Is the Peculiar Galactic Center Transient Swift J174610.4-290018 A Recurrent Nova?

TL;DR

Swift J174610.4-290018, a peculiar GC X-ray transient detected in 2024, is analyzed with archival Chandra data and contemporaneous Swift, NuSTAR, and XRISM observations to determine its nature. The authors find a quiescent spectrum consistent with a high-temperature thermal plasma and Fe line complex, and identify two outbursts (2005 and 2024) with Fe lines and Cr-like features, but they argue that an accretion-disk corona LMXB origin is unlikely. Instead, the temporal and spectral evolution closely resemble recurrent nova behavior, akin to RS Oph, suggesting a RN in a symbiotic-like system near the GC and implying the presence of massive white dwarfs and wide binaries close to Sgr A*. The results motivate targeted, multi-wavelength follow-up to confirm the RN scenario and to illuminate the GC binary demographics in this extreme environment.

Abstract

Swift J174610.4-290018, a peculiar transient X-ray source originally discovered by the Swift satellite in February 2024 at the onset of its outburst, recently attracted intentional or coincident follow-up X-ray observations using Swift, NuSTAR and Chandra. We have performed a comprehensive analysis of the relevant X-ray data to investigate the spectral and temporal properties of this source between October 2000 and July 2024. Archival Chandra data reveal a plausible additional outburst in 2005, followed by a quiescent state in the next $\sim$19 years. The X-ray spectra in both the quiescent and outburst phases are consistent with a thermal plasma spectrum with relatively high temperatures ($\sim$10 keV) and prominent emission lines from both neutral and highly-ionized iron. A previously proposed low-mass X-ray binary/accretion disk corona scenario for Swift J174610, based on XRISM observations near the peak of the 2024 outburst, is examined against the newly derived X-ray properties and is disfavored, in particular due to its difficulty in explaining the quiescent state spectrum. Instead, we suggest a symbiotic binary/recurrent nova scenario, which gains support from many of the observed X-ray properties of Swift J174610 and a close comparison with the famous Galactic recurrent nova RS Oph. If confirmed, Swift J174610 would represent the first nova detected in the Galactic center, offering new insights into the otherwise elusive population of massive white dwarfs as well as wide binaries in the close vicinity of Sgr A*. Our findings call for multi-wavelength follow-up observations for this highly unusual X-ray source.

Figures (3)

• Figure 1: (a) Combined 2–8 keV Chandra image of the GC, with the horizontal axis aligned with the right ascension direction. The position of Swift J174610 is marked by a green circle of $5^{\prime\prime}$ radius, and Sgr A* is indicated by a black cross. (b) Zoomed-in Chandra view of Swift J174610, showing the only likely Chandra counterpart placed at the center of the panel. (c) Combined 3–79 keV NuSTAR image showing the same region as in panel (b). A circle of $30^{\prime\prime}$ radius denotes the source extraction region, and the central cross marks the Chandra-source position of Swift J174610. (d) Combined 2–8 keV Swift image, shown with the same $30^{\prime\prime}$-radius circle and position marker as in panel (c).
• Figure 2: Main panel: The 2--8 keV X-ray light curve of Swift J174610, spanning from 2000 to 2025 as observed by Chandra/ACIS. The energy flux and its associated $1 \sigma$ error (denoted by crosses), or its 3$\sigma$ upper limits in the case of a non-detection (denoted by downward arrows), are derived using CIAO tool aprates for each observation, taking into account the Poisson statistics in the low-count regime. The observations utilized for spectral extraction of the 2005 outburst, 2024 outburst, and the quiescent state are denoted by blue, orange, and black markers, respectively, while the remaining observations are shown in grey. The shaded region encloses the timespan of the insert, which presents a cross-observatory view of the 2--8 keV flux evolution of the 2024 outburst. Observed count rates from Swift (black crosses), NuSTAR (red triangles), and Chandra (blue circles) and XRISM (magenta stars, as reported by Yoshimoto+25) have been converted to 2--8 keV energy fluxes using their respective best-fit spectral models. To enhance the signal-to-noise ratio during the source's fading phase, we grouped every five consecutive Swift observations taken since April 2024. For comparison, the 1--10 keV light curves of two historical outbursts (2006 and 2021; Page+22) from the recurrent nova RS Oph are plotted by light blue and orange symbols, with their fluxes rescaled for a distance of 8 kpc and peak times aligned with the Swift J174610 flux maximum (MJD 60365.5).
• Figure 3: Background-subtracted spectra of Swift J174610 at different flux levels. (a) The quiescent state observed by Chandra. (b) The two outbursts observed by Chandra, black for 2005 and red for 2024. (c) The 2024 outburst observed by Swift before April. (d) The 2024 outburst observed by NuSTAR. The combined spectra of FPMA and FPMB are shown in red and black, respectively. The spectra are adaptively binned to ensure a minimum of 10 counts per spectral bin and a S/N $\geq$ 3. The best-fit model in XSPEC, TBABS*(BREMSS+4*GAUSS) is overlaid as solid curves. A maximum of four Gaussian lines with fixed centroids at 5.65, 6.4, 6.7, and 7.0 keV, are shown by the dotted curves. The bottom panels display the residuals, with 1$\sigma$ error bars.