Discovery of a polar candidate towards the Galactic disk: XMMU J180332.4-232230

ATel #16736; Samaresh Mondal (INAF-OAB), Gabriele Ponti (INAF-OAB; MPE), Frank Haberl (MPE), Sergio Campana (INAF-OAB), Nanda Rea (ICE, CSIC; IEEC), Tong Bao (INAF-OAB), Mark R. Morris (UCLA)
on 26 Jul 2024; 10:55 UT
Credential Certification: Sergio Campana (sergio.campana@brera.inaf.it)

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We are performing an X-ray scan of the inner galactic disk using XMM-Newton (Mondal et al. 2022, A&A, 666, A150; Mondal et al. 2023, A&A, 671, A120). We report the discovery of a periodic X-ray source towards the Galactic disk. The source located at R.A. = 18h 03m 32.47s, DEC = -23d 22' 30.0'' (J2000) with a one sigma positional uncertainty of 0.5 arcsec is designated as XMMU J180332.4-232230. The source was discovered by XMM-Newton in our ongoing Heritage survey of the inner Galactic disk (Date: 02-04-2024, ObsID: 0886110901, PI: G. Ponti). The EPIC-pn 2-10 keV mean count rate of the source is 0.006 c/s and the 2-10 keV source flux is 1.5x10^{-13} erg/s/cm^2. The Lomb-Scargle periodogram of the X-ray light curve shows a peak at the frequency of 1.97x10^{-4} Hz above a three-sigma confidence level, indicating the source's periodic nature with a period of 5068+/-260 s. The 2-10 keV pulse fraction of the source is 82+/-17%. The hardness ratio (HR) of the source is -0.2+/-0.09. We computed the HR using the net count in the soft (2-5 keV) and hard (5-10 keV) bands, given by the formula HR= (hard count - soft count)/(hard count + soft count). The source is located in region 2 of the HR vs. period diagram of Mondal et al. 2024, A&A, 686, A125 suggesting it to be polar. The 0.2-10 keV spectrum of the source is best fitted by a simple power law model with absorption column density NH=(3+/-1)x10^{21} cm^{-2} and photon index Gamma=1.1+/-0.4. The absorption column density estimated from the X-ray spectral fitting is significantly smaller than one would expect for the Galactic Center X-ray sources. This indicates the source should be located at a distance between 1-2 kpc. No X-ray counterparts were found in other catalogues. We also searched for an optical and infrared association; however, no counterparts were found within three-sigma X-ray positional uncertainty.