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 .