Siegel et al. 2023: Investigating the Lower Mass Gap with Low-mass X-Ray Binary Population Synthesis

Investigating the Lower Mass Gap with Low-mass X-Ray Binary Population Synthesis

Jared C. Siegel, Ilia Kiato, Vicky Kalogera, Christopher P. L. Berry, Thomas J. Maccarone, Katelyn Breivik, Jeff J. Andrews, Simone S. Bavera, Aaron Dotter, Tassos Fragos, Konstantinos Kovlakas, Devina Misra, Kyle A. Rocha, Philipp M. Srivastava, Meng Sun, Zepei Xing, Emmanouil Zapartas

Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio pulsars have been used to identify a gap between the most massive neutron stars (NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs are typically only possible for transient systems: outburst periods enable detection via all-sky X-ray monitors, while quiescent periods enable radial velocity measurements of the low-mass donor. We quantitatively study selection biases due to the requirement of transient behavior for BH mass measurements. Using rapid population synthesis simulations (COSMIC), detailed binary stellar-evolution models (MESA), and the disk instability model of transient behavior, we demonstrate that transient LMXB selection effects introduce observational biases, and can suppress mass-gap BHs in the observed sample. However, we find a population of transient LMXBs with mass-gap BHs form through accretion-induced collapse of an NS during the LMXB phase, which is inconsistent with observations. These results are robust against variations of binary evolution prescriptions. The significance of this accretion-induced collapse population depends upon the maximum NS birth mass MNS,birth−max . To reflect the observed dearth of low-mass BHs, COSMIC and MESA models favor MNS,birth−max≲2M⊙ . In the absence of further observational biases against LMXBs with mass-gap BHs, our results indicate the need for additional physics connected to the modeling of LMXB formation and evolution.

Figure 3. From population synthesis, each combination of binary evolution prescription and disk instability model yields an observed LMXB mass distribution with a substantial fraction of mass-gap BHs (for MNS,birth-max = 3M⊙ ). Left: the detection probability-weighted fraction of LMXBs with mass gap BHs, flow(pdetect), is presented against the unweighted fraction, flow−intrinsic , for each model (using either no transition to RIA, a sharp transition, or a smooth transition). For points that fall above the dashed black line, detection effects lead to a dearth of mass-gap BH detections relative to the intrinsic population. Right: the pdiscover detection-weighted fraction of LMXBs with a mass-gap BH is presented against the ptransition -weighted fraction. This panel breaks down the two components of the LMXB detection probability: (i) being bright enough during outburst for discovery pdiscover and (ii) transitioning from outburst to quiescence during the survey ptransition . Relative to ptransition , weighting by pdiscover suppresses observation of mass-gap BHs.

NASA/ADS: 2023ApJ…954..212S

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