On the recurrence times of neutron star X-ray binary transients and the nature of the Galactic Centre quiescent X-ray binaries

The presence of some X-ray sources in the Galactic Centre region which show variability, but do not show outbursts in over a decade of monitoring has been used to argue for the presence of a large population of stellar mass black holes in this region. A core element of the arguments that these objects are accreting black holes is the claim that neutron stars (NSs) in low mass X-ray binaries (LMXBs) do not have long transient recurrence times. We demonstrate in this paper that about half of the known transient LMXBs with clear signatures for NS primaries have recurrence times in excess of a decade for outbursts at the sensitivity of MAXI. We furthermore show that, in order to reconcile the expected total population of NS LMXBs with the observed one and with the millisecond radio pulsar (MSRP) population of the Galaxy, systems with recurrence times well in excess of a century for outbursts detectable by instruments like MAXI must be the dominant population of NS LMXBs, and that few of these systems have yet been discovered.Comment: 7 pages, accepted to MNRAS, small correction made to abstract from originally posted version to remove an ambiguit

Chasing the break: tracing the full evolution of a black hole X-ray binary jet with multiwavelength spectral modeling

Black hole (BH) X-ray binaries (XRBs) are ideal targets to study the connection between accretion inflow and jet outflow. Here we present quasi-simultaneous, multiwavelength observations of the Galactic BH system MAXI J1820+070, throughout its 2018–2019 outburst. Our data set includes coverage from the radio through X-ray bands from 17 different instruments/telescopes, and encompasses 19 epochs over a 7 month period, resulting in one of the most well-sampled multiwavelength data sets of a BH XRB outburst to date. With our data, we compile and model the broadband spectra of this source using a phenomenological model that includes emission from the jet, a companion star, and an accretion flow. This modeling allows us to track the evolution of the spectral break in the jet spectrum, a key observable that samples the jet launching region. We find that the spectral break location changes over at least ≈3 orders of magnitude in electromagnetic frequency over this period. Using these spectral break measurements, we link the full cycle of jet behavior, including the rising, quenching, and reignition, to the changing accretion flow properties as the source evolves through its different accretion states. Our analysis shows consistent jet behavior with other sources in similar phases of their outbursts, reinforcing the idea that jet quenching and recovery may be a global feature of BH XRB systems in outburst. Our results also provide valuable evidence supporting a close connection between the geometry of the inner accretion flow and the base of the jet

The impact of thermal winds on the outburst lightcurves of black hole X-ray binaries

Context. The observed signatures of winds from X-ray binaries are broadly consistent with thermal winds, which are driven by X-ray irradiation of the outer accretion disc. Thermal winds produce mass outflow rates that can exceed the accretion rate in the disc. Aims. We aim to study the impact of thermal wind mass loss on the stability and lightcurves of black hole X-ray binaries subject to the thermal-viscous instability driving their outbursts. Strong mass loss could stop outbursts early, as proposed for the 2015 outburst of V404 Cyg. Methods. We used an analytical model for thermal (Compton) wind mass loss as a function of radius, X-ray spectrum, and luminosity that was calibrated against numerical simulations. We also estimated the fraction of the X-rays, emitted close to the compact object, that are scattered back to the outer disc in the wind. Scattering in the thermal wind couples irradiation to the disc size and inner mass accretion rate. The disc evolution equations were modified to include this wind mass loss and the varying irradiation fraction. Results. Scattering in the strong wind expected of long Porb systems enhances the irradiation heating of the outer disc, keeping it stable against the thermal-viscous instability. This accounts very well for the existence of persistently bright systems with large discs, such as Cyg X-2, 1E 1740.7−2942, or GRS 1758−258. Mass loss from the thermal wind shortens the outburst, as expected, but it is insufficient in explaining the rapid decay timescale of black-hole X-ray binary outbursts. However, including the wind-related varying irradiation fraction produces lightcurves with plateaus in long Porb systems like GRO J1655−40. Plateau lightcurves may be a dynamical signature of enhanced irradiation due to scattering in thermal winds. Conclusions. Mass loss due to thermal winds is not a major driver for the outburst dynamics up to luminosities of 0.1 − 0.2 LEdd. Higher luminosities may produce stronger mass loss but studying them is complicated since the wind becomes opaque. Magnetic winds, which extract angular momentum with little mass loss, seem more promising to explain the fast decay timescales generically seen in black-hole X-ray binaries. Thermal winds can play an important role in the outburst dynamics through the varying irradiation heating. This may be evidenced by relating changes in wind properties, X-ray spectra, or luminosity with changes in the optical emission that traces the outer disc. Simulations should enable more accurate estimates of the dependence of the irradiation onto the disc as a function of irradiation spectrum, radius, and disc wind properties

Chasing the break: Tracing the full evolution of a black hole X-ray binary jet with multi-wavelength spectral modeling

International audienceBlack hole X-ray binaries (BH XRBs) are ideal targets to study the connection between accretion inflow and jet outflow. Here we present quasi-simultaneous, multi-wavelength observations of the Galactic black hole system MAXI J1820+070, throughout its 2018-2019 outburst. Our data set includes coverage from the radio through X-ray bands from 17 different instruments/telescopes, and encompasses 19 epochs over a 7 month time period, resulting in one of the most well-sampled multi-wavelength data sets of a BH XRB outburst to date. With our data, we compile and model the broad-band spectra of this source using a phenomenological model that includes emission from the jet, companion star, and accretion flow. This modeling allows us to track the evolution of the spectral break in the jet spectrum, a key observable that samples the jet launching region. We find that the spectral break location changes over at least $\approx3$ orders of magnitude in electromagnetic frequency over this period. Using these spectral break measurements, we link the full cycle of jet behavior, including the rising, quenching, and re-ignition, to the changing accretion flow properties as the source evolves through its different accretion states. Our analyses show a consistent jet behavior with other sources in similar phases of their outbursts, reinforcing that the jet quenching and recovery may be a global feature of BH XRB systems in outburst. Our results also provide valuable evidence supporting a close connection between the geometry of the inner accretion flow and the base of the jet

Paving the way to simultaneous multi-wavelength astronomy

Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field