Identification of compact objects in X-ray/gamma-ray binaries and the exploration of long-period exoplanets by high-precision astrometry

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1 X-ray binary Gaia Gaia-JASMINE Joint 6-9 Dec exoplanet ESA/ATG NAOJ Identification of compact objects in X-ray/gamma-ray binaries and the exploration of long-period exoplanets by high-precision astrometry Lagrange et al Masaki YAMAGUCHI Institute of Astronomy, University of Tokyo Collaborators T. Yano, N. Gouda (NAOJ), and T. Matsuo (Osaka U.)

2 Outline Introduction Astrometric observation of an orbital motion Science objectives of astrometric binaries WD or NS for γ Cas (HMXB) NS or BH for gamma-ray binaries Exploration of Long-period exoplanets Summary

3 Astrometry for a star in a binary/planetary system The orbital motion can affect trajectories of stars due to its finite size of orbit = orbital motion can be extracted Kraus et al. 2009, A&A, 497, 195 lectures/mass/mass.html Stellar motion on the celestial sphere Helical motion Orbital motion (projected)

4 Two kinds of effects of the orbital motion Two cases in which the orbital motion affects its trajectory on the celestial sphere Case 1: periodic deviation from helical motion (smaller orbit) Case 2: Accelerated motion (larger orbit) Case 1: smaller orbit Case 2: larger orbit

5 X-ray/gamma-ray binaries Case 1: smaller orbit

6 Astrometry for X-ray/gamma-ray binaries Celestial sphere Optical star Compact object All orbital elements a = e = i = ω = Ω = P = + Primary mass, distance Mass of the compact object High precision White dwarf? Neutron star? Black hole?

7 Object WD or NS? : γ Cas (HMXB) Distance (kpc) period (days) Stellar mass (M o ) Consists of γ Cas Be +?? X-ray source associated with a Be star Wikipedia Shows peculiar behaviors, different from usual X- ray binaries: e.g., very low luminosity: L x ~ erg/s Candidate scenarios for explaining this X-ray: (1) Be-WD (or NS) (1) Be star (2) e.g., Lopes de Oliveira et al WDorNS (2) single Be X-ray Be star e.g., Robinson et al The compact object is not identified; WD or NS X-ray

8 NS or BH? : Gamma-ray binaries Object Distance (kpc) period (days) Stellar mass (M o ) Consists of LS O6.5+?? 1FGL J O6 +?? LSI B0e +?? HESS J B0e +?? What is gamma-ray binaries? Massive star & compact object Gamma rays are synchronized with binary phase Two emission scenarios are proposed (A) Accretion & Jet (BH is favored) (B) Wind collision (NS is favored) But which scenario is realized is under discussion The compact object is not identified BH scenario Mirabel 2012 NS scenario

9 Mass precision of X-ray/gamma-ray binaries Object The relation between the precision of mass σ m and that of SMA σ a (~10μas for Gaia or SJ) Distance (kpc) period (days) Stellar mass (M o ) distance Consists of Mass precision (M o ) γ Cas Be +?? 0.03 LS O6.5+?? 6 1FGL J O6 +?? 6 LSI B0e +?? 1 HESS J B0e +?? 0.5 γ Cas: high enough to identify the compact object (WD or NS) Latter two gamma-ray binaries: high enough to identify the compact object (NS or BH)

10 Long-period exoplanets Case 2: large binary orbit

11 Planet mass(m J ) Exoplanet exploration by astrometric method Astrometric method Detecting the projected motion by gravity of planets We can determine the mass! But high precision is required All sky survey by Gaia ~20,000 detections are predicted for five years Perryman et al Planets with semi-major axis~10au are detectable Casertano et al These planets can also be detected by the direct imaging method Astrometric method Gaia (launched in 2013) Casertano et al M o, 200pc M o, 25pc 1 10 Semi-major axis(au)

12 Planetary mass(m J ) Synergy with the direct imaging method Direct imaging method The number of planets ever detected: ~70 Typical separation>~3au Typical planetary mass>~3m J Gaia detectable region Partially overlapped by detectable region of direct imaging method Gaia may bring many planets observable by direct imaging True for specific instruments and for specific distance? In this talk Gaia detectable region Some instruments for direct imaging β Pic b (Period ~20 years) Gaia detectable region Lagrange et al Direct imaging Radial velocity mircolensing Exoplanet.eu Semi-major axis(au)

13 Companion mass(m J ) Results@10pc Probability that Gaia detect the acceleration Displacement by acceleration > 0.2mas (Central star is 0.3M TMT/PFI E-ELT/EPICS VLT/SPHERE 5-year period Double detection is possible for wide region Semi-major axis(au) follow-up observation by direct imaging method is possible for Planetary systems detected by Gaia

14 Companion mass(m J ) Results@140pc Central star is 0.3M Molecular Cloud) VLT/SPHERE E-ELT/EPICS TMT/PFI TMT/PFI E-ELT/EPICS Semi-major axis(au) 30-40m class telescopes are required for follow-up observations of direct imaging for targets in TMC

15 Issues of feasibility Gaia s cadence: ~2months Can Gaia detect binaries with a known period? Object Distance (kpc) period (days) Stellar mass (M o ) Small-JASMINE might be OK for these binaries Gaia s detectable acceleration? Consists of Is the smallest detectable δ ~10μas? 100μas? In this study, we adopt δ>3 x (7μas x N) (N=70) Mass precision (M o ) γ Cas Be +?? 0.03 LS O6.5+?? 6 1FGL J O6 +?? 6 LSI B0e +?? 1 HESS J B0e +?? 0.5 δ

16 Summary Astrometric measurement enable us to find the companion mass The precision of the compact object mass for X- ray/gamma-ray binaries The compact objects in γ Cas and two gamma-ray binaries will be identified by high-precision (~10μas) astrometry The parameter region where Gaia and some instruments for direct imaging can detect planets Double detection will be possible for large parameter region

17 Estimation of the compact object mass If we can observe the large part of the binary orbit, all orbital elements can be obtained (e.g., Aitken 1964) Relation between a companion mass M c and semi-major axis a derived using Kepler s 3 rd law M tot : total mass = M star + M c distance a = e = i = ω = Ω = P = + Primary mass, distance Compact object mass

18 Precision of the compact object mass By replacing Mc and a to their precisions, we obtain depends on P, d, M tot, and σ a (~10μas) This is valid when uncertainties of orbital period, distance, and primary mass are small Orbital periods are usually determined with a good precision Distances will be accurately measured by Gaia Primary masses are sufficiently small in this estimation