The Macronova in GRB 050709 and the GRB-macronova connection

Abstract

GRB 050709 was the first short Gamma-ray Burst (sGRB) with an identified optical counterpart. Here we report a reanalysis of the publicly available data of this event and the discovery of a Li-Paczynski macronova/kilonova that dominates the optical/infrared signal at t>2.5 days. Such a signal would arise from 0.05 r-process material launched by a compact binary merger. The implied mass ejection supports the suggestion that compact binary mergers are significant and possibly main sites of heavy r-process nucleosynthesis. Furthermore, we have reanalysed all afterglow data from nearby short and hybrid GRBs (shGRBs). A statistical study of shGRB/macronova connection reveals that macronova may have taken place in all these GRBs, although the fraction as low as 0.18 cannot be ruled out. The identification of two of the three macronova candidates in the I-band implies a more promising detection prospect for ground-based surveys.

Figure 1. The optical observations of sGRB 050709 and the macronova component.

(a) The fits to the R-band emission (green dashed line) and to the I-band observations from the VLT I-band data as well as the first two HST F814W-band data points (red dash-dotted line) yield the declines of t^{−1.63±0.16} and t^{−1.12±0.09}, respectively. The dotted lines represent the ‘suggested' afterglow emission lightcurves of the GRB outflow after the jet break (that is, t^−2.5 for the energy distribution index of the shock-accelerated electrons p∼2.5). (b) Shown are the residuals of the optical emission after the subtraction of a suggested fast-declining forward shock afterglow after t=1.4 days (dotted lines in panel (a)). The simulated I/R/V-band macronova light curves are for the ejecta from a black hole–neutron star merger, corresponding to an ejection mass of and a velocity of V_ej∼0.2c. An uncertainty of ∼0.75 mag (the shaded region) has been adopted following Hotokezaka et al. (c) The SED of the macronova signal of sGRB 050709 measured by VLT on 12 July 2005 compared with a possible Iron line-like spectral structure adopted from Kasen et al. It is worth noting that all errors are 1σ statistical errors and the upper limits are at the 3σ confidence level.

Figure 2. Comparison of the lightcurves of macronova candidates and theoretical models.

Absolute Vega magnitudes versus rest frame time of the macronova candidates in sGRB 050709, hGRB 060614 (ref. 26) and sGRB 130603B. The red dashed line is the same as the dynamical ejecta macronova model I-band emission presented in Fig. 1 (the green dashed line represents the H-band emission), whereas the red dotted line is the disk-wind ejecta macronova model I-band emission light curve for and V_ej=0.07c (the green dotted line represents the H-band emission). It is noteworthy that all errors are 1σ statistical errors and the upper limits are at the 3σ confidence level.

Figure 3. Comparison of the limits of macronova in some sGRBs and theoretical models.

Absolute Vega magnitudes versus rest frame time of the I-band/F814W-band observations of sGRB 050509B, hGRB 060505 (ref. 52) and sGRB 061201 (refs 49, 66). The HST F814W 3σ upper limits of GRB 080503 (ref. 54) are also shown for an assumed redshift of z=0.25, following Kasen et al.. It is worth noting that the Gemini i-band 3σ upper limit of sGRB 060505 was reanalysed in this work. The red dashed line is the dynamical ejecta macronova model I-band emission while the red solid line is the disk-wind ejecta macronova model I-band emission light curve, where the same model parameters in Fig. 2 are chosen. The black dotted line represents the macronova I-band emission expected for a double neutron star merger with and V_ej∼0.1c, implying that the 3σ upper limits reported in sGRB 050509B, hGRB 060505 (ref. 52) and sGRB 061201 are not deep enough to exclude the compact object merger origin.

Figure 4. The VLT I-band images of the afterglow of GRB 050709.

The data were taken on 12 July 2005 (a) and 30 July 2005 (b), and the signal resulted in the image substraction (c). The afterglow position has been circled and the afterglow emission is clearly visible on 12 July 2005. The images are magnified only for demonstration.