A 2.4-12 Microns Spectrophotometric Study with ISO of Cygnus X-3 in Quiescence
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1 Abstract. We present mid-infrared spectrophotometric results obtained with the ISO on the peculiar X-ray bi- nary Cygnus X-3 in quiescence, at orbital phases 0.83 to 1.04. The 2.4 - 12 µm continuum radiation observed with ISOPHOT-S can be explained by thermal free-free emis- sion in an expanding wind with, above 6.5 µm, a possi- ble additional black-body component with temperature T ∼ 250K and radius R ∼ 5000R⊙ at 10 kpc, likely due to thermal emission by circumstellar dust. The observed brightness and continuum spectrum closely match that of the Wolf-Rayet star WR 147, a WN8+B0.5 binary system, when rescaled at the same 10 kpc distance as Cygnus X- 3. A rough mass loss estimate assuming a WN wind gives −4 −1 ∼ 1.2 × 10 M⊙.yr . A line at ∼ 4.3 µm with a more than 4.3 σ detection level, and with a dereddened flux of 126 mJy, is interpreted as the expected He I 3p-3s line at 4.295 µm, a prominent line in the WR 147 spectrum. These results are consistent with a Wolf-Rayet-like com- panion to the compact object in Cyg X-3 of WN8 type, a later type than suggested by earlier works. Key words: binaries: close - stars: individual: Cyg X-3 - stars: Wolf-Rayet - stars: mass loss - infrared: stars arXiv:astro-ph/0207466v1 22 Jul 2002 A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: missing; you have not inserted them ASTROPHYSICS A 2.4 - 12 µm spectrophotometric study with ISO of CygnusX-3 in quiescence ⋆ Lydie Koch-Miramond1, P´eter Abrah´am´ 2,3, Ya¨el Fuchs1,4, Jean-Marc Bonnet-Bidaud1, and Arnaud Claret1 1 DAPNIA/Service d’Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France 2 Konkoly Observatory, P.O. Box 67, 1525 Budapest, Hungary 3 Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany 4 Universit´eParis VII, France Received 5 June 2002; Accepted 28 June 2002 1. Introduction removed, and a 2 - 2.5 M⊙ Wolf-Rayet like star remains with P = 0.2 d. A system containing a black hole and an CygnusX-3 has been known as a binary system since its He core burning star is also favored by Ergma & van den discovery by Giacconi et al. (1967), but there is still debate Heuvel (1998). In addition, Cygnus X-3 undergoes giant about the masses of the two stars and the morphology of radio bursts and there is evidence of jet-like structures the system (for a review see Bonnet-Bidaud & Chardin moving away from Cygnus X-3 at 0.3 - 0.9 c (Mioduszewski 1988). The distance of the object is 8-12.5kpc with an ab- et al. 1998, 2001; Mart´i et al. 2001). sorption on the line of sight A ∼ 20 mag (van Kerkwijk V The main objective of the Infrared Space Observatory et al. 1996). The flux modulation at a period of 4.8 hours, (ISO) spectrophotometric measurements in the 2.4 - 12 µm first discovered in X-rays (Parsignault 1972), then at near range was to constrain further the nature of the com- infrared wavelengths (Becklin et al. 1973), and observed panion star to the compact object: the expected strong simultaneously at X-ray and near-IR wavelengths by Ma- He lines as well as the metallic lines in different ioniza- son et al. (1986), is believed to be the orbital period of the tion states are important clues, together with the spectral binary system. Following infrared spectroscopic measure- shape of the continuum in a wavelength range as large as ments (van Kerkwijk et al. 1992), where WR-like features possible. An additional motivation for the imaging pho- have been detected in I and K band spectra, the nature of tometry with ISOCAM was to provide spatial resolution the mass-donating star is suggested to be a Wolf-Rayet- to a possible extended emission feature as a remnant of like star, but an unambiguous classification, similar to the the expected high mass loss from the system. The pa- other WR stars, is still lacking. Mitra (1996) and Van- per is laid out as follows. In Section 2 observational as- beveren et al. (1998) pointed out that it is not possible pects are reviewed. Section 3 summarizes the results on to find a model that meets all the observed properties of the continuum and line emissions from Cygnus X-3 and Cygnus X-3 where the companion star is a normal Pop- four Wolf-Rayet stars of WN6, 7 and 8 types. Section 4 ulation I Wolf-Rayet star with a spherically symmetric reviews the constraints set by the present observations on stellar wind. In the evolution model originally proposed the wind and on the nature of the companion to the com- by van den Heuvel & de Loore (1973) a final period of pact object in Cygnux X-3. Finally, Section 5 summarizes the order of 4.8 h may result from a system with initial 0 0 0 the conclusions of this paper. masses M1 =15 M⊙, M2 = 1 M⊙, P = 5 d, the final sys- tem being a neutron star accreting at a limited rate of ∼ −7 −1 2. Observations and data reduction 10 M⊙.yr , from the wind of a core He burning star of about 3.8 M⊙. Vanbeveren et al. (1998) proposed that We observed Cygnus X-3 with the Infrared Space Ob- the progenitor of Cygnus X-3 is a 50 M⊙ +10 M⊙ system servatory (ISO, see Kessler et al. 1996) on April7, 1996 0 with P = 6 d; after spiral-in of the black hole into the en- corresponding to JD 2450180.8033 to 2450180.8519. The velope of the companion, the hydrogen reach layers are subsequent observing modes were: ISOCAM imaging µ Send offprint requests to: Lydie Koch-Miramond (e-mail: photometry at 11.5 m (LW10 filter, bandwith 8 to [email protected]) 15 µm), ISOPHOT-S spectrophotometry in the range ⋆ Based on observations with ISO, an ESA project with in- 2.4 - 12 µm, for 4096s, covering the orbital phases 0.83 to struments funded by ESA Member States (especially the PI 1.04 (according to the parabolic ephemeris of Kitamoto countries: France, Germany, the Netherlands and the United et al. 1992); ISOPHOT multi-filter photometry at central Kingdom) and with the participation of ISAS and NASA. wavelengths 3.6, 10, 25 and 60 µm. Observing modes and Lydie Koch-Miramond et al.: Spectrophotometry of Cygnus X-3 in quiescence 3 Table 1. The ISO observing modes, ISO identifications, wavelength ranges and observing times on April7, 1996, used on CygnusX-3 Instrument TDTNUM Wavelength Aperture Total observing range (µm) arcsec time (s) TU (start) ISOCAM-LW10 14200701 8-15 1.5 1134 6:57:09 ISOPHOT-SS 2.4-4.9 24x24 14200803 4096 7:16:47 ISOPHOT-SL 5.9-11.7 24x24 ISOPHOT-P 3.6 2.9-4.1 10 ISOPHOT-P 10 9-10.9 23 14200802 1100 8:27:33 ISOPHOT-P 25 20-29 52 ISOPHOT-P 60 48-73 99 observation times are summarized in Table 1. Preliminary In order to verify our data reduction scheme (which results were presented in Koch-Miramond et al. (2001). is not completely standard due to the application of the FFT algorithm) and to estimate the level of calibration uncertainties, we reduced HD184400, an ISOPHOT stan- dard star observed in a similar way as CygnusX-3. The 2.1. ISOPHOT-S data reduction results were very consistent with the model prediction of A low resolution mid-infrared spectrum of Cygnus X-3 was the star, and we estimate that the systematic uncertainty obtained with the ISOPHOT-S sub-instrument. The spec- of our calibration is less than 10%. trum covered the 2.4 - 4.9 and 5.9 - 11.7 µm wavelength ranges simultaneously with a spectral resolution of about 2.2. ISOPHOT spectral energy distribution 0.04 and 0.1 µm, respectively. The observation was per- formed in the triangular chopped mode with two back- The observed spectral energy distribution is shown on ground positions located at ±120′′, and with a dwelling Fig. 1. The observed (not dereddened) continuum flux in time of 128 s per chopper position. The field of view is the range 2.4-7 µm is 20 ±10mJy in good agreement with 24”x24”. The whole measurement consisted of 8 OFF1– that observed by Ogley et al. (2001) with ISOCAM on the ON–OFF2–ON cycles and lasted 4096s. same day (the dereddened fluxes are shown on Fig. 2) ; the ± The ISOPHOT-S data were reduced in three steps. We observed flux decreases to about 10 8mJy around 9 µm. first used the Phot Interactive Analysis (PIA1, Gabriel An unresolved line is observed at about 4.3 µm peaking ± et al. 1997) software (Version 8.2) to filter out cosmic at 57 10 mJy. The linewidth is 0.04 µm, consistent with ∼ glitches in the raw data and to determine signals by per- the instrumental response and corresponding to 2500 −1 forming linear fits to the integration ramps. After a second km.s . Note that the measured line flux might be under- deglitching step, performed on the signals, a dark cur- estimated because the ISOPHOT-S pixels are separated rent value appropriate to the satellite orbital position of by small gaps, and a narrow line might falls into a gap. the individual signal was subtracted.