Spectra of GRB 970228 from the Transient Gamma-Ray Spectrometer
  Visible afterglow counterparts have now been detected for two GRBs (970228 and 970508) but are absent, with $L_{opt}/L_{\gamma}$ ratios at least two orders of magnitude lower, for other GRBs, e.g., 970828. The causes of this variation are unknown. Any correspondence which could be discovered between the gamma-ray properties of a GRB and its $L_{opt}/L_{\gamma}$ would be useful, both in determining the GRB mechanisms, and in allocating resources for counterpart searches and studies. This paper presents the gamma-ray spectra of GRB 970228 as measured by the Transient Gamma-Ray Spectrometer and comments on characteristics of this GRB compared to others that do and do not have observable counterparts. 
D. M. Palmer, T. L. Cline, N. Gehrels, K. Hurley, P. Kurczynski, N.   Madden, R. Pehl, R. Ramaty, H. Seifert, B. J. Teegarden