The broadband spectrum of Cygnus X-1
S. Fritz, J. Wilms, K. Pottschmidt, M. A. Nowak, E. Kendziorra, M. G. Kirsch, I. Kreykenbohm, A. Santangelo
aa r X i v : . [ a s t r o - ph ] J un THE BROADBAND SPECTRUM OF CYGNUS X-1
S. Fritz , J. Wilms , K. Pottschmidt , M. A. Nowak , E. Kendziorra , M. G. Kirsch , I. Kreykenbohm , andA. Santangelo Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany Dr. Remeis Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg,Germany Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093-0424, USA MIT-CXC, NE80-6077, 77 Massachusetts Ave., Cambridge, MA 02139, USA European Space Astronomy Centre (ESA), Madrid, Spain INTEGRAL Science Data Centre, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
ABSTRACT
The Black Hole (BH) binary Cygnus X-1 has beenobserved simultaneously by INTEGRAL, RXTE, andXMM-Newton for four times in November and Decem-ber 2004, when Cyg X-1 became first observable withXMM-Newton. During these observations the sourcewas found in one of its transitional states between thehard state and the soft state. We obtained a high signal tonoise spectrum of Cyg X-1 from 3 keV to 1 MeV whichallows us to put constraints on the nature of the Comp-tonizing plasma by modeling the continuum with Comp-tonization models as eqpair [3]. Using XMM-Newtonwe were also able to confirm the presence of a relativisti-cally broadened Fe K α line.Key words: stars: individual (Cyg X-1), black holephysics.
1. INTRODUCTION
Being one of the brightest sources in the X-ray sky,Cyg X-1 has become also one of the best studied GalacticBHs. The system consists of the O9.7I star HDE 226868of 40 M ⊙ [18] and a compact object with a mass of about10 M ⊙ . Most of the time (90% up to MJD 51300, 75%from then on [17]) the source can be found in its hardstate, where the X-ray spectrum is characterized by apower law with photon index Γ ≈ . ff at ≈
150 keV [8, and references therein]. Fur-ther spectral characteristics are reflection features fromthe accretion disk and a relativistically broadened Fe K α emission line at about 6.4 keV. The other canonical statein which Cyg X-1 can be found is the soft state duringwhich the spectrum is dominated by a soft componentpeaking at ∼ Γ ≈ − Table 1. Observing log for the three instruments used.Times are given in ksec.
Date XMM RXTE INTEGRAL14 /
15 Nov 04 (obs1) 17.6 12.0 58.020 /
21 Nov 04 (obs2) 17.7 16.7 80.026 /
27 Nov 04 (obs3) 20.0 26.0 79.002 /
03 Dec 04 (obs4) 10.0 10.0 80.0 AS M C oun t R a t e [ c p s ], − k e V JD−2450000 year
Figure 1.
RXTEASM light curve of Cyg X-1. The verti-cal lines mark the times of our observations. states” which exhibit properties of both canonical states[1].Cyg X-1 was observed in 2004 with INTEGRAL,RXTE, and XMM-Newton simultaneously on Novem-ber 14 /
15 (hereafter called obs1), 20 /
21 (obs2), 26 / / / ASM lightcurve of Cyg X-1 with the four obser-vations indicated by vertical bars. The log of observationsis given in Table 1. The observations took place duringone of the transitional states of Cyg X-1 which are veryinteresting for a further study due to the fact that they arecharacterized by radio flaring, the presence of a relativis-tically broadened iron line as well as a complex X-raytiming behavior [e.g. 4, 12, 17, 11, 2].n this contribution we present some results of ourongoing analysis. For RXTE we used data from thePCA and HEXTE, covering an energy range from 3 to120 keV. The data extraction was done using HEASOFT5.3.1. The INTEGRAL spectra comprise information ofthe three instruments JEM-X, IBIS (ISGRI), and SPI,including energies up to 1 MeV and were reduced usingINTEGRAL OSA 5.1. The data reduction followedthe standard procedures as described in the cookbooks( http://isdc.unige.ch/?Support+documents ).For XMM-Newton we only use data from the EPIC-pnas the EPIC-MOS was switched o ff to allocate maximumtelemetry to the EPIC-pn (see also the paragraph on theModified Timing Mode below). The energy range usedfor the XMM-Newton analysis is 2.8 to 9.4 keV.Due to the not yet finalized calibration of the XMM-Newton Modified Timing Mode we divide our analysis intwo parts. For the first part we combined the INTEGRALand RXTE data to study the broadband continuum. Theanalysis of the relativistically broadened Fe K α line wasdone independently using the XMM-Newton data only.
2. CONTINUUM
To model the broadband continuum we used the hy-brid thermal / non-thermal Comptonization code eqpair by [3] which also includes electron-positron pair produc-tion. In this model the temperature of the Comptoniz-ing medium is computed self-consistently by balancingCompton cooling with external heating. The amount ofheating is specified by the the ratio ℓ h /ℓ s of the compact-ness of the Comptonizing medium and the seed photondistribution. We modeled the soft emission by adding a diskbb component which provides the seed photons forthe Comptonization (therefore we set the temperature ofthe eqpair seed photons equal to the temperature at theinner edge of the disk). This continuum is partly reflectedo ff the accretion disk (XSPEC model reflect ) and mod-ified at lower energies by interstellar absorption.We first performed pure thermal eqpair fits to the fourobservations independently [7]. In all cases they showedevidence of a spectral hardening above ≈
300 keV whichcould be an indicator for the presence of a non-thermalelectron component in the plasma. In order to get bet-ter statistics for SPI in the crucial energy range we de-cided to sum up a time averaged spectrum comprising allINTEGRAL and RXTE observations although Cyg X-1was highly variable during the observations. For this timeaveraged spectrum we first considered again a pure ther-mal plasma. We fixed ℓ s =
10 to force the seed photonspectrum to be dominated by disk radiation and assumeda disk inclination of 45 deg [e.g. 9]. As the inner radius ofthe disk could not be constrained it was fixed to its defaultvalue R in = R g . Table 2 shows the best fit parameters.The resulting disk parameters kT in = . + . − . keV andnorm = + − as well as the values for the compactness ra-tio ( ℓ h /ℓ s = . + . − . ), the optical depth ( τ = . + . − . ), −6 −4 −2 no r m a li z ed c oun t s s − k e V − −4−2024−4−2024 Energy [keV] χ Figure 2. Best fit to the time averaged spectrum using the eqpair model (
PCA : 3–25 keV, dark blue;
HEXTE : 15–120 keV, light blue;
JEM-X : 4–26 keV, orange;
IBIS(IS-GRI) : 20–250 keV, yellow;
SPI : 25–1000 keV, red). Theupper panel shows the residuals to the pure thermal fit,the lower one those of the hybrid thermal / non-thermalmodel. and the reflection covering factor ( Ω / π = . + . − . )are consistent with the results obtained from analyzingthe four observations independently. Also the Iron lineparameters match the values obtained before. The χ yields a value of 1.72 with 350 dof. As shown in Fig. 2a deviation between the data points and this pure thermalmodel is still present in the time averaged spectrum.Therefore we added a non-thermal component in the eqpair model by allowing the parameter ℓ nth /ℓ h to vary.We found that 57% of the power supplied to the electronsin the plasma is contained in the non-thermal distribu-tion of the electrons, which was chosen to be a power law(Table 2). This additional component improves the χ to 1.62 (349 dof). The dimensionless parameter ℓ h /ℓ s in-creases to a value of 4 . + . − . , τ = . + . − . and Ω / π = . + . − . are also slightly higher than in the pure thermalmodel. The disk parameters ( kT in = . + . − . keV andnorm = + − ) as well as the values found for the Fe K α line do not change significantly with respect to the ther-mal model.
3. IRON LINE3.1. Modified Timing Mode
Due to telemetry restrictions it is not always trivial tostudy bright sources with the maximum possible time res- able 2. Best fit parameters for the eqpair model. Theiron line is modeled as a simple Gaussian here. eqpair (th.) eqpair (th. / nth.) N H [10 cm − ] 0 + . − + . − kT in [keV ] 1 . + . − . . + . − . norm 45 + − + − E K α [keV ] 6 . + . − . . + . − . σ K α [keV ] 0 . + . − . . + . − . ℓ h /ℓ s . + . − . . + . − . ℓ nth /ℓ h – 0 . + . − . τ . + . − . . + . − . Ω / π . + . − . . + . − . ξ + − + − χ / dof 1.72 /
350 1.62 / ff the EPIC-MOS camera and run-ning the EPIC-pn in a Modified Timing Mode [10]. Inthis mode the lower energy threshold was increased to2.8 keV (the standard value is 200 eV). This implies thata re-calibration of the instrument is required because thecombination of split events is not done on-board but dur-ing the first step of the EPIC-pn data analysis. Due tothe increased lower threshold a large fraction of the splitpartners is not transmitted and therefore the spectrum ap-pears to be softer. By comparison with former TimingMode observations we built a new detector response ma-trix [16]. However, there are still some e ff ects not in-cluded yet, for example the improvement of the ChargeTransfer E ffi ciency of the detector due to the high countrates. The special calibration for the Modified TimingMode will be made public through the XMM-NewtonSOC as soon as full confidence on the calibration hasbeen reached. The Fe K α line observed in BH candidates is intrinsicallynarrow with a rest frame energy of 6.4–6.97 keV depend-ing on the ionization state. It originates from materialwhich is just a few gravitational radii away from the BHand therefore it is broadened by gravitational redshift ef-fects as well as by Doppler shifts. The exact shape of the no r m a li z ed c oun t s c m − s − k e V − −10 0 10 −10 0 10 Energy [keV] χ Figure 3. Fit to the obs2
XMM-Newton data.
Upperpanel:
Power-law fit to the data outside the 5–8 keVband. There are strong residuals remaining in the Fe K α region. Lower panel:
Fit to the whole band using apower-law and a narrow line and a relativistic line. line depends on the accretion geometry, namely on theFe K α emissivity of the disk, the angular momentum ofthe BH and the observer’s viewing angle. Relativisticallybroadened Iron lines are therefore an important observa-tional tool for the understanding of the accretion geom-etry. For more information on relativistically broadenedIron lines see for example [15, 6]. Due to the increased soft X-ray emission during the tran-sitional state there is an increased pile-up in the centerof the point spread function. We therefore excluded datafrom the innermost 3 CCD columns from our analysis,however the signal to noise ratio of the data is still su ffi -cient for spectral analysis as could be seen in Fig. 3.A power-law fit to the XMM-Newton data outside the5–8 keV band reveals strong residuals in the Fe K α re-gion ( χ = . σ = ±
35 eV) line at E = . ± .
02 keV with anequivalent width of 14 eV and a relativistic Kerr line at E = . ± . χ dramatically to 1.3. The emissivity of the rela-tivistic line is found to be ∝ r − . ± . . These parametersare similar to earlier Chandra intermediate state observa-tions by Miller et al. [12]. Note however that the cali-bration of the timing mode is not yet fully completed sothat the energy values obtained from our analysis couldbe slightly too high. . SUMMARY AND OUTLOOK The 3 keV–1 MeV broadband spectrum of Cyg X-1 canbe well described by the eqpair model. The parame-ters we obtain from our analysis are consistent with pre-vious results e.g. from the RXTE monitoring campaign[17, 7, 14]. Furthermore our data show evidence for thepresence of a non-thermal component in the distributionof the electrons with 57% of the power supplied to theelectrons going into the non-thermal acceleration.The simultaneous XMM-Newton observation confirmsthe presence of a relativistic line [12] during the inter-mediate state of Cyg X-1 and the parameters obtained forthe broad and narrow components lead to the conclusionthat the accretion disk seems to extend down to the inner-most stable orbit at 1 . R g .As soon as the calibration of the Modified Timing Modeis completed we will combine all three instruments tomodel the whole broadband spectrum from 2.8 keV upto 1 MeV. REFERENCES [1] T. Belloni. In L. Burderi, L. A. Antonelli,F. D’Antona, T. di Salvo, G. L. Israel, L. Piersanti,A. Tornambè, and O. Straniero, editors,
InteractingBinaries: Accretion, Evolution, and Outcomes , vol-ume 797 of
American Institute of Physics Confer-ence Series , pages 197–204, 2005.[2] M. Cadolle Bel, P. Sizun, A. Goldwurm, J. Ro-driguez, P. Laurent, A. A. Zdziarski, L. Foschini,P. Goldoni, C. Goui ff ès, J. Malzac, E. Jourdain,and J.-P. Roques. Astron. Astrophys. , 446:591–602,2006.[3] P. S. Coppi.
Mon. Not. R. Astron. Soc. , 258:657–683, 1992.[4] W. Cui, S. N. Zhang, W. Focke, and J. H. Swank.
Astrophys. J. , 484:383, 1997.[5] J. F. Dolan, C. J. Crannell, B. R. Dennis, K. J. Frost,and L. E. Orwig.
Nature , 267:813–815, 1977.[6] A. C. Fabian.
Astronomische Nachrichten , 327:943,2006.[7] S. Fritz, J. Wilms, K. Pottschmidt, M.A. Nowak,I. Kreykenbohm, and A. Santangelo. In A. Wilson,editor,
Proceedings of the X-ray Universe 2005 ,number 604 in ESA SP, pages 267–268, Noordwijk,2006. ESA Publications Division.[8] M. Gierli´nski, A. A. Zdziarski, C. Done, W. N.Johnson, K. Ebisawa, Y. Ueda, F. Haardt, and B. F.Phlips.
Mon. Not. R. Astron. Soc. , 288:958–964,1997.[9] M. Gierli´nski, A. A. Zdziarski, J. Poutanen, P. S.Coppi, K. Ebisawa, and W. N. Johnson.
Mon. Not.R. Astron. Soc. , 309:496–512, 1999. [10] E. Kendziorra, J. Wilms, F. Haberl, M. G. F. Kirsch,M. Martin, and M. A. Nowak. In G. Hasinger andM. J. L. Turner, editors,
Proceedings of the SPIE,Volume 5488 , pages 613–622, 2004.[11] J. Malzac, P. O. Petrucci, E. Jourdain, M. CadolleBel, P. Sizun, G. Pooley, C. Cabanac, S. Chaty,T. Belloni, J. Rodriguez, J. P. Roques, P. Durou-choux, A. Goldwurm, and P. Laurent.
Astron. As-trophys. , 448:1125–1137, 2006.[12] J. M. Miller, A. C. Fabian, R. Wijnands, R. A.Remillard, P. Wojdowski, N. S. Schulz, T. Di Mat-teo, H. L. Marshall, C. R. Canizares, D. Pooley, andW. H. G. Lewin.
Astrophys. J. , 578:348–356, 2002.[13] Y. Ogawara, K. Mitsuda, K. Masai, J. V. Vallerga,L. R. Cominsky, J. M. Grunsfeld, J. S. Kruper, andG. R. Ricker.
Nature , 295:675, 1982.[14] K. Pottschmidt, J. Wilms, M. A. Nowak, S. Larsson,A. A. Zdziarski, and G. G. Pooley.
Advances inSpace Research , 38:1350–1353, 2006.[15] C. S. Reynolds and M. A. Nowak.
Phys. Rep ,377:389–466, 2003.[16] J. Wilms, E. Kendziorra, M.A. Nowak,K. Pottschmidt, F. Haberl, M. Kirsch, and S. Fritz.In A. Wilson, editor,
Proceedings of the X-rayUniverse 2005 , number 604 in ESA SP, pages217–222, Noordwijk, 2006. ESA PublicationsDivision.[17] J. Wilms, M. A. Nowak, K. Pottschmidt, G. G. Poo-ley, and S. Fritz.
Astron. Astrophys. , 447:245–261,2006.[18] J. Ziółkowski.