The coalescence of a binary neutron star ( NS ) system ( a ‘ NS merger ’ or NSM ) may in some cases produce a massive NS remnant that is long-lived and , potentially , indefinitely stable to gravitational collapse .
Such a remnant has been proposed as an explanation for the late X-ray emission observed following some short duration gamma-ray bursts ( GRBs ) and as possible electromagnetic counterparts to the gravitational wave chirp .
A stable NS merger remnant necessarily possesses a large rotational energy \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 10 ^ % { 52 } erg , the majority of which is ultimately deposited into the surrounding circumburst medium ( CBM ) at mildly relativistic velocities .
We present Very Large Array ( VLA ) radio observations of 7 short GRBs , some of which possessed temporally extended X-ray emission , on timescales of \sim 1 - 3 years following the initial burst .
No radio sources were detected , with typical upper limits \sim 0.3 mJy at \nu = 1.4 GHz .
A basic model for the synchrotron emission from the blast wave is used to constrain the presence of a long-lived NSM remnant in each system .
Depending on the GRB , our non-detections translate into upper limits on the CBM density n \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 3 % \times 10 ^ { -2 } -3 cm ^ { -3 } required for consistency with the remnant hypothesis .
Our upper limits rule out a long-lived remnant in GRB 050724 and 060505 , but can not rule out such a remnant in other systems due to their lower inferred CMB densities based on afterglow modeling or the lack of such constraints .