA Tab. 2). Below the break point the model spectra are excep-
G.Lamer&S.J.Wagner:Markarian501inX-raybrightstate–RXTEobservations 3
beenobservedinanyotherXBL(Ciliegietal.1995,Lamerat andamplitudeshavebeenfound.Inaninvestgationofthespec-
al. 1996). The spectrum measured on 16 April 1997 by SAX tral variability of 6 BL Lac objectsGiommiet al. foundtight
in aboutthe same energyband (2.14-10keV) is even slightly correlationsoffluxandspectralhardnessforanyoftheobjects,
harder(α=0.59,Pianetal.1997).AsthePCAhasnosensitiv- includingMrk501.Thishighlightsthepeculiarityofthespec-
itybelow2keV,wearenotabletoverifythebreakat∼2keV tralvariabilityreportedhere.
intheSAXspectrum.
4. Conclusions
OurRXTEobservationsfromJuly1997showthattheperiodof
increasedX-raybrightnesscontinuedthroughout1997andwas
notlimited to a shortflare in April.The objectclearly was in
along-lastinghigh-statein1997ratherthanexhibitingashort,
spectacularX-rayflareduringtheepochofthe SAX observa-
tions.WeconfirmtheextraordinaryhardX-rayspectrumwhich
extendsupto100keV.Thepeakofthesynchrotronemissionis
observedinthehardX-rays,morethan2ordersofmagnitude
higher than in earlier observations (e.g. by ROSAT in 1991,
Lameretal.1996).
Fig.2. Top panel: PCA and HEXTE countrate spectra of the DuringtheRXTEobservationsinJulythetotalfluxandthe
entire observation fitted with a broken power law model (see spectral hardness show an anti-correlation.This is a very un-
Tab. 2 for the modelparameters).Bottom panel: Fit residuals usualspectralbehaviourforanyBLLacobject,ithasnotbeen
with1%systematicuncertaintyallowed. observed previously in Mrk 501 and is in marked contrast to
thefactthattheSAXobservationsofPianetal.showedaflatter
spectrumduringthebrighterstage(thereareonlytwodifferent
AbovethebreakpointthePCAspectrumsteepensbyupto brightnessandspectrallevelsintheSAXdatasetratherthana
0.3 in energyindex. In some of the spectra α2 exceeds unity, well-establishedcorrelation).Theflux-spectralindexrelations
indicatingthatthemaximumofthesynchrotronpoweroutputis should hence not be regarded as universal and viable models
reachedinthehardX-rayrange.However,thereisnoevidence shouldexplainbothkindsofbehaviour.
forfurthersteepeningofthespectrumupto100keV. Thebrokenpowerlaw(orgradualsteepeningofthe spec-
Duringthe RXTE observationsthe PCA count-ratevaried trum) is consistent with synchrotroncoolingof a single com-
by ∼ 30%, but in contrast to the results from the April 1997 ponentanddoesnotrequirethesuperpositionofdifferentpar-
SAXobservationsananti-correlationoffluxandspectralhard- ticle distributions.A homogeneousjetandamagneticfieldof
ness was observed both below and above the spectral break. 0.4G are capable of producing the integrated spectral signa-
Such an anti-correlation has never been reported in Mrk 501 ture.Wehavefoundratherdramaticspectraldifferencesinthe
andisquiteunusualforX-rayBLLacobjectsaltogether.Pian lightcurveswhichcannotbeexplainedbyasinglepopulation
et al. found a spectral hardening by ∆α = 0.32 in the SAX butrequiresthecontributionsofatleasttwo,ifnotmorespec-
MECS 2-10 keV band during an increase of the flux in this tralcomponents.Thisillustratesthegreatimportanceofwell-
energybandbyafactorof2.4. sampledmonitoringwithinstrumentscoveringawidespectral
Thelargedegreeofspectralvariabilityanditsanti-correlat- range.
ion with total flux implies greatdifferencesbetween the light
curvesintheindividualX-rayenergybands.Wehencederived Acknowledgements. WethankJ.Swankforacceptingtherequestfor
lightcurvesforsevenandfourdifferentchannelrangesofthe RXTETOOobservationsofMrk501,andtheRXTEschedulersfor
PCA and HEXTE detectors,respectively(Figs. 3 and 4). The the flexible observation planning. D. Gruber and W. Heindl are ac-
light curves in the softer bands show a decline by 20% over knowledged for their assistance with the HEXTE data analysis. We
thankR.Staubertforacarefulreadingofthemanuscript.RXTEASM
3 days with a well defined minimum at MJD 50643.2 and a
dataareprovidedbytheASM/RXTEteamsatMITandGSFC.PCA
subsequentincrease by 30% during the following 2 days. On
andHEXTEdatawereobtainedthroughtheHEASARCOnlineSer-
the other hand the hardest bands are dominated by a feature
vice,providedbyNASA/GSFC.ThisworkwassupportedbyDARA
with a broadmaximumaroundMJD 50642.A smoothtransi-
undergrant50OR96031,andtheDFG(SFB328).
tion with superpositions of both morphologies is observed in
the intermediateenergybands.The independentvariabilityof
softandhardX-raysrequirestheexistenceofatleasttwoemis- References
sioncomponentswhichisnotevidentfromtheX-rayspectrum
Bradbury,S.M.etal.,1997,A&A320,L5
alone.
Ciliegi,P.,Bassani,L.,andCaroli,E.,1995,ApJ439,80
The sharpest feature in the 2-20 keV lightcurves is a de- Giommi,P.,Barr,P.,Garilli,B.,etal.,1990,ApJ356,432
4 G.Lamer&S.J.Wagner:Markarian501inX-raybrightstate–RXTEobservations
Protheroe, R.J., Bhat, C.L., Fleury, P., et al., 1997, Proc. 25th. Int.
cosmicRayConf.,Durban1997,inpress
Rothschild,R.E.,Blanco,P.R.,Gruber,D.E.etal.,1997,ApJ,inpress
Quinn,J.etal.1996,ApJ456,L83
Stark,A.A.,Gammie,C.F.,Wilson,R.W.,etal.,1992,ApJS79,77
Fig.3. Background subtracted PCA light curves in different
energybands.
G.Lamer&S.J.Wagner:Markarian501inX-raybrightstate–RXTEobservations 5
Fig.4. BackgroundsubtractedHEXTElightcurvesindifferent
energy bands, the count-rates of clusters A and B have been
added.NotethegoodagreementofthePCAandHEXTElight
curvesinthe3energybandsbetween15and80keV.
