The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs

Abstract: We present a first look at the Galactic population of heretofore known HMXBs as observed by SRG/eROSITA during its first four surveys. eROSITA's sensitivity of $$\sim10<sup>{-13}\,\mathrm{erg}\,\mathrm{s}<sup>{-1}\,\mathrm{cm}<sup>{-2}$$, translating to $10<sup>{32}$-$10<sup>{34},\mathrm{erg}\,\mathrm{s}<sup>{-1}$ in luminosity for most known HMXBs in the Milky Way, has thus far never been reached by any wide-area survey instrument. We present the extended log N-log L distribution of known HMXBs reaching down to $10<sup>{32}\,\mathrm{erg}\,\mathrm{s}<sup>{-1}$ using eROSITA, and show the large scatter that can be induced by source intrinsic variability. We present sub-type resolved luminosity distributions, showing that the Supergiant X-ray binaries (SgXBs) and Be X-ray binaries (BeXRBs) occupy different parts of the overall distribution, and reanalyse RXTE/ASM data and MAXI for comparison to eROSITA. The luminosity regime uncovered by eROSITA allows a systematic study of the "transient" BeXRBs, which are typically below the detection threshold of monitors outside of outburst, and whose low luminosity behavior has been a longstanding question. Signatures of stable accretion at low luminosities have been observed with pointed instruments for a fraction of the overall sample, so far. With the eROSITA results, we posit that accretion outside of outburst is likely the norm, since a vast majority (> 80%) of BeXRBs are detected at luminosities at least an order of magnitude higher than expected for the most X-ray luminous Be stars. We discuss the observed luminosity in the context of cold disk accretion and the "propeller" mechanism. We highlight a small subpopulation of "isolated" Be-stars that reach luminosities comparable to the least luminous BeXRBs, hinting at the presence of compact object companions.