A scientist's diagram illustrating the layout of a BeXRB system, showing the decretion disk around the Be star and a neutron star on an eccentric (highly elliptical) orbit around it, but still maintaining steady accretion despite being far away from the material of the disk. Image credit: P. Thalhammer/Remeis Observatory. Download Image
During the past week, NuSTAR observed IGR J06074+2205, the latest in a series of observations of a sample of Be X-ray binaries (BeXRBs) — binary systems with a neutron star orbiting and accreting matter from its Be star companion, a type of massive star characterized by unique equatorial outflows. BeXRBs display occasional but exceptionally bright outbursts, partly resulting from their eccentric orbits, and are well-studied during these bright states. Their behaviour when the neutron star is farther away from the Be star has been a longstanding question. A handful of BeXRBs — the most well-known of them — have already been confirmed to accrete steadily at low luminosities, challenging previous models. The current observations, by targeting those systems that have not yet been observed at low luminosities, aim to complement our understanding of BeXRBs by performing an unbiased hard X-ray survey to probe the universality of prolonged stable accretion outside of outbursts in these systems, and to identify exceptions to what is becoming the prevalent scenario. The data obtained will not only shed light on the properties of individual accreting neutron stars, such as their magnetic fields and spin periods, but will also contribute towards sharpening constraints on the X-ray luminosity function of High Mass X-ray Binaries, which are crucial to understanding the star formation history of our Galaxy.
During the past week, NuSTAR observed IGR J06074+2205, the latest in a series of observations of a sample of Be X-ray binaries (BeXRBs) — binary systems with a neutron star orbiting and accreting matter from its Be star companion, a type of massive star characterized by unique equatorial outflows. BeXRBs display occasional but exceptionally bright outbursts, partly resulting from their eccentric orbits, and are well-studied during these bright states. Their behaviour when the neutron star is farther away from the Be star has been a longstanding question. A handful of BeXRBs — the most well-known of them — have already been confirmed to accrete steadily at low luminosities, challenging previous models. The current observations, by targeting those systems that have not yet been observed at low luminosities, aim to complement our understanding of BeXRBs by performing an unbiased hard X-ray survey to probe the universality of prolonged stable accretion outside of outbursts in these systems, and to identify exceptions to what is becoming the prevalent scenario. The data obtained will not only shed light on the properties of individual accreting neutron stars, such as their magnetic fields and spin periods, but will also contribute towards sharpening constraints on the X-ray luminosity function of High Mass X-ray Binaries, which are crucial to understanding the star formation history of our Galaxy.
Author: Aafia Ansar Mohideen (Research Scientist, Dr. Karl Remeis Sternwarte Astronomical Institute, Germany)
