Sara Motta

Abstract:

The black hole transient GRS 1915+105 entered a new phase of activity in 2018, generally characterised by low X-ray and radio fluxes. This phase has only been interrupted by episodes of strong and variable radio emission, where high levels of X-ray absorption local to the source were measured. We present 18 epochs of near-infrared spectroscopy (2018–2023) obtained with GTC/EMIR and VLT/X-shooter, spanning both radio-loud and radio-quiet periods. We demonstrate that radio-loud phases are characterised by strong P-Cygni line profiles, indicative of accretion disc winds with velocities of up to ∼3000 km s−1. This velocity is consistent with those measured in other black hole transients. It is also comparable to the velocity of the X-ray winds detected during the peak outburst phases in GRS 1915+105, reinforcing the idea that massive, multi-phase outflows are characteristic features of the largest and most powerful black hole accretion discs. Conversely, the evolution of the Brγ line profile during the radio-quiet phases follows the expected trend for accretion disc lines in a system that is gradually decreasing its intrinsic luminosity, exhibiting weaker intensities and more pronounced double-peaks

Abstract:

We present Monitor of All-sky X-ray Image (MAXI) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the Be X-ray binary, MAXI J0655−013, in outburst. NuSTAR observed the source once early in the outburst, when spectral analysis yields a bolometric (0.1–100 keV), unabsorbed source luminosity of L bol = 5.6 × 1036 erg s−1, and a second time 54 days later, by which time the luminosity had dropped to L bol = 6.5 × 1034 erg s−1 after first undergoing a dramatic increase. Timing analysis of the NuSTAR data reveals a neutron star spin period of 1129.09 ± 0.04 s during the first observation, which decreased to 1085 ± 1 s by the time of the second observation, indicating spin-up due to accretion throughout the outburst. Furthermore, during the first NuSTAR observation, we observed quasi-periodic oscillations (QPOs) with centroid frequency ν 0 = 89 ± 1 mHz, which exhibited a second harmonic feature. By combining the MAXI and NuSTAR data with pulse period measurements reported by Fermi/GBM, we are able to show that apparent flaring behavior in the MAXI light curve is an artifact introduced by uneven sampling of the pulse profile, which has a large pulsed fraction. Finally, we estimate the magnetic field strength at the neutron star surface via three independent methods, invoking a tentative cyclotron resonance scattering feature at 44 keV, QPO production at the inner edge of the accretion disk, and spin-up via interaction of the neutron star magnetic field with accreting material. Each of these result in a significantly different value. We discuss the strengths and weaknesses of each method and infer that MAXI J0655−013 is likely to have a high surface magnetic field strength, B s > 1013 G.