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Physics of Extreme Massive Stars Conference, Rio de Janeiro

June 28, 2024

Koushik Sen

Institute of Astronomy, Nicolaus Copernicus University, Torun

Whispering in the dark: Faint X-ray emission from BH+OB star binaries

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

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Massive binaries

M1 = 30 M

M2 = 20 M

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Black hole + OB star binaries

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Langer+2020, Sen+2021

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Searching techniques for BH+OB binaries

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  • 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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Accretion onto stellar mass BHs

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RO

Racc

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

Accretion radius of the compact object (Davidson+1973)

Bondi-Hoyle mass accretion rate (Bondi+1944)

figure not to scale

a

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Radiative efficiency of accreting BHs

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Xie & Yuan (2012)

𝛿 = electron heating parameter

𝝐 = radiative efficiency

​

𝝐 = 𝝐( , 𝛿)

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Grid of binary evolution models

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Mdonor,i = 10 - 90 Msun

qi = Maccretor,i/Mdonor,i = 0.25 - 0.95

Porb,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

arXiv:2406.08596

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X-ray luminosity of BH+OB star binaries

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

β‡’ β€œX-ray-faint” BH+OB star binaries

arXiv:2406.08596

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X-ray luminosity of BH+OB star binaries

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Sen, el Mellah, Langer et al. (2024, arXiv:2406.08596)

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Number predictions for the LMC

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β‰… 28 X-ray-faint systems in the LMC

β‰… 72 X-ray-faint systems in the LMC

arXiv:2406.08596

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A smoking gun: HD 96670

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MBH = 6.2 M

MOB = 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

*stellar parameters not well-constrained (Gomez+2021, Wang+2022)

=> excellent target for follow-up spectroscopy and a Chandra proposal

MBH = 6.2 Msun, MOB = 22.7 Msun

​

Porb = 5.28 d, ROB = 17.1 Rsun

​

Teff = 38000 K (Hohle+2010)

arXiv:2406.08596

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Observable properties of X-ray-faint systems

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arXiv:2406.08596

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Summary

  • 20-50% of BH+OB star binaries can be observed in faint X-rays.
  • Our simulations predict 28-72 BH+OB star binaries to be observable in the LMC from the SRG/eROSITA survey.
  • HD96670 is a smoking gun in the Milky Way.

X-ray-quiet

X-ray-bright

X-ray-faint

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Whispering in the dark: Faint X-ray emission from BH+OB star binaries

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

Physics of Extreme Massive Stars Conference, Rio de Janeiro

June 28, 2024

Koushik Sen

Institute of Astronomy, Nicolaus Copernicus University, Torun

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Number predictions for the LMC

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  • 120 BH+OB star binaries in 10-40 Msun range (Langer+2020).
  • 136 BH+OB star binaries in 10-90 Msun range (this work).
  • 10 BH+OB are X-ray-bright.
  • For 𝛿 = 0.001, 28 are X-ray-faint.
  • For 𝛿 = 0.1, 44 are X-ray-faint.
  • For 𝛿 = 0.5, 72 are X-ray-faint.

arXiv:2406.08596

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Whispering in the dark: Faint X-ray emission from BH+OB star binaries

Koushik Sen

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

β‡’ β€œX-ray-faint” BH+OB star binaries