Jun-Ping Chen | Search arXiv

arXiv2026

The retirement party of GRS 1915+105: the radio fading to quiescence of the archetypal microquasar

After more than 25 years of near-Eddington activity and over six years in a Compton-thick obscured phase, the black-hole X-ray binary GRS 1915+105 has begun to fade in the radio to a level lower than ever previously recorded, possibly marking the onset of its final descent towards quiescence. With the accretion flow hidden in the X-rays, the radio emission is currently the best tracer of the accretion activity. We aim to establish whether the fading marks the end of the outburst that began in 1992 or a temporary switch-off. We present the long-term MeerKAT monitoring of the source from the ThunderKAT and X-KAT programmes up to 2026 July, deep dual-band (L- and S-band) MeerKAT Director's Discretionary Time observations, and three Einstein Probe/FXT X-ray pointings bracketing them. Following a last Jy-level flare in early 2025, the 1.28 GHz flux density declined monotonically to $\sim$0.2-0.4 mJy by 2026 March, and after 2026 May 9 it was no longer formally detected in our 10-min monitoring observations. Deep MeerKAT observations in 2026 July detect the source at $\sim$150 $μ$Jy in L band and $\sim$90 $μ$Jy in S band, the lowest radio flux densities ever recorded from the system, corresponding to $L_{\rm R}\simeq2\times10^{28}$ erg s$^{-1}$ at 1.28 GHz, with a spectral index $α=-0.67\pm0.41$ across 1.28-2.62 GHz. The radio decline is the first direct evidence that the central engine itself is fading rather than merely receding further behind the absorber. The steep spectrum is consistent with either a combination of optically thick and thin emission, or an unusual, intrinsically steep core-jet spectrum. Should this mark the onset of quiescence, disc-instability models predict it to last of order $10^4$ yr. The switch-off of the central engine may shed light on the nature of the donor star and on the origin of the very-high-energy gamma-ray emission in this system.

astro-ph.HE↗

arXiv2026

Simulating the late stages of WD-BH/NS mergers: an origin for fast X-ray transients and GRBs with periodic modulations

Recent studies indicate that mergers of a white dwarf (WD) with a neutron star (NS) or a stellar-mass black hole (BH) may be a potential progenitor channel for certain merger-kind, but long-duration $γ$-ray bursts (GRBs), e.g., GRBs 230307A and 211211A. The relatively large tidal disruption radius of the WD can result in non-negligible residual orbital eccentricity ($0 \lesssim e \lesssim 0.2$), causing episodic mass transfer, i.e., repeated tidal disruptions (RPDs) of the WD. We perform smoothed-particle-hydrodynamics simulations of RPDs in sixteen WD-BH/NS systems, capturing the subsequent mass transfer and accretion. The WD undergoes RPDs near the orbital periastron, modulating the ensuing accretion process, leading to variations of the accretion rate on the orbital period. Across all simulations, the peak accretion rates range from $4 \times10^{-4}$ to 0.2 $M_{\odot} \rm \ s^{-1}$, while the RPD duration spans from $\sim$ 10 s to an hour. More compact systems, i.e., those with a higher mass ratio (higher WD mass and lower accretor mass), tend to undergo fewer RPD cycles, resulting in shorter durations and higher accretion rates. If such events can launch relativistic jets, three categories of non-thermal X/$γ$-ray transients are predicted, in decreasing order of their mean accretion rates: (1) an X-ray transient with a simultaneous GRB, both lasting for $10^{1-2}$ s; (2) a longer X-ray transient lasting up to $10^{2-3}$ s that has a GRB appearing only at its later phase ; (3) an ultra-long X-ray transient lasting for $\sim 10^{3}$ s without a GRB. A generic feature of these transients is that their prompt emission light curves are probably periodically modulated with periods of a few to tens of seconds.

astro-ph.HE↗