HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105

We report infrared observations of the microquasar GRS 1915+105 using the NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June 2003. During epochs of high X-ray/radio activity near the beginning and end of this period, we find that the $1.87$\um infrared flux is generally low ($\sim 2$ mJy) and relatively steady. However, during the X-ray/radio ``plateau'' state between these epochs, we find that the infrared flux is significantly higher ($\sim 4-6$ mJy), and strongly variable. In particular, we find events with amplitudes $\sim 20-30$% occurring on timescales of $\sim 10-20$s (e-folding timescales of $\sim 30$s). These flickering timescales are several times faster than any previously-observed infrared variability in GRS 1915+105 and the IR variations exceed corresponding X-ray variations at the same ($\sim 8s$) timescale. These results suggest an entirely new type of infrared variability from this object. Based on the properties of this flickering, we conclude that it arises in the plateau-state jet outflow itself, at a distance $<2.5$ AU from the accretion disk. We discuss the implications of this work and the potential of further flickering observations for understanding jet formation around black holes.Comment: 19 pages, incl. 4 figures; accepted for publication in Ap

Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs

Diskoseismic c-modes in accretion discs have been invoked to explain low-frequency variabilities observed in black-hole X-ray binaries. These modes are trapped in the inner-most region of the disc and have frequencies much lower than the rotation frequency at the disc inner radius. We show that because the trapped waves can tunnel through the evanescent barrier to the corotational wave zone, the c-modes are damped due to wave absorption at the corotation resonance. We calculate the corotational damping rates of various c-modes using the WKB approximation. The damping rate varies widely depending on the mode frequency, the black hole spin parameter and the disc sound speed, and is generally much less than 10% of the mode frequency. A sufficiently strong excitation mechanism is needed to overcome this corotational damping and make the mode observable.Comment: 10 pages, 5 figures, MNRAS in pres

NICER/NuSTAR Characterization of 4U 1957+11: A Near Maximally Spinning Black Hole Potentially in the Mass Gap

© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/4U 1957+11 is a black hole candidate system that has been in a soft X-ray spectral state since its discovery. We present analyses of recent joint NICER and NuSTAR spectra, which are extremely well described by a highly inclined disk accreting into a near maximally spinning black hole. Owing to the broad X-ray coverage of NuSTAR, the fitted spin and inclination are strongly constrained for our hypothesized disk models. The faintest spectra are observed out to 20 keV, even though their hard tail components are almost absent when described with a simple corona. The hard tail increases with luminosity, but shows clear two-track behavior with one track having appreciably stronger tails. The disk spectrum color-correction factor is anticorrelated with the strength of the hard tail (e.g., as measured by the Compton y parameter). Although the spin and inclination parameters are strongly constrained for our chosen model, the mass and distance are degenerate parameters. We use our spectral fits, along with a theoretical prior on color-correction, an observational prior on likely fractional Eddington luminosity, and an observational prior on distance obtained from Gaia studies, to present mass and distance contours for this system. The most likely parameters, given our presumed disk model, suggest a 4.6 M ⊙ black hole at 7.8 kpc observed at luminosities ranging from ≈1.7% to 9% of Eddington. This would place 4U 1957+11 as one of the few actively accreting sources within the mass gap of ≈2–5 M ⊙ where there are few known massive neutron stars or low-mass black holes. Higher mass and distance, however, remain viable.Peer reviewe

Spacecraft System Design for an Advanced X-Ray Monitor (AXM) Mission

This paper describes a system concept for a NASA Small Explorer Mission to develop an all-skyviewing Advanced X-ray Monitor (AXM). The spacecraft is configured to be launched from a Pegasus XL vehicle. AXM is designed to provide unprecedented sensitivity to cosmic explosions seen in X-rays. These include the ejections of relativistic jets by black holes in the Galaxy, and the fireballs of gamma ray bursts that originate in distant Galaxies. Such events are captured with 31 cameras mounted on the AXM spacecraft to continuously view 97% of the celestial sphere, excluding occultations by the Earth. The camera detectors are Gas Electron Multiplier (GEM) devices, developed at CERN and used with coded masks for X-ray astronomy. The pointing orientations for the cameras presented a challenge to provide 4p steradian viewing, while accommodating spacecraft subsystems and deployable solar arrays for power. The mounting orientation resembles the 32 faces and vertices of a soccer ball, with one camera eliminated to avoid the saturating effect of solar X-rays. The objective of continuous, all-sky viewing is accomplished with a three-axis stabilized attitude control subsystem with the solar panels pointed close to the Sun. The AXM mission is designed for launch into a ~600-km altitude, circular, equatorial orbit. An approximately 1 degree spacecraft maneuver once per day will maintain the solar panels aligned with the Sun. The spacecraft is powered by solar arrays that deploy after launch and are then fixed for the mission duration. Within limitations, the AXM spacecraft has been designed to gracefully tolerate many kinds of anomalies

Corotational Instability of Inertial-Acoustic Modes in Black-Hole Accretion Discs: Non-Barotropic Flows

We study the effect of corotation resonance on the inertial-acoustic oscillations (p-modes) of black-hole accretion discs. Previous works have shown that for barotropic flows (where the pressure depends only on the density), wave absorption at the corotation resonance can lead to mode growth when the disc vortensity, $\zeta=\kappa^2/(2\Omega\Sigma)$ (where $\Omega, \kappa, \Sigma$ are the rotation rate, radial epicyclic frequency and surface density of the disc, respectively), has a positive gradient at the corotation radius. Here we generalize the analysis of the corotation resonance effect to non-barotropic fluids. We show that the mode instability criterion is modified by the finite radial Brunt-V\"as\"al\"a frequency of the disc. We derive an analytic expression for the reflectivity when a density wave impinges upon the corotation barrier, and calculate the frequencies and growth rates of global p-modes for disc models with various $\alpha$-viscosity parameterizations. We find that for disc fluids with constant adiabatic index $\Gamma$, super-reflection and mode growth depend on the gradient of the effective vortensity, $\zeta_{\rm eff} = \zeta/S^{2/\Gamma}$ (where $S \equiv P/\Sigma^{\Gamma}$ measures the entropy): when $d\zeta_{\rm eff}/dr > 0$ at the corotation radius, wave absorption leads to amplification of the p-mode. Our calculations show that the lowest-order p-modes with azimuthal wave number $m=2, 3, 4,...$ have the largest growth rates, with the frequencies approximately in (but distinct from) the $2:3:4...$ commensurate ratios. We discuss the implications of our results for the high-frequency quasi-periodic oscillations observed in accreting black-hole systems.Comment: 12 pages, 5 figures, published in MNRA

A Radio Pulsar/X-ray Binary Link

Radio pulsars with millisecond spin periods are thought to have been spun up by transfer of matter and angular momentum from a low-mass companion star during an X-ray-emitting phase. The spin periods of the neutron stars in several such low-mass X-ray binary (LMXB) systems have been shown to be in the millisecond regime, but no radio pulsations have been detected. Here we report on detection and follow-up observations of a nearby radio millisecond pulsar (MSP) in a circular binary orbit with an optically identified companion star. Optical observations indicate that an accretion disk was present in this system within the last decade. Our optical data show no evidence that one exists today, suggesting that the radio MSP has turned on after a recent LMXB phase.Comment: published in Scienc

Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

We study the global stability of non-axisymmetric p-modes (also called inertial-acoustic modes) trapped in the inner-most regions of accretion discs around black holes. We show that the lowest-order (highest-frequency) p-modes, with frequencies $\omega=(0.5-0.7) m\Omega_{\rm ISCO}$, can be overstable due to general relativistic effects, according to which the radial epicyclic frequency is a non-monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius and carries a negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity. When the mode frequency is sufficiently high, such that the slope of the vortensity is positive at corotation positive wave energy is absorbed at the resonance, leading to the growth of mode amplitude. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our analysis of the behavior of the fluid perturbations in the transonic flow near the ISCO indicates that, while the inflow tends to damp the mode, the damping effect is sufficiently small under some conditions so that net mode growth can still be achieved. We further clarify the role of the Rossby wave instability and show that it does not operate for black hole accretion discs with smooth-varying vortensity profiles. Overstable non-axisymmetric p-modes driven by the corotational instability provide a plausible explanation for the high-frequency (> 100 Hz) quasi-periodic oscillations (HFQPOs) observed from a number of black-hole X-ray binaries in the very high state. The absence of HFQPOs in the soft (thermal) state may result from mode damping due to the radial infall at the ISCO.Comment: 19 pages, 11 figures, MNRAS in pres

Interface Modes and Their Instabilities in Accretion Disc Boundary Layers

We study global non-axisymmetric oscillation modes trapped near the inner boundary of an accretion disc. Observations indicate that some of the quasi-periodic oscillations (QPOs) observed in the luminosities of accreting compact objects (neutron stars, black holes and white dwarfs) are produced in the inner-most regions of accretion discs or boundary layers. Two simple models are considered in this paper: The magnetosphere-disc model consists of a thin Keplerian disc in contact with a uniformly rotating magnetosphere with and low plasma density, while the star-disc model involves a Keplerian disc terminated at the stellar atomosphere with high density and small density scale height. We find that the interface modes at the magnetosphere-disc boundary are generally unstable due to Rayleigh-Taylor and/or Kelvin-Helmholtz instabilities. However, differential rotation of the disc tends to suppress Rayleigh-Taylor instability and a sufficiently high disc sound speed (or temperature) is needed to overcome this suppression and to attain net mode growth. On the other hand, Kelvin-Helmholtz instability may be active at low disc sound speeds. We also find that the interface modes trapped at the boundary between a thin disc and an unmagnetized star do not suffer Rayleigh-Taylor or Kelvin-Helmholtz instability, but can become unstable due to wave leakage to large disc radii and, for sufficiently steep disc density distributions, due to wave absorption at the corotation resonance in the disc. The non-axisymmetric interface modes studied in this paper may be relevant to the high-frequency QPOs observed in some X-ray binaries and in cataclysmic variables.Comment: 14 pages, 9 figures, submitted to MNRA