Radio properties of the quasi-periodic eruption source RXJ1301.9+2747 at parsec scales

TL;DR

This study uses VLBI to probe parsec-scale radio emission in the QPE host RXJ1301.9+2747, testing competing emission scenarios. The 5.0 GHz observations reveal a compact source with size limits $< (0.9\times0.4)\,\mathrm{pc}$ and $T_{\rm B} > 2.2\times 10^{7}\,\mathrm{K}$, while the spectrum remains negative ($F_{\nu} \propto \nu^{\sim -0.8}$) and the compact flux accounts for the total radio output at 5 GHz. Astrometric analysis after calibrator core-shift corrections shows a potential small offset between 1.7 and 5.0 GHz emission relative to Gaia, though systematics may contribute. Collectively, the results disfavor a star-formation or canonical AGN core origin, and favor a remnant jet or outflow scenario, which is consistent with broader QPE radio properties and supports the view that QPE hosts often harbor recently ignited, low-activity nuclei.

Abstract

Quasi-periodic eruptions (QPEs) are repeating soft X-ray flares associated with galactic nuclei. Several recent works have found evidence that the accretion flow in the galactic nuclei of QPEs is of recent origin, and that it is unlike canonical active galactic nuclei (AGN). A precursor tidal disruption event has been observed in a few cases. In this work we report new radio observations of the QPE host galaxy RXJ\,1301.9+2747 taken at 5.0\,GHz with the High Sensitivity Array (HSA), to complement archival 1.7\,GHz observations reported previously. Our new observations confirm the presence of a highly compact radio source in RXJ\,1301.9+2747, which is smaller than $0.9\times0.4~\mathrm{pc}$ at 5.0\,GHz. The nonsimultaneous very long baseline interferometry (VLBI) compact flux of the source is consistent with a negative spectral index, and thus is similar to the larger non-VLBI scale radio spectral index. Contrary to earlier results at 1.7\,GHz, we find the 5\,GHz emission offset from the optical Gaia position, which may be due to dust extinction in the host galaxy. In addition, there is a significant offset between the 1.7 and 5.0\,GHz data, which may result from astrophysical uncertainties in the calibration source. This sheds new light on the elusive properties of the radio-detected QPE sources. Consistent with previous results, our observations disfavor a star formation or jet-core-region origin of the radio emission. We cannot rule out a reconnection-driven scenario for the radio emission, but we favor a remnant jet or outflow scenario. This is overall in agreement with the radio properties of radio-detected QPE sources at lower angular resolution.

Figures (8)

• Figure 1: Time averaged visibilities as observed by the HSA in February 2023 at 5 GHz. The blue points illustrate the best fit ehtim image model, shown in \ref{['fig:clean_map']}.
• Figure 2: Map of RXJ obtained with ehtim, showing the 5.0 GHz HSA observations. The data are offset to the observation phase center at R.A.$=13~02~ 00.138$, Dec.$=+27~46 ~57.855$. The restoring beam sized use in the ehtim image is $(2.9\times1.4)~\mathrm{mas}^2$.
• Figure 3: Astrometry of Gaia, and the 1.7 GHz and 5.0 GHz observations, as annotated. The Gaia optical position is placed at (0,0); all other positions are plotted relative to this position. The ellipses indicates the best fit Gaussian ellipse derived from a model fit to the VLBI data. Uncertainties are at $1\sigma$ level. The dashed line indicates the calibrator core shift axis along the apparent 1.7GHz position (violet dot) is shifted; the translucent gray ellipses indicate the systematic uncertainty of that core shift (see text for details).
• Figure 4: Clean map of RXJ obtained with our new 5.0 GHz HSA observations. We note that the peak flux derived is likely biased by amplitude self-calibration.
• Figure 5: Difmap image of the phase-referencing calibrator J1300+2830.