eROSITA discovery of a new BeXRB pulsar eRASSU J012422.9-724248 near the SMC wing

ATel #15886; C. Maitra, F. Haberl, D. Kaltenbrunner (all MPE), V. Doroshenko, L. Ducci (all IAAT Tuebingen), A. Udalski (Univ. of Warsaw)
on 3 Feb 2023; 09:48 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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eRASSU J012422.9-724248 was discovered in the Magellanic Bridge, near the Eastern Wing of the SMC during the first four all-sky surveys with eROSITA onboard SRG (eRASS1-4). The source was detected at the following position (after astrometrical corrections):

RA (J2000) = 01:24:22.9, DEC (J2000) = -72:42:48.7

with a 1.4" statistical uncertainty. As likely optical counterpart we identify an early type star (V ~ 15 mag and U-B and V-B compatible with a Be star), coincident with the X-ray position.

Optical spectroscopy with LCO/FLOYDS revealed a strong H-alpha line in emission with an equivalent width of -25.0 ± 0.2 Å. This confirms eRASSU J012422.9-724248 as a newly identified Be X-ray binary. A Lomb-Scargle analysis of the OGLE-IV I-band light curve of the counterpart (smc732.10.7) revealed a clear signal at 63.83 days, which we interpret as the orbital period of the binary system.

eROSITA scanned the source for a total exposure time of 1.3 ks (after vignetting corrections). The average spectrum can be described by an absorbed power-law model with a photon index of 1.33 (±0.8) and an absorption column density fixed to the line of sight value of 1.6 ×10²¹ H cm^-2 (Willingale et al. 2013). The average source flux was ~1.9×10^-13 erg cm^-2 s^-1 in the 0.2-8.0 keV band, corresponding to an absorption-corrected luminosity of 8.1×10³⁴ erg s^-1, assuming a distance of 62 kpc. For all spectral parameters 90% confidence ranges are given.

From recent X-ray observations using the Neil Gehrels Swift observatory on 2023-01-15 between 18:12 and 23:08 UTC (MJD 59959.76 - 59959.96, 2.2 ks exposure) the source was detected with an XRT count rate of 2.01±0.33 ×10^-2 cts s^-1 in the 0.2-10.0 keV band. Assuming the spectral parameters from above, this corresponds to an observed flux of 1.2×10^-12 erg cm^-2 s^-1 and a luminosity of 5.0×10³⁵ erg s^-1 (0.2-10.0 keV), which is at a factor of ~6 higher brightness level as during the eROSITA observations.

Following this, a NuSTAR observation was performed to further investigate the broad-band X-ray properties of the source. The NuSTAR observation was performed between 2023-01-22 (MJD 59967.42) and 2023-01-23 (MJD 59968.42) with an effective exposure of 75.3 ks on FPMA and 75.8 ks on FPMB. Strong pulsations are detected independently in the 3.0 -78.0 keV light curves obtained from FPMA and FPMB thereby confirming the source as an X-ray pulsar. The detected spin period is 341.7±0.1 s where the error is determined using a block bootstrapping method. The 3.0-50.0 keV spectrum can be described by an absorbed cutoff-powerlaw model, a photon index of 0.39 (±0.51) , a folding energy of 6.8 (±2.8) keV. A putative cyclotron line is detected at ~12 keV but needs a detailed analysis for confirmation. The observed source flux was 1.5×10^-12 erg cm^-2 s^-1 corresponding to a similar luminosity during the Swift observations. The detailed analysis of NuSTAR and eROSITA data will be published elsewhere.

We thank the Swift and NuSTAR SOC teams for the quick scheduling of the DDT observations.