A Be-type star with a black hole companion

Transcription

1 A Be-type star with a black hole companion Jorge Casares (IAC) I. Negueruela (UA) M. Ribó (UB) I. Ribas (IEEC, CSIC) J.M. Paredes (UB) A. Herrero (IAC) S. Simón-Díaz (IAC) Nature (2014; 16 Dec)

2 Stellar-Mass BHs All BHs discovered through X-ray emission triggered by accretion in interactive binaries. Vast majority are fed by low-mass stars and detected as X-ray Transients (XRTs) OUTBURST: viscous instability in accretion disc QUIESCENCE (L X /L Edd <10-5 ): companion can be detected radial velocity studies 17 dynamical BHs

3 Dynamical BHs + 4 HMXBs Massive OB donors Wind fed X-ray persistent 17 XRTs McClintock et al CQGra But no BHs fed by Be stars Remarkable because ~80 BeXBs known in the Galaxy (~150 including Magellanic Clouds), hosting pulsars

4 38 extragalactic 60 galactic LS I Fermi&AGILE Cygnus X-1 AGILE LS 5039 PSR B Fermi&AGILE AGILE 19 PWN 1 Pulsar 9 SNR 4 BS 1 WR 1 OC GC 24 UNID At HE (E > 100 MeV) Cygnus X-3: Fermi & AGILE

5 Mirabel 2006, (Perspective) Science LS 5039 O6.5V P orb =3.9 d e=0.35 1FGL J O6V 16.6 d e=?? LSI B0V(e) 26.5 d e=0.54 HESS J B0V(e) 321 d e=0.83 PSR B O9.5V(e) 1237 d e=0.87

6 Short γ-ray flare detected by AGILE Lucarelli et al. 2010, Atel 2761 V=8.8 mag Hipparcos+TAAS+NSVS light curve P= ± 0.04 d Williams et al. 2010, ApJ 723 L93

7 LT campaign 2011 Equatorial disc emission (mainly HI, FeII) Broad photospheric lines (Vsini ~346 km/s) Absorption line velocities Casares et al. 2012, MNRAS

8 Unexpected HeII 4686 emission Mercator spectrum (R=80000) Moves in antiphase with HeI absorptions: orbits the companion and not the Be Double-peaked profile: Keplerian disc Use HeII 4686 to trace the orbit of the companion to the Be star

9 Refined Orbit of Be star FeII lines have very symmetric/sharp double peaks and formed in innermost regions of the equatorial Be disc (Arias et al. 2006) We fit 2-Gaussian template to double peak in FeII 4583 improved Radial Velocity curve of Be star

10 Orbital Solutions Independent orbital fits to HeII 4686 and FeII 4583 velocities yield same eccentricity, Time of Periastron and also ω HeII =ω FeII +180 o Proves the 2 lines display the reflex motion of two stars in a binary New fit to the ensemble of 72 velocities with a double-line orbital model FeII

11 Spectral Classification and Mass of Be star MWC 656 B1.5 III B2 III Narrow Balmer wings indicate class III Relative strength of metallic lines strongly supports B1.5 III Sp.Type-Mass calibrations: detached eclipsing binaries (e.g. B1.5III in V380 Cyg yield dynamical mass 13.1±0.2 M ) We conservatively adopt M to encompass uncertainties in Spect. Class. Mass likely underestimated ~10-15% because large rotation makes star appear cooler and less luminous

12 Nature of Be companion Orbital solution M 2 /M 1 =0.41±0.07 Spectral Classification M 1 =10-16 M M 2 = M MS star with such mass would be a B3-B9 and easily detected Stripped He core (WR) has strong winds with high excitation lines And too massive for a Subdwarf, WD or NS Only alternative is a BH Further, none of the BeXBs known shows evidence for a stable accretion disc, many with same spect. class. as MWC 656. This suggests a fundamental difference in the nature of the compact star.

13 Implications (I) NS/BeXBs are transients: easily detected through X-ray outbursts caused by enhanced accretion during periastron (Type I) or disruption of Be disc (Type II) Two possible explanations for the lack of BH/BeXBs 1) Selection bias (Zhang et al. 2004): BH/BeXBs are transient with very long recurrence times. BPS by Podsiadlowsky et al (2003) BH/Bes form with short (<10 d) periods and low eccentricity Be discs tidally truncated by BHs, suppressing mass transfer/x-ray activity. 2) Evolutionary bias (Belczynski et al. 2009): BH/BeXBs rarely survive CE phase. BPS predict a few tens BH BeXBs in the Galaxy, and a ratio NS/ BH ~50. There should be 0-2 BHs among the ~80 BeXBs currently known. Hence MWC 656 fulfils the statistics

14 Implications (II) Unlikely we have discovered the only BH BeXB At d=2.6±0.6 kpc MWC 656 is relatively close and one of the (optically) brightest BeXBs. Many other MWC656-clones may exist in the Galaxy. Key property is the faint X-ray luminosity: L X 2x10 32 erg/s (ROSAT) or 3x 10-7 L Edd. Comparable to quiescent BH transients. Similarly, the accretion disc in MWC 656 is in a quiescent state. BeXBs have very low mass transfer rates (peak values M /yr near periastron) which imply extremely long outburst recurrence periods. Dormant condition + lack of solid surface explains why BH BeXBs are very difficult to detect through X-ray surveys.

15 CONCLUSIONS Be/BH may be more abundant than predicted by BPS