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Register a new userNGC 300 X-1 is a Wolf-Rayet/Black-Hole binary
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Added by
Dr Paul A. Crowther
Publication date
2010
fields
Physics
and research's language is
English
Authors
P. A. Crowther
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We present VLT/FORS2 time-series spectroscopy of the Wolf-Rayet star #41 in the Sculptor group galaxy NGC 300. We confirm a physical association with NGC 300 X-1, since radial velocity variations of the HeII 4686 line indicate an orbital period of 32.3 +/- 0.2 hr which agrees at the 2 sigma level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 +/- 8 km/s, from which a mass function of 2.6 +/- 0.3 Msun is obtained. A revised spectroscopic mass for the WN-type companion of 26+7-5 Msun yields a black hole mass of 20 +/- 4 Msun for a preferred inclination of 60-75 deg. If the WR star provides half of the measured visual continuum flux, a reduced WR (black hole) mass of 15 +4 -2.5 Msun (14.5 +3 -2.5 Msun) would be inferred. As such, #41/NGC 300 X-1 represents only the second extragalactic Wolf-Rayet plus black-hole binary system, after IC 10 X-1. In addition, the compact object responsible for NGC 300 X-1 is the second highest stellar-mass black hole known to date, exceeded only by IC 10 X-1.
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Context. NGC 300 X-1 is the second extragalactic candidate, after IC 10 X-1, in the rare class of Wolf-Rayet/compact object X-ray binary systems exemplified in the Galaxy by Cyg X-3. From a theoretical point of view, accretion onto a black hole in a detached system is possible for large orbital periods only if the mass of the relativistic object is high or the velocity of the accreted wind is low. Aims. We analysed a 2 week SWIFT XRT light curve of NGC 300 X-1 and searched for periodicities. Methods. Period searches were made using Lomb-Scargle periodogram analysis. We evaluated the confidence level using Monte Carlo simulations. Results. A period of 32.8+-0.4h (3 sigma error) was found for NGC 300 X-1 with a confidence level >99%. Furthermore, we confirm the high irregular variability during the high flux level, as already observed in the XMM-Newton observations of the source. A folded XMM-Newton light curve is shown, with a profile that is in agreement with SWIFT. The mean absorbed X-ray luminosity in the SWIFT observations was 1.5x10^38 erg/s, close to the value derived from the XMM-Newton data. Conclusions. While Cyg X-3 has a short period of 4.8 h, the period of NGC 300 X-1 is very close to that of IC 10 X-1 (34.8+-0.9 h). These are likely orbital periods. Possibility of formation of accretion disk for such high orbital periods strongly depends on the terminal velocity of the Wolf-Rayet star wind and black-hole mass. While low masses are possible for wind velocities < 1000 km/s, these increase to several tens of solar masses for velocities > 1600 km/s and no accretion disk may form for terminal velocities larger than 1900 km/s.
We have discovered a persistent, but highly variable X-ray source in the nearby starburst galaxy NGC 253. The source varies at the level of a factor of about 5 in count rate on timescales of a few hours. Two long observations of the source with Chandra and XMM-Newton show suggestive evidence for the source having a period of about 14-15 hours, but the time sampling in existing data is insufficient to allow a firm determination that the source is periodic. Given the amplitude of variation and the location in a nuclear starburst, the source is likely to be a Wolf-Rayet X-ray binary, with the tentative period being the orbital period of the system. In light of the fact that we have demonstrated that careful examination of the variability of moderately bright X-ray sources in nearby galaxies can turn up candidate Wolf-Rayet X-ray binaries, we discuss the implications of Wolf-Rayet X-ray binaries for predictions of the gravitational wave source event rate, and, potentially, interpretations of the events.
IC 10 X-1 is a bright (Lx=10^38 ergs/s) variable X-ray source in the local group starburst galaxy IC 10. The most plausible optical counterpart is a luminous Wolf-Rayet star, making IC 10 X-1 a rare example of a Wolf-Rayet X-ray binary. In this paper, we report on the detection of an X-ray orbital period for IC 10 X-1of 34.4 hours. This result, combined with a re-examination of optical spectra, allow us to determine a mass function for the system f(m)=7.8 Msun and a probable mass for the compact object of 24-36 Msun. If this analysis is correct, the compact object is the most massive known stellar black black hole. We further show that the observed period is inconsistent with Roche lobe overflow, suggesting that the binary is detached and the black hole is accreting the wind of the Wolf-Rayet star. The observed mass loss rate of [MAC92] 17-A is sufficient to power the X-ray luminosity of IC 10 X-1.
Using XMM-Newton, we undertook a dedicated project to search for X-ray bright wind-wind collisions in 18 WR+OB systems. We complemented these observations with Swift and Chandra datasets, allowing for the study of two additional systems. We also improved the ephemerides, for these systems displaying photometric changes, using TESS, Kepler, and ASAS-SN data. Five systems displayed a very faint X-ray emission ($log [L_{rm X}/L_{rm BOL}]<-8$) and three a faint one ($log [L_{rm X}/L_{rm BOL}]sim-7$), incompatible with typical colliding wind emission: not all WR binaries are thus X-ray bright. In a few other systems, X-rays from the O-star companion cannot be excluded as being the true source of X-rays (or a large contributor). In two additional cases, the emission appears faint but the observations were taken with the WR wind obscuring the line-of-sight, which could hide a colliding wind emission. Clear evidence of colliding winds was however found in the remaining six systems (WR19, 21, 31, 97, 105, 127). In WR19, increased absorption and larger emission at periastron are even detected, in line with expectations of adiabatic collisions.
We show that black-hole High-Mass X-ray Binaries (HMXBs) with O- or B-type donor stars and relatively short orbital periods, of order one week to several months may survive spiral in, to then form Wolf-Rayet (WR) X-ray binaries with orbital periods of order a day to a few days; while in systems where the compact star is a neutron star, HMXBs with these orbital periods never survive spiral-in. We therefore predict that WR X-ray binaries can only harbor black holes. The reason why black-hole HMXBs with these orbital periods may survive spiral in is: the combination of a radiative envelope of the donor star, and a high mass of the compact star. In this case, when the donor begins to overflow its Roche lobe, the systems are able to spiral in slowly with stable Roche-lobe overflow, as is shown by the system SS433. In this case the transferred mass is ejected from the vicinity of the compact star (so-called isotropic re-emission mass loss mode, or SS433-like mass loss), leading to gradual spiral-in. If the mass ratio of donor and black hole is $>3.5$, these systems will go into CE evolution and are less likely to survive. If they survive, they produce WR X-ray binaries with orbital periods of a few hours to one day. Several of the well-known WR+O binaries in our Galaxy and the Magellanic Clouds, with orbital periods in the range between a week and several months, are expected to evolve into close WR-Black-Hole binaries,which may later produce close double black holes. The galactic formation rate of double black holes resulting from such systems is still uncertain, as it depends on several poorly known factors in this evolutionary picture. It might possibly be as high as $sim 10^{-5}$ per year.
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