Broadband study of the Be/X-ray binary pulsar eRASSU J012422.9-724248 in the Magellanic Bridge, near the Eastern Wing of the Small Magellanic Cloud

TL;DR

This study identifies eRASSU J012422.9-724248 as a Be/X-ray binary pulsar in the Magellanic Bridge by combining eROSITA, Swift, NuSTAR X-ray data with OGLE and LCO optical observations. It measures a spin period of $P_{ ext{spin}} = 341.71 ext{ s}$ and an orbital period of $P_{ ext{orb}} = 63.65 ext{ d}$, and detects a tentative cyclotron line at $E_{ ext{CRSF}} \,\approx\, 12.3\text{ keV}$, which would imply a magnetic field of $B \,\approx\ 1.4\times10^{12}\text{ G}$ if confirmed. The X-ray spectrum is well described by an absorbed power law with a high-energy cutoff across instruments, and the source shows persistent X-ray emission at a few $\times 10^{35}\text{ erg s}^{-1}$ alongside long-term optical disk variability. The findings place eRASSU J012422.9-724248 among the persistent BeXRBs in the Magellanic system and highlight a close link between disk evolution and X-ray activity in such systems.

Abstract

The first four all-sky surveys with eROSITA the soft X-ray instrument on board the Spektrum-Roentgen-Gamma (SRG) satellite revealed a new X-ray source, eRASSU J012422.9-724248, in the Magellanic Bridge, near the Eastern Wing of the Small Magellanic Cloud (SMC). We performed a broadband timing and spectral analysis using the optical and X-ray data of eRASSU J012422.9-724248. Using the X-ray observations with eROSITA, Swift, NuSTAR and optical data from the optical Gravitational Lensing Experiment (OGLE) and the Las Cumbres Observatory (LCO), we confirm the nature of eRASSU J012422.9-724248 as a Be/X-ray binary (BeXRB) pulsar in the Magellanic bridge. The position is coincident with that of an early-type star (OGLE ID SMC732.10.7). We detect the spin period at 341.71 s in NuSTAR data and infer a period of 63.65 days from the 15 year monitoring with OGLE, that we interpret as the orbital period of the system. A tentative CRSF at ~12.3 keV is identified in NuSTAR spectra with ~1.8-sigma. The source appears to show a persistent X-ray luminosity and an optical magnitude transition on the long timescale. We propose eRASSU J012422.9-724248 is a new member of the class of persistent BeXRBs.

Figures (13)

• Figure 1: eROSITA light curve of eRASSU J012422.9$-$724248, where the blue points represent time binning per eROSITA scan and the black points show results in which each bin contains a minimum of ten counts to improve statistics. Error bars in the $x$-direction give the length of time bins. For the bins with fewer than ten counts, the 1-$\sigma$ upper limits are plotted.
• Figure 2: The Lomb-Scargle periodogram of eRASSU J012422.9$-$724248 obtained from the combined NuSTAR data (3.0-22.0 keV). The dashed red line marks the 99.73% (3-$\sigma$) confidence level obtained from the simulation.
• Figure 3: NuSTAR pulse profiles obtained from folding the combined light curves from two observations in different energy bands and the hardness ratio. There is no statistically significant difference between the individual pulse profiles of the two observations.
• Figure 4: Upper panel: NuSTAR phase energy heat-map obtained from the combined light curve from observation 90901301002. Lower panel: The heat-map obtained in the same way from observation 90901301004. Each energy bin is normalised by subtracting the average pulse intensity and subtracted by the standard deviation of the energy bin. The 3-22 keV pulsed profile is plotted as red dashed histogram.
• Figure 5: NuSTAR pulsed fraction as a function of energy. The combined data of two observations are used, which reveals a drop at around 10 keV. Such a feature could not be significantly constrained using data from either observation alone due to low statistics.