A strongly truncated inner accretion disk in the Rapid Burster

The neutron star (NS) low-mass X-ray binary (LMXB) the Rapid Burster (RB; MXB 1730-335) uniquely shows both Type I and Type II X-ray bursts. The origin of the latter is ill-understood but has been linked to magnetospheric gating of the accretion flow. We present a spectral analysis of simultaneous $\textit{Swift}$, $\textit{NuSTAR}$ and $\textit{XMM–Newton}$ observations of the RB during its 2015 outburst. Although a broad Fe K line has been observed before, the high quality of our observations allows us to model this line using relativistic reflection models for the first time. We find that the disc is strongly truncated at 41.8$^{+6.7}_{−5.3}$ gravitational radii (∼87 km), which supports magnetospheric Type II burst models and strongly disfavours models involving instabilities at the innermost stable circular orbit. Assuming that the RB magnetic field indeed truncates the disc, we find $\textit{B}$ = (6.2 ± 1.5) × 10$^{8}$ G, larger than typically inferred for NS LMXBs. In addition, we find a low inclination ($\textit{i}$=29$^{\circ}$±2$^{\circ}$). Finally, we comment on the origin of the Comptonized and thermal components in the RB spectrum.We thank the referee for comments on this Letter. JvdE and ND are supported by a Vidi grant from the Netherlands Organization for Scientific Research (NWO) awarded to ND. ND also acknowledges support via a Marie Curie fellowship (FP-PEOPLE-2013-IEF-627148) from the European Commission. ACF, AL and MP are supported by Advanced Grant Feedback 340442 from the European Research Counsil (ERC). TB acknowledges support from NewCompStar (COST Action MP1304). JvdE and TB acknowledge the hospitality of the Institute of Astronomy in Cambridge, where this research was carried out

A NICER discovery of a low-frequency quasi-periodic oscillation in the soft-intermediate state of MAXI J1535–571

We present the discovery of a low-frequency ≈5.7 Hz quasi-periodic oscillation (QPO) feature in observations of the black hole X-ray binary MAXI J1535-571 in its soft-intermediate state, obtained in 2017 September-October by the Neutron Star Interior Composition Explorer. The feature is relatively broad (compared to other low-frequency QPOs; quality factor Q ≈ 2) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak harmonic and low-amplitude broadband noise. These characteristics identify it as a weak Type A/B QPO, similar to ones previously identified in the soft-intermediate state of the transient black hole X-ray binary XTE J1550-564. The lag-energy spectrum of the QPO shows increasing soft lags toward lower energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum). This large phase shift has similar amplitude but opposite sign to that seen in Rossi X-Ray Timing Explorer data for a Type B QPO from the transient black hole X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the Type B QPO in GX 339-4 pointed toward a precessing jet-like corona illuminating the accretion disk as the origin of the QPO signal. We suggest that this QPO in MAXI J1535-571 may have the same origin, with the different lag sign depending on the scale height of the emitting region and the observer inclination angle

Disk-Jet Coupling in the 2017/2018 Outburst of the Galactic Black Hole Candidate X-Ray Binary MAXI J1535-571

MAXI J1535-571 is a Galactic black hole candidate X-ray binary that was discovered going into outburst in 2017 September. In this paper, we present comprehensive radio monitoring of this system using the Australia Telescope Compact Array, as well as the MeerKAT radio observatory, showing the evolution of the radio jet during its outburst. Our radio observations show the early rise and subsequent quenching of the compact jet as the outburst brightened and then evolved toward the soft state. We constrain the compact jet quenching factor to be more than 3.5 orders of magnitude. We also detected and tracked (for 303 days) a discrete, relativistically moving jet knot that was launched from the system. From the motion of the apparently superluminal knot, we constrain the jet inclination (at the time of ejection) and speed to ≤45° and ≥0.69 c, respectively. Extrapolating its motion back in time, our results suggest that the jet knot was ejected close in time to the transition from the hard intermediate state to soft intermediate state. The launching event also occurred contemporaneously with a short increase in X-ray count rate, a rapid drop in the strength of the X-ray variability, and a change in the type-C quasi-periodic oscillation (QPO) frequency that occurs >2.5 days before the first appearance of a possible type-B QPO