Rapid jet production and suppression during fast state transitions in the black hole X-ray binary MAXI J1348$-$630

TL;DR

MAXI J1348--630's 2019/2020 outburst is used to probe jet–accretion coupling during rapid state transitions in a black hole X-ray binary. By combining dense NICER X-ray timing with MeerKAT/ATCA radio monitoring, the authors identify a brief reactivation of compact jets during a short hard-intermediate-like interval, followed by the launch of two discrete ejecta (RK2 and RK3). They show a tentative temporal link between ejecta launches and sharp drops in X-ray rms variability, suggesting the ejection of coronal material as a mechanism for jet production and suppression. The results illuminate the dynamic corona–jet connection and motivate high-resolution VLBI and dense X-ray timing campaigns to constrain jet-launching processes.

Abstract

Black hole X-ray binaries (BH XRBs) launch powerful relativistic jets during bright outburst phases. The properties of these outflows change dramatically between different spectral/accretion states. Compact jets are observed during the hard state and are quenched during the soft state, while discrete ejecta are mainly launched during the hard-to-soft state transition. Currently, we do not understand what triggers the formation/destruction of compact jets or the launch of discrete ejecta. In this context, finding a unique link between the jet evolution and the properties of the X-ray emission, such as its fast variability, would imply major progress in our understanding of the fundamental mechanisms that drive relativistic outflows in BH XRBs. Here we show that a brief, strong radio re-brightening during a predominantly soft state of the BH XRB MAXI J1348$-$630 was contemporaneous with a significant increase in the X-ray rms variability observed with NICER in 2019. During this phase, the variability displayed significant changes and, at the same time, MAXI J1348$-$630 launched two relativistic discrete ejecta that we detected with the MeerKAT and ATCA radio-interferometers. We propose that short-lived compact jets were reactivated during this excursion to the hard-intermediate state and were switched off before the ejecta launch, a behavior that has been very rarely observed in these systems. Interestingly, with the caveat of gaps in our radio and X-ray coverage, we suggest a tentative correspondence between the launch of ejecta and the drop in X-ray rms variability in this source, while other typical X-ray signatures associated with discrete ejections are not detected. We discuss how these results provide us with insights into the complex dynamic coupling between the jets and hot corona in BH XRBs.

Figures (4)

• Figure 1: Top panel: NICER 0.5--12 keV X-ray light curve (count rate) of MAXI J1348--630 during its 2019/2020 outburst. The white, light-gray, light-orange and light-pink regions denote, respectively the hard, HIMS, SIMS and soft state, from Zhang2020. The purple vertical dashed lines region marks the presence of intermittent strong band-limited noise during the soft state. Type-B and A QPOs are marked with red and light-blue points, respectively. The colored stars mark the times in the light curve at which the power spectra shown in Figure \ref{['fig:PSD']} were obtained. Second panel: X-ray fractional rms variability, calculated in the 0.5–64 Hz frequency range. The inset on the right shows in detail the evolution of the variability between MJD 58570 and 58590. Third panel: MeerKAT (1.3 GHz) and ATCA (5.5 and 9 GHz) core and RK2 radio light curves, from Carotenuto2021. Bottom panel: Angular separation in arcsec between RK2 and MAXI J1348--630 from the same radio observations. Detections of the core are shown as dark blue points. The single detection of RK3 is also marked with a cyan point. A linear motion was used to fit the RK2 data. The red vertical line marks the RK2 inferred ejection date: $t_{\rm ej} = \rm MJD \ 58578.8 \pm 3.2$, where the two red dashed lines represent the upper and lower bounds of the $t_{\rm ej}$ confidence interval. The same is shown in blue for RK1, with $t_{\rm ej} = \rm MJD \ 58521.5^{+1.8}_{-3.0}$ from Carotenuto_2022.
• Figure 2: Left panels: Representative NICER power spectra at different phases of the outburst, calculated in the 0.5--12 keV band and labeled from a to f. Right panels: NICER hardness-intensity diagram (HID, top) and hardness-rms diagram (HRD, bottom) of the outburst. The hardness ratio is calculated as $(6 - 12)/(2 - 3.5)$ keV, while the total fractional rms is calculated in the 0.5–64 Hz frequency range (and 0.5--12 keV energy range). In both rows, the colored stars mark the position in the diagrams at which the power spectra were obtained. The presence of strong band-limited noise typical of the hard state supports the reactivation of compact jets during the short HIMS phase around MJD 58570. The two drops in X-ray rms variability happening after 58576 and after MJD 58582 could be consistent with the launch of the RK2 and RK3 discrete ejecta, respectively.
• Figure 3: NICER 0.6--10 keV unfolded energy spectra associated to the PSDs shown in Figure \ref{['fig:PSD']}, with the same color-code. The unfolded spectra, dominated by the disk component, have been deconvolved against a power law with $\Gamma = 2$, without any fitting, and are shown for illustrative purposes. We note that the spectra with broadband noise (magenta and cyan points) have slightly harder tails at high energies than spectra without variability (e.g. yellow and white points), supporting the scenario in which the variability is connected to the hot corona, which may disappear in the aftermath of a discrete ejection.
• Figure 4: ATCA 9 GHz image of the 2019 April 16 observation (MJD 58589), with two radio sources identified as RK1 and RK2 in Carotenuto2021. Contours start at 3 times the rms ($\sim$25 $\mu$Jy beam$^{-1}$). The position of MAXI J1348--630 is marked with a magenta cross. Top: the original RK2 emission is produced by two point-sources whose fitted positions are marked with a yellow and black cross. We identify the yellow cross with the true position of RK2, at an angular distance of $\sim$1.1 arcsec from the core, while the black cross is an additional component (RK3) or core emission from MAXI J1348--630. Center: residual image after the image-plane subtraction of the two above-mentioned point-sources, showing only gaussian-like noise at the position of MAXI J1348--630. Bottom: residual image after the image-plane subtraction of a single point-source with the location free to vary, and marked with a blue cross, showing residuals above three sigma level near MAXI J1348--630.