Pulsars, transients and relativistic astrophysics

Abstract:

We present the discovery of a correlation, in a sample of 16 gamma-ray burst 8.5 GHz radio afterglows, between the intrinsic luminosity measured at 10 days in the rest frame, LRadio, 10d, and the average rate of decay past this time, α>10d. The correlation has a Spearman’s rank coefficient of −0.70 ± 0.13 at a significance of >3σ and a linear regression fit of $\alpha _{>10d} = -0.29^{+0.19}_{-0.28} \log \left(L_{\mathrm{Radio,10d}} \right) + 8.12^{+8.86}_{-5.88}$. This finding suggests that more luminous radio afterglows have higher average rates of decay than less luminous ones. We use a Monte Carlo simulation to show the correlation is not produced by chance or selection effects at a confidence level of >3σ. Previous studies found this relation in optical/UV, X-ray and GeV afterglow light curves, and we have now extended it to radio light curves. The Spearman’s rank coefficients and the linear regression slopes for the correlation in each waveband are all consistent within 1σ. We discuss how these new results in the radio band support the effects of observer viewing geometry, and time-varying microphysical parameters, as possible causes of the correlation as suggested in previous works.

Abstract:

Abstract We investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGN), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering 9 AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the midplane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the midplane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the SMBH’s role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte-Carlo simulations of AGN evolution.

Abstract:

Accretion onto strongly magnetized neutron stars in X-ray pulsars (XRPs) produces intense X-ray emission and gamma-ray photons, the latter arising from nuclear reactions and high-energy particle collisions in the stellar atmosphere. These gamma-rays interact with the magnetic field via one- and two-photon pair creation processes, generating electron-positron pairs. We investigate one-photon pair production in sub-critical XRPs, with a focus on how surface magnetic field strength affects gamma-ray absorption in the magnetosphere. Using general relativistic photon trajectory simulations, we map the spatial distribution of pair creation sites and quantify absorption efficiencies. We find that XRPs with surface fields B ≲ 10 12 G are largely transparent to 2.2MeV gamma-rays, while fields B ≳ 3 × 10 12 G lead to efficient absorption within a few tens of centimeters from the surface. For lower field strengths, absorption can occur at larger distances and outside the accretion column, offering a potential channel for radio emission. Our results provide new insight into the interplay between nuclear processes, magnetospheric structure, and multiwavelength radiation in XRPs.

Abstract:

The sensitivity and field of view of the MeerKAT radio telescope provide excellent opportunities for commensal transient searches. We carry out a commensal transient search in supernova and short gamma-ray burst fields using methodologies established by S. I. Chastain et al. We search for transients in MeerKAT L-band images with integration times of 30 minutes, finding 13 variable sources. We compare these sources to the VLASS and RACS survey data, and examine possible explanations for the variability. Additionally, for one of these sources we examine archival Chandra ACIS data. We find that 12 of these sources are consistent with variability due to interstellar scintillation. The remaining source could possibly have some intrinsic variability. We also split the MeerKAT L band into upper and lower halves, and search for transients in images with an integration time of 8 s. We find a source with a duration of 8–16 s that is highly polarized at the lowest frequencies. This source is spatially coincident with a star detected by the Transiting Exoplanet Survey Satellite. We conclude that this source may be consistent with a stellar flare. Finally, we calculate accurate upper and lower limits on the transient rate using transient simulations.

Abstract:

The young neutron star X-ray binary, Cir X-1, resides within its natal supernova remnant and experiences ongoing outbursts every 16.5 days, likely due to periastron passage in an eccentric orbit. We present the deepest ever radio image of the field, which reveals relativistic jet-punched bubbles that are aligned with the mean axis of the smaller-scale jets observed close to the X-ray binary core. We are able to measure the minimum energy for the bubble, which is around Emin = 1045 erg. The nature and morphological structure of the source were investigated through spectral index mapping and numerical simulations. The spectral index map reveals a large fraction of the nebula’s radio continuum has a steep slope, associated with optically thin synchrotron emission, although there are distinct regions with flatter spectra. Our data are not sensitive enough to measure the spectral index of the protruding bubbles. We used the PLUTO code to run relativistic hydrodynamic simulations to try and qualitatively reproduce the observations with a combined supernova-plus-jet system. We are able to do so using a simplified model in which the asymmetrical bubbles are best represented by supernova explosion which is closely followed (within 100 years) by a phase of very powerful jets lasting less than 1000 years. These are the first observations revealing the initial breakout of neutron star jets from their natal supernova remnant, and further support the scenario in which Cir X-1 is a younger relation of the archetypal jet source SS433.