A 78 Day X-ray Period Detected from NGC 5907 ULX1 by Swift

TL;DR

This paper reports the discovery of a 78-day periodicity in the X-ray emission from NGC 5907 ULX1, indicating potential orbital or super-orbital modulation, based on extensive Swift observations.

Contribution

First detection of a 78-day X-ray periodicity in NGC 5907 ULX1 using Swift, with detailed analysis of flux variations and significance assessment.

Findings

Detected a 78.1-day period with high significance

Flux varies by a factor of 3-4 over the cycle

Periodic signal is robust against simulations

Abstract

We report the detection of a $78.1\pm0.5$ day period in the X-ray lightcurve of the extreme ultraluminous X-ray source NGC 5907 ULX1 ($L_{\rm{X,peak}}\sim5\times10^{40}$ erg s$^{-1}$), discovered during an extensive monitoring program with Swift. These periodic variations are strong, with the observed flux changing by a factor of $\sim$3-4 between the peaks and the troughs of the cycle; our simulations suggest that the observed periodicity is detected comfortably in excess of 3$\sigma$ significance. We discuss possible origins for this X-ray period, but conclude that at the current time we cannot robustly distinguish between orbital and super-orbital variations.