NICER and Fermi GBM Observations of the First Galactic Ultraluminous X-ray Pulsar Swift J0243.6+6124.

Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed in late September 2017 in a giant outburst with a peak luminosity of 2 × 1039(d/7 kpc)2 erg s-1 (0.1-10 keV), with no formerly reported activity. At this luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray pulsar. We describe Neutron star Interior Composition Explorer (NICER) and Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this source. A new orbital ephemeris is obtained for the binary system using spin-frequencies measured with GBM and 15-50 keV fluxes measured with the Neil Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic spin-up. Power spectra measured with NICER show considerable evolution with luminosity, including a quasi-periodic oscillation (QPO) near 50 mHz that is omnipresent at low luminosity and has an evolving central frequency. Pulse profiles measured over the combined 0.2-100 keV range show complex evolution that is both luminosity and energy dependent. Near the critical luminosity of L ~ 1038 erg s-1, the pulse profiles transition from single-peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both NICER and GBM show a turn-over to softening as the intensity increases. This behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. These two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a Coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. This is the highest observed (to date) value of the critical luminosity, suggesting a magnetic field of B ~ 1013 G.

A 400-solar-mass black hole in the galaxy M82.

M82 X-1, the brightest X-ray source in the galaxy M82, has been thought to be an intermediate-mass black hole (100 to 10,000 solar masses) because of its extremely high luminosity and variability characteristics, although some models suggest that its mass may be only about 20 solar masses. The previous mass estimates were based on scaling relations that use low-frequency characteristic timescales which have large intrinsic uncertainties. For stellar-mass black holes, we know that the high-frequency quasi-periodic oscillations (100-450 hertz) in the X-ray emission that occur in a 3:2 frequency ratio are stable and scale in frequency inversely with black hole mass with a reasonably small dispersion. The discovery of such stable oscillations thus potentially offers an alternative and less ambiguous means of mass determination for intermediate-mass black holes, but has hitherto not been realized. Here we report stable, twin-peak (3:2 frequency ratio) X-ray quasi-periodic oscillations from M82 X-1 at frequencies of 3.32 ± 0.06 hertz and 5.07 ± 0.06 hertz. Assuming that we can extrapolate the inverse-mass scaling that holds for stellar-mass black holes, we estimate the black hole mass of M82 X-1 to be 428 ± 105 solar masses. In addition, we can estimate the mass using the relativistic precession model, from which we get a value of 415 ± 63 solar masses.