Pulsars, transients and relativistic astrophysics

Abstract:

We review the existing distance estimates to the black hole X-ray binary Swift J1727.8–1613, present new radio and near-UV spectra to update the distance constraints, and discuss the accuracies and caveats of the associated methodologies. We use line-of-sight H i absorption spectra captured using the MeerKAT radio telescope to estimate a maximum radial velocity with respect to the local standard of rest of 24.8 ± 2.8 km s−1 for Swift J1727.8−1613, which is significantly lower than that of a nearby extragalactic reference source. From this, we derive a near-kinematic distance of dnear = 3.6 ± 0.3 (stat) ± 2.3 (sys) kpc as a lower bound after accounting for additional uncertainties given its Galactic longitude and latitude, (l, b) ≈ (8.6°, 10.3°). Near-UV spectra from the Hubble Space Telescope’s Space Telescope Imaging Spectrograph allows us to constrain the line-of-sight color excess to E(B – V) = 0.37 ± 0.01 (stat) ± 0.025 (sys). We then implement this in Monte Carlo simulations and present a distance to Swift J1727.8−1613 of 5.5−1.1+1.4 kpc, under the assumption that the donor star is an unevolved, main-sequence K4(±1)V star. This distance implies a natal kick velocity of 190 ± 30 km s−1 and therefore an asymmetrical supernova explosion within the Galactic disk as the expected birth mechanism. A lower distance is implied if the donor star has instead lost significant mass during the binary evolution. Hence, more accurate measurements of the binary inclination angle or donor star rotational broadening from future observations would help to better constrain the distance.

Abstract:

Jets from stellar-mass and supermassive black holes provide the unique opportunity to study similar processes in two very different mass regimes. Historically, the apparent speeds of black hole X-ray binary (BHXRBs) jets have been observed to be lower than jet speeds from active galactic nuclei (AGNs) and specifically blazars. In this work, we show that selection effects could be the primary cause of the observed population differences. For the first time, it is possible to perform a statistical analysis of the underlying BHXRB jet Lorentz factor distribution. We use both the Anderson–Darling test and apply nested sampling to this problem. With Bayes factors, we confirm that the Lorentz factor distribution of BHXRBs is best described with a power law, the same model that has been applied to AGN jets. For a Lorentz factor distribution following we find a value for the exponent of . This exponent is consistent with values found in AGN population studies, within for Swift-BAT and Fermi-LAT selected AGNs. The best-fitting exponent for the radio selected MOJAVE sample is just above our limit. This is a remarkable agreement given the different scales at which the jets are observed. The observed slower apparent speeds in BHXRBs are largely due to the much larger inclinations in this sample. Furthermore, nested sampling confirms that is completely unconstrained using this method. Therefore, based on kinematics alone, BHXRB jets are broadly consistent with being just as relativistic as those from supermassive black holes.