First Search for an X-ray -- Optical Reverberation Signal in an Ultraluminous X-ray Source

Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to AGN broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1's X-rays are variable on a timescale of a few hundred seconds (RMS of 9.0$\pm$0.5%), the optical emission does not show any statistically significant variations. We set a 3$\sigma$ upper limit on the RMS optical variability of 3.3%. The ratio of the X-ray to the optical variability is an indicator of X-ray reprocessing efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar ratio for NGC 5408 X-1, the expected RMS optical variability is $\approx$2% which is still a factor of roughly two lower than what was possible with the VLT observations in this study. We find marginal evidence (3$\sigma$) for optical variability on a $\sim$ 24 hour timescale. Our results demonstrate that such measurements can be made, but photometric conditions, low sky background levels and longer simultaneous observations will be required to reach the optical variability levels similar to X-ray binaries.