Elliptic flow of thermal photons and formation time of quark gluon   plasma at RHIC
  We calculate the elliptic flow of thermal photons from Au+Au collisions at RHIC energies for a range of values for the formation time $\tau_0$ but fixed entropy (or particle rapidity density). The results are found to be quite sensitive to $\tau_0$. The $v_2$ for photons decreases as $\tau_0$ decreases and admits a larger contribution from the QGP phase which has a smaller $v_2$. The elliptic flow coefficient for hadrons, on the other hand, is only marginally dependent on $\tau_0$. 
Rupa Chatterjee and Dinesh K. Srivastava