GCN Circular 346

Gravitational collapse of neutron stars (NS) to (di)quark stars (QS) due
to phase transition or mass accretion are expected to generate highly
relativistic jets plus a mildly relativistic spherical explosion (e.g.,
Dar 1999a,b). The jets, when pointing in our direction, produce GRBs, and
afterglows that decline with time like

             F(t)\sim t^{-\alpha}/[1+(t/t_0)^{\beta-\alpha}]

The spherical explosion is expected to add to it an afterglow like that of
GRB 980425/SN 1998bw (Galama et al 1998; , time-dilated and redshifted by 
1+z. Assuming ``a standard explosion'' = 1998bw at redshift 1+z: 
              
 F(t)\sim t^{-\alpha}/[1+(t/t_0)^{\beta-\alpha}]+ SN1998bw[1+z] 

For GRB 990510, \alpha= 0.76, \beta=2.4, and t_0=1.57 days is the
time when the jet begins to spread. The spherical explosion is
expected to produce an up turn in the time-decline of the afterglow of GRB
990510 in the R band before the end of May, reach a maximum with R
magnitude m=26.7\pm 0.3 between early and mid June, decline fastly by two
additional magnitudes by mid August and then decline exponentially 
with a much smaller rate, with a half time (1+z)=2.62 longer than that of
GRB 980425/SN 1998bw, i.e., t(half)=140 days.  An identical behavior
is expected from the late afterglow (t>20 days after explosion) of GRB
990123 (z=1.61, Kulkarni et al. 1999). 

[The above behavior explains better the behavior of the afterglow of GRB
970228 (z=0.69; Kulkarni 1999), t_max=20 days, m(max)=25 in the R band)
than a single power-law and consistent with all the other measured GRB
afterglows]. 

HST is urged to conduct observations of GRBs 990510 and 990123 in order to 
test these predictions and verify/falsify the proposed NS-->QS origin of 
GRBs. 


Dar, A. 1999a, A\&A, in press (astro-ph/9902017)
Dar, A. 199b astro-ph/9905315
Galama, T. J. et al. 1998, Nature 395, 670 
Kulkarni, S. R. et al 1999a, Nature, 398, 389
McKenzie, E. H. \& Schaefer, B. E. 1999 astro-ph/9904397 
Stanek, K. Z. 1999, astro-ph/9905304   
Vreeswijk, P. M. et al. 1999 GCN Circ. No. 324