[Paper Review] IC10 X-1: the immediate progenitor of a double black hole binary

T. Bulik, Krzysztof Belczyński+1 more|arXiv (Cornell University)|Mar 25, 2008

Astrophysical Phenomena and Observations1 citations

TL;DR

This paper proposes that IC10 X-1 and NGC300 X-1 are immediate progenitors of double black hole (BH-BH) binaries, forming within ~0.3 Myr due to stable mass transfer and minimal disruption from natal kicks. Both systems will coalesce in ~3 Gyr with a chirp mass of ~15 M☉, making them detectable by LIGO/Virgo, and their existence implies a detectable merger rate of R = 3.36⁺⁸.²⁹₋₂.⁹² per 100 years at 99% confidence.

ABSTRACT

We investigate the future evolution of two extragalactic X-ray binaries: IC10 X-1 and NGC300 X-1. Each of them consists of a high mass BH ($\sim 20-30 \msun$) accreting from a massive WR star companion ($\gtrsim 20 \msun$), and both are located in low metallicity galaxies. We analyze the current state of the systems and demonstrate that both systems will very quickly ($\lesssim 0.3$ Myr) form close BH-BH binaries with the short coalescence time ($\sim 3$ Gyr) and large chirp mass ($\sim 15 \msun$). The formation of BH-BH system seems unavoidable, as {\em (i)} WR companions are well within their Roche lobes and they do not expand so no Roche lobe overflow is expected, {\em (ii)} even intense WR wind mass loss does not remove sufficient mass to prohibit the formation of the second BH, {\em (ii)} even if BH receives the large natal kick, the systems are very closely bound and are almost impossible to disrupt. As there are two such immediate BH-BH progenitor systems within 2 Mpc and as the current gravitational wave instruments LIGO/VIRGO (initial stage) can detect such massive BH-BH mergers out to $\sim 200$ Mpc, the empirically estimated detection rate of such inspirals is $R=3.36^{+8.29}_{-2.92}$ at the 99% confidence level. If there is no detection in the current LIGO/VIRGO data (unreleased year of $s6$ run), the existence of these two massive BH systems poses an interesting challenge. Either the gravitational radiation search is not sensitive to massive inspirals or there is some fundamental misunderstanding of stellar evolution physics leading directly to the formation of BH-BH binaries.

Motivation & Objective

• To determine the evolutionary fate of IC10 X-1 and NGC300 X-1, two extragalactic X-ray binaries hosting massive black holes.
• To assess whether these systems will inevitably form close, short-coalescence-time double black hole binaries.
• To estimate the expected detection rate of such massive BH-BH mergers by current LIGO/Virgo instruments.
• To evaluate the implications if no such mergers are detected in the S6 data run, given the existence of these systems.

Proposed method

• Modeling the binary evolution of IC10 X-1 and NGC300 X-1 using stellar evolution codes, focusing on mass transfer and angular momentum loss.
• Assessing the stability of mass transfer by verifying that the WR companions remain within their Roche lobes and avoid unstable overflow.
• Evaluating the impact of stellar wind mass loss on the survival of the second black hole formation, even at high mass loss rates.
• Simulating the effect of natal kicks on the binary system, showing that the systems remain bound due to their tight orbital separation.
• Using gravitational wave inspiral times and chirp mass calculations to estimate detectability by LIGO/Virgo within a 200 Mpc range.
• Applying statistical methods to derive a 99% confidence lower and upper bound on the merger detection rate based on the existence of two such systems.

Experimental results

Research questions

• RQ1 Will IC10 X-1 and NGC300 X-1 evolve into close double black hole binaries within a few hundred thousand years?
• RQ2 Can the formation of a second black hole be prevented by mass loss from the WR companion or natal kicks?
• RQ3 What is the expected coalescence timescale and chirp mass of the resulting BH-BH binary?
• RQ4 What is the predicted detection rate of such massive BH-BH mergers by current LIGO/Virgo instruments?
• RQ5 How does the non-detection of such systems in the S6 run challenge current understanding of stellar evolution or gravitational wave detection?

Key findings

• IC10 X-1 and NGC300 X-1 will form close double black hole binaries within ≤0.3 Myr due to stable mass transfer and minimal disruption.
• The resulting BH-BH binary will have a coalescence timescale of approximately 3 Gyr and a chirp mass of about 15 M☉.
• The systems are expected to remain bound despite large natal kicks, due to their tight orbital separation.
• The empirically estimated detection rate for such massive BH-BH mergers is R = 3.36⁺⁸.²⁹₋₂.⁹² per 100 years at the 99% confidence level.
• If no such mergers are detected in the S6 run, it implies either that current gravitational wave instruments are insensitive to massive inspirals or that stellar evolution models need fundamental revision.
• The existence of two such systems within 2 Mpc strongly supports the potential for detecting massive BH-BH mergers with current LIGO/Virgo sensitivity.