Detection of a quasi-periodic oscillation from a ULX in NGC 4736

We present our findings on a new quasi-periodic oscillation (QPO) and a long period from the transient ultraluminous X-ray source (ULX) X-2 in nearby galaxy NGC 4736 based on the Chandra and XMM-Newton archival data. Timing properties of ULX X-2 have been examined by obtaining power density spectrum (PDS) using Fast Fourier Transform technique. The PDS of X-2 reveals a QPO peak at 0.73_{-0.14}(+0.16) mHz with an fractional rms variability of 16% using the Chandra data (ObsID 808). Additionally, the PDS obtained by using XMM-Newton-pn data (ObsID 0404980101) shows a peak at 0.53_{-0.35}(+0.09) mHz with a fractional rms variation of 5%. We note that these frequencies overlap within their error limits and may be the same oscillation. Also, we detect a long periodicity or a QPO in the Chandra data of about (5.2±2.0)×10(-5) Hz (∼ 5.4 hrs) over 3 sigma confidence level. If this is a QPO, it is the lowest QPO detected from a ULX. The mass of the compact object in ULX X-2 is estimated from Eddington luminosity and disk blackbody model in the range of (11-82) M_{sun}. In order to understand the exact nature of this transient source we will examine the spectral properties of the optical counterpart using 6-m BTA telescope of the SAO RAS.