Gamma-ray astronomy

Abstract:

Abstract Disc winds play a crucial role in many accreting astrophysical systems across all scales. In accreting white dwarfs (AWDs) and active galactic nuclei (AGN), radiation pressure on spectral lines is a promising wind-driving mechanism. However, the efficiency of line driving is extremely sensitive to the ionization state of the flow, making it difficult to construct a reliable physical picture of these winds. Recently, we presented the first radiation-hydrodynamic (RHD) simulations for AWDs that incorporated detailed, multi-dimensional ionization calculations via fully frequency-dependent radiative transfer, using the Sirocco code coupled to Pluto. These simulations produced much weaker line-driven winds ($\dot{M}_{\rm wind}/\dot{M}_{\rm acc} < 10^{-5}$ for our adopted parameters) than earlier studies using more approximate treatments of ionization and radiative transfer (which yielded $\dot{M}_{\rm wind}/\dot{M}_{\rm acc} \simeq 10^{-4}$). One remaining limitation of our work was the assumption of an isothermal outflow. Here, we relax this by adopting an ideal gas equation of state and explicitly solving for the multi-dimensional temperature structure of the flow. In the AWD setting, accounting for the thermal state of the wind does not change the overall conclusions drawn from the isothermal approximation. Our new simulations confirm the line-driving efficiency problem: the predicted outflows are too highly ionized, meaning they neither create optimal driving conditions, nor reproduce the observed ultraviolet wind signatures. Possible solutions include wind clumping on sub-grid scales, a softer-than-expected spectral energy distribution or additional driving mechanisms. With the physics now built into our simulations, we are well-equipped to also explore line-driven disc winds in AGN.

Abstract:

Abstract Black hole X-ray binaries in outburst launch discrete, large-scale jet ejections which can propagate to parsec scales. The kinematics of these ejecta appear to be well described by relativistic blast wave models original devised for gamma-ray burst afterglows. In previous kinematic-only modelling, a crucial degeneracy prevented the initial ejecta energy and the interstellar medium density from being accurately determined. In this work, we present the first joint Bayesian modelling of the radiation and kinematics of a large-scale jet ejection from the X-ray binary MAXI J1535-571. We demonstrate that a reverse shock powers the bright, early ejecta emission. The joint model breaks the energetic degeneracy, and we find the ejecta has an initial energy of E0 ∼ 3 × 1043 erg, and propagates into a low density interstellar medium of nism ∼ 4 × 10−5 cm−3. The ejecta is consistent with being launched perpendicular to the disc and could be powered by an efficient conversion of available accretion power alone. This work lays the foundation for future parameter estimation studies using all available data of X-ray binary jet ejecta.

Abstract:

ABSTRACT One of the potential sources of repeating fast radio bursts (FRBs) is a rotating magnetosphere of a compact object, as suggested by the similarities in the polarization properties of FRBs and radio pulsars. Attempts to measure an underlying period in the times of arrival of repeating FRBs have nevertheless been unsuccessful. To explain this lack of observed periodicity, it is often suggested that the line of sight towards the source must be sampling active parts of the emitting magnetosphere throughout the rotation of the compact object, i.e. has a large duty cycle, as can be the case in a neutron star with near-aligned magnetic and rotation axes. This may lead to apparently aperiodic bursts; however, the polarization angle of the bursts should be tied to the rotational phase from which they occur. This is true for radio pulsars. We therefore propose a new test to identify a possible stable rotation period under the assumptions above, based on a periodogram of the measured polarization angle time series for repeating FRBs. We show that this test is highly sensitive when the duty cycle is large, where standard time-of-arrival periodicity searches fail. Therefore, we can directly test the hypothesis of repeating FRBs of magnetospheric origin with a stable rotation period. Both positive and negative results of the test applied to FRB data will provide important information.

Abstract:

In low-mass X-ray binaries (LMXBs), accretion flows are often associated with either jet outflows or disk winds. Studies of LMXBs with luminosities up to roughly 20% of the Eddington limit indicate that these outflows generally do not co-occur, suggesting that disk winds might inhibit jets. However, previous observations of LMXBs accreting near or above the Eddington limit show that jets and winds can potentially coexist. To investigate this phenomenon, we carried out a comprehensive multiwavelength campaign (using the Very Large Array (VLA), Chandra/High Energy Transmission Grating Spectrometer (HETG), and NICER) on the near-Eddington neutron-star Z-source LMXB GX 13+1. NICER and Chandra/HETG observations tracked GX 13+1 across the entire Z track during high Eddington rates, detecting substantial resonance absorption features originating from the accretion disk wind in all X-ray spectra, which implies a persistent wind presence. Simultaneous VLA observations captured a variable radio jet, with radio emission notably strong during all flaring branch observations—contrary to typical behavior in Z sources—and weaker when the source was on the normal branch. Interestingly, no clear correlation was found between the radio emission and the wind features. Analysis of VLA radio light curves and simultaneous Chandra/HETG spectra demonstrates that an ionized disk wind and jet outflow can indeed coexist in GX 13+1, suggesting that their launching mechanisms are not necessarily linked in this system.