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Koushik Sen

Nicolaus Copernicus University in Torun

Whispering in the dark: X-ray faint BHs around OB stars

with I. El Mellah, N. Langer, X.-T. Xu, Martin Quast, D. Pauli

September 16, 2024

VFTS Meeting, ESA, Madrid

arXiv:2406.08596, A&A, in press

A collection of horror stories

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Q: How many BHs will we find in BLOeM?

Answer format: Name - number - $$ amount (optional)

929 targets (Shenar+)

45% show RVV (Sana+) -> 418

130 B5–F5 supergiants (Lee+)

4-7 Oe, 9-16 Be binaries (Julia+)

3% of MS binaries -> BH comp (Langer+)

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Black hole + OB star binaries

see also Hirai+2021, Romero-Shaw+2023, Misra+2023

adapted from Kruckow+2018

Specific angular momentum (j) of the accreted matter (Shapiro+1976)

Specific angular momentum of test particle at ISCO radius (Sen+2024)

Observability of a BH+OB star binary as a High Mass X-ray Binary (HMXB)

Criterion for the formation of an accretion disk around the BH (Iben & Tutukov 1996)

Sen+2021: An accretion disk seldom forms around BHs in BH+OB systems

i) high wind velocity of the OB star companion, ii) first BH has negligible spin

adapted from Sen+2024

Isolated binary evolution

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Searching techniques for BH+OB binaries

strong X-ray emission (e.g. Walter et al. 2015; Motta et al. 2021).
astrometric variations (e.g Breivik et al. 2017; Mashian & Loeb 2017; Yamaguchi et al. 2018; Andrews et al. 2019).
photometric variability (Zucker et al. 2007; Masuda & Hotokezaka 2019).
spectroscopic monitoring (e.g. Geisers et al. 2018, Thompson et al. 2019, Mahy et al. 2022, Shenar et al. 2022).

Large population of X-ray quiet BHs remain to be discovered!!

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Revisiting accretion onto stellar mass BHs

2D, 3D GRMHD simulations

1D stellar evolution (MESA)

Ichimaru 1977; Narayan & Yi 1994; Bisnovatyi-Kogan & Lovelace 1997; Quataert & Narayan 1999; Blandford & Begelman 1999; Yuan 2001; ud-Doula & Owocki 2002; Sharma+2007; Yuan et al. 2012; Xie & Yuan 2012; Cunningham+2012; Cangemi+2021; El Mellah+2022

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Revisiting accretion onto stellar mass BHs

R_O

R_acc

Matter is accreted from the stellar wind (v_wind) of the O star

Accretion radius of the compact object (Davidson+1973)

Bondi-Hoyle mass accretion rate (Bondi+1944)

figure not to scale

So, when can a wind-fed accretion disk form? This diagram shows an O star with radius Rzero separated from the BH by a distance a. The matter accreted is from the stellar wind of the O star, where the wind velocity is given by this equation. Mzero is the mass of the O star. The BH can accrete matter from within an accretion radius definition as the region where the kinetic energy of the wind matter is smaller than the absolute value of its potential energy in the gravitational field of the BH. If the BH was stationary, then the denominator would be equal to the wind velocity, but since it is in an orbit, we have to take the vector sum of the wind velocity and orbital velocity of the BH. Now, the wind velocity across the accretion radius is not exactly the same as it depends on the exact distance. So, there will be a net influx of angular momentum through the accretion radius which can be calculated by a neat integration over the entire surface. Then, the amount of accreted angular momentum by the BH is given by this formula, where eta is an efficiency factor and omega orb is the orbital angular velocity of the binary. From detailed hydrodynamical simulations, eta is about one-third for BHs accreting matter from a stellar wind.

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Radiative efficiency of BHs w/o disks (ADAF)

Xie & Yuan (2012)

𝛿 = electron heating parameter

𝝐 = radiative efficiency

𝝐 = 𝝐( , 𝛿)

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Grid of binary evolution models

M_donor,i = 10 - 90 M_sun

q_i = M_accretor,i/M_donor,i = 0.25 - 0.95

P_orb,i ∼ 1 - 3162 days

*weighted by the Salpeter IMF, initial binary distribution functions, and the time spent in the BH+OB phase

Population syn* of BH+OB binaries

adapted from Kruckow+2018

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X-ray luminosity of BH+OB star binaries

Lx > 10³¹ erg/s can be detected (Crowther+2022)

⇒ “X-ray-faint” BH+OB star binaries

Sen+2024

Accretion disk present

Accretion disk absent

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MWC 656

XMM-Newton; for 14 ks

Munar-Adrover+2014

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X-ray luminosity of BH+OB star binaries

Source of 𝜹 = 0.5:

particle acceleration through magnetic reconnection (Quataert & Gruzinov 1999)

𝝈 of 10²-10⁵ possible for an OB star with B = 10 G)

Non-thermal synchrotron emission from the BH corona

Sen+2024

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A closer look at a BH+OB binary

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A smoking gun: HD 96670

M_BH = 6.2 M

M_OB = 22.7 M

Observed Lx = 2.2e32 erg/s (NuSTAR, Gomez+2021) to 2.4e34 erg/s (XMM-Newton, Saxton+2008)

Predicted Lx = 8e33 erg/s

M_BH= 6.2 Msun, M_OB= 22.7 Msun

P_orb= 5.28 d, R_OB = 17.1 Rsun

Teff = 38000 K (Hohle+2010)

Sen+2024

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Number predictions for the LMC

≅ 28 X-ray-faint systems in the LMC

≅ 72 X-ray-faint systems in the LMC

Sen+2024

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Observable properties of X-ray-faint systems

Sen+2024

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Observable properties of X-ray-faint systems

Sen+2024

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Predictions for the SMC

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Future directions

Chandra Legacy Program for the LMC (Proposal ID #24400340, PI: Vallia Antoniou, Lx >= 10**32 erg/s).

Deep Chandra Survey of the SMC (Antoniou+2019, Lx >= 10³² erg/s).

XRISM for spectra (energy resolution 7 eV).

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20-50% of BH+OB star binaries can be observed in faint X-rays.
Our simulations predict 28-72 BH+OB star binaries may be observable in the LMC from long exposure surveys.
HD96670 is a smoking gun in the Milky Way.

Summary

X-ray-quiet

X-ray-bright

X-ray-faint

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