A possible macronova in the late afterglow of the long–short burst GRB 060614

Long-duration (>2 s) γ-ray bursts that are believed to originate from the death of massive stars are expected to be accompanied by supernovae. GRB 060614, that lasted 102 s, lacks a supernova-like emission down to very stringent limits and its physical origin is still debated. Here we report the discovery of near-infrared bump that is significantly above the regular decaying afterglow. This red bump is inconsistent with even the weakest known supernova. However, it can arise from a Li-Paczyński macronova—the radioactive decay of debris following a compact binary merger. If this interpretation is correct, GRB 060614 arose from a compact binary merger rather than from the death of a massive star and it was a site of a significant production of heavy r-process elements. The significant ejected mass favours a black hole–neutron star merger but a double neutron star merger cannot be ruled out.

L ong-duration (42 s) g-ray bursts (GRBs) are believed to originate from Collapsars that involve death of massive stars and are expected to be accompanied by luminous supernovae (SNe). GRB 060614 was a nearby burst with a duration of 102 s at a redshift of 0.125(ref. 1). While it is classified as a long burst according to its duration, extensive searches did not find any SNe-like emission down to limits hundreds of times fainter 2-4 than SN 1998bw, the archetypal hypernova that accompanied long GRBs 5 . Moreover, the temporal lag and peak luminosity of GRB 060614 fell entirely within the short duration subclass and the properties of the host galaxy distinguish it from other long-duration GRB hosts. Thus, GRB 060614 did not fit into the standard picture in which long-duration GRBs arise from the collapse of massive stars while short ones arise from compact binary mergers. It was nicknamed the 'long-short burst' as its origin was unclear. Some speculated that it originated from compact binary merger and thus it is intrinsically a 'short' GRB 1,4,[6][7][8] . Others proposed that it was formed in a new type of a Collapsar which produces an energetic g-ray burst that is not accompanied by an SNe [2][3][4] .
Two recent developments may shed a new light on the origin of this object. The first is the detection of a few very weak SNe (for example, SN 2008ha 9 ) with peak bolometric luminosities as low as LB10 41 erg s À 1 . The second is the detection of an infrared bump, again with a LB10 41 erg s À 1 , in the late afterglow of the short burst GRB 130603B 10,11 . This was interpreted as a Li-Paczyński macronova (also called kilonova) 12-19 -a nearinfrared/optical transient powered by the radioactive decay of heavy elements synthesized in the ejecta of a compact binary merger. Motivated by these discoveries, we re-examined the afterglow data of this peculiar burst searching for a signal characteristic to one of these events.
The X-ray and UV/optical afterglow data of GRB 060614, were extensively examined in the literature 20,21 and found to follow very well the fireball afterglow model up to tB20 days 22 . The J-band has been disregarded because only upper limits B19-20 th mag with a sizeable scatter are available at t 42.7 days, and these are too bright to significantly constrain even supernovae as luminous as SN 1998bw 23 . In this work we focus on the optical emission. We have re-analysed all the late time (that is, t Z1.7 days) very large telescope (VLT) V, R and I -band archival data and the Hubble space telescope (HST) F606W and F814W archival data, including those reported in the literature 3,4 and several unpublished data points. Details on data reduction are given in the Methods.

Results
The discovery of a significant F814W-band excess. Figure 1 depicts the most complete late-time optical light curves (see Supplementary Table 1; the late VLT upper limits are not shown in Fig. 1) of this burst. The VLT V, R and I-band fluxes decrease with time as pt À 2.30±0.03 (see Fig. 1, in which the VLT V/I band data have been calibrated to the F606W/F814W filters of HST with proper k-corrections), consistent with that found earlier 3,20,21 . However, the first HST F814W data point is significantly above the same extrapolated power-law decline. The significance of the deviation is B6s (see the estimate in the Methods). No statistically significant excess is present in both the F606W and the R bands. The F814W-band excess is made most forcibly by considering the colour evolution of the transient, defined as the difference between the magnitudes in each filter, which evolves from V-IE0.65 mag by the VLT (correspondingly for HST we have F606W-F814WE0.55 mag) at about t B1.7 days to F606W-F814WE1.5 mag by HST at about 13.6 days after the trigger of the burst. With proper/minor extinction corrections, the optical to X-ray spectrum energy distribution for GRB 060614 at the epoch of B1.9 days is nicely fitted by a single power law 3,20,21 F v pv À 0.8 . In the standard external forward shock afterglow model, the cooling frequency is expected to drop with time as 22 v c pt À 1/2 . Thus, it cannot change the optical spectrum in the time interval of 1.9-13.6 days. Hence, the remarkable colour change and the F814W-band excess of B1 mag suggest a new component. Like in GRB 130603B this component was observed at one epoch only. After the subtraction of the power-law decay component, the flux of the excess component decreased with time faster than t À 3.2 for t 413.6 days. Note that an unexpected optical re-brightening was also detected in GRB080503, another 'long-short' burst 24 . However, unlike the excess component identified here, that re-brightening was achromatic in optical to X-ray bands and therefore likely originated by a different process.

Discussion
Shortly after the discovery of GRB 060614 it was speculated that it is powered by an 'unusual' core collapse of a massive star 2,3 . We turn now to explore whether the F814W-band excess can be powered by a weak supernova. Figure 2 depicts the colour F606W-F814W of the excess component (we take F606W-F814WE1.5 mag as a conservative lower limit of the colour of the 'excess' component due to the lack of simultaneous excess in F606W-band) with that of SN 2006aj 25 , SN 2008ha (i.e., the extremely dim event) 26 and SN 2010bh 27 . The excess component has a much redder spectrum than the three supernovae. If the 'excess component' was thermal it had a low effective temperature T eff o3,000 K to yield the very soft spectrum. Such unusually low effective temperature is also needed to account for the very rapid decline of the excess component. The expansion velocity can be estimated as uB1.2 Â 10 4 km s À 1 (L/10 41 erg s À 1 ) 1/2 (T eff /3,000 K) À 2 (t/13.6 days) À 1 . The implied 56 Ni mass is $ 10 À 3 M if this was a supernova-like event that peaked at B13.6 days 9 . We take a standard cosmology model with H 0 ¼ 71 km s À 1 Mpc The low luminosity as well as the low effective temperature of the transient emission are typical characteristics of a macronova, a transient arising from the radioactive b-decay of material ejected in a compact binary merger. The opacity of the macronova material is determined by the Lanthanides that are produced via r-process in the neutron-rich outflow. This opacity is very large (kE10 cm 2 g À 1 ) resulting in a weak, late and red emission. The emerging flux is greatly diminished by line blanketing, with the radiation peaking in the near-infrared and being produced over a timescale of B1-2 weeks 17,18 . Simple analytic estimates, using a radioactive b-decay heating rate 16,28 of 10 10 erg s À 1 g À 1 [t/(1 þ z)1 day] À 1.3 , suggest that in order to explain the observed F814W-band excess, the required ejecta mass and expansion velocity are: M ej $ 0:13M ðL=10 41 erg s À 1 Þðt=13:6 dayÞ 1:3 and uB0.1c (L/10 41 erg s À 1 ) 1/2 (T eff /2,000 K) À 2 (t/13.6 days) À 1 , respectively. Note that the macronova outflow is quite cold at such a late time 17,18 . The effective temperature is T eff E2,000 K and the observer's F814W-band is above the peak of the black body spectrum. The emitting radius and the corresponding expansion velocity are much larger than in a supernova at this stage. Scaled up numerical simulations of lighter ejecta from black holeneutron star mergers 28 suggest that M ej $ 0:1M and a velocity B02c can account for the observed F814W-band excess. This numerical example is presented in Fig. 1 in dashed lines.
The implied ejecta mass is large compared with the mass ejection estimated numerically to take place in double neutron star mergers. However, it is within the possible range of dynamical ejecta of black hole-neutron star mergers with some extreme parameters (a large neutron star radius and a high black hole spin aligned with the orbital angular momentum) 14,[29][30][31][32] . An accretion disk wind may contribute some additional mass as well 15,33,34 . However, the radioactive heating due to fission of the heavy r-process nuclei, which is quite uncertain and subdominant in current heating estimates 16 , may play an important role in the energy deposition. It may increase the energy deposition rate at around 10 days by a significant factor 35 . This may reduce the required ejecta mass to $ 0:03 À 0:05M . This range of the ejecta masses is well within the range of the dynamical ejecta of black hole-neutron star mergers and it is even compatible with some estimates of double neutron star mergers.
We conclude that while a weak supernova cannot explain the observations, a high mass ejection macronova may. Like in GRB 130603B we must caution here that this interpretation is based on a single data point. However, if this interpretation is correct, it has far reaching implications. First, the presence of macronovae in both the canonical short burst GRB 130603B and in this 'long-short' one, GRB 060614, suggests that the phenomenon is common and the prospects of detecting these transients are promising. A more conclusive detection based on more than a single data point could be achieved in the future provided that denser HST observations are carried out. Moreover, as a black holeneutron star merger is favoured in explaining the large ejected mass this implies that such binary systems may exist and their mergers are also responsible for GRBs. It also suggests that the 'long-short' burst was in fact 'short' in nature, namely, it arose from a merger and not from a Collapsar. The fact that a merger generates a 100 s long burst is interesting and puzzling by itself.
Clearly such events would contribute a significant fraction of the r-process material 36 . The actual contribution relative to the contribution of 130603B-like events is difficult to estimate as it is unclear which fraction of the macronovae/kilonovae behave as each type. Because of beaming most mergers will not be observed as GRBs. However, they emit omnidirectional gravitational radiation that can be detected by the upcoming Advanced LIGO/VIRGO/KAGRA detectors. These near-infrared/optical macronovae could serve as promising electromagnetic counterparts of gravitational wave triggers in the upcoming Advanced LIGO/VIRGO/KAGRA era.

Methods
Data reduction. We retrieved the public VLT imaging data of GRB 060614 from European Southern Observatory (ESO) Science Archive Facility (http://archive.eso.org). The raw data were reduced following standard procedures, including bias subtraction, flat fielding, bad pixel removal and combination. Observations made with the same instrument and filter at different epochs are compared with that of the last epoch. The software package ISIS (http://www2.iap.fr/users/alard/package.html) is used to subtract images and measure the GRB afterglow from the residual images. Photometric errors are estimated from the photon noise and the sky variance to 1s confidence level. The 3s of the background root mean square of the residual images is taken as the limiting magnitude. Finally, standard stars  Table 1, being well consistent with these given by other groups 3,21 . We assumed that the afterglow is characterized by the same power-law spectrum with index b ¼ 0.80 during these observations 20 , with which we get the k-corrections between the VLT V/I and HST F606W/F814W magnitudes, namely 0.12 mag and 0.02 mag, respectively. Such corrections have been taken into account in Fig. 1.
HST archive data of GRB 060614 are available from the Mikulski Archive for Space Telescopes (MAST; http://archive.stsci.edu), including one observation with the Wide Field and Planetary Camera 2 (WFPC2) and four observations with the Advanced Camera for Surveys (ACS) in F606W and F814W bands. The reduced data provided by MAST were used in our analysis. The last observation has been taken as the reference and the other images of the same filter are subtracted in order to directly measure fluxes of the afterglow from the residual images. Empirical point spread functions (PSFs) were built with bright stars in each image. Bright compact objects in the same field were used to align and relatively calibrate these images. WFPC2 image differs from ACS image in PSF. Before image subtraction, the WFPC2 and ACS images were matched to the same resolution by convolving each with the other's PSF. The PSF-matched WFPC2 and ACS images were aligned and subtracted. Aperture photometry was carried out for the afterglow in the residual image. The aperture correction derived from the empirical PSF was applied to yield the total flux. The host galaxy was used to relatively calibrate the afterglow between images, and the ACS zeropoints were used for absolute calibration. If the signal of the afterglow is too faint to be a secure detection, an upper limit of 3s background root mean square is adopted. The results are reported in Supplementary Table 1, being well in agreement with these published in the literature 4 . The magnitudes of the host galaxy are measured in the last observation of all filters and can well be fitted by an Sc type galaxy template ( Supplementary Fig. 1), demonstrating the self-consistence of our results.
VLT light curve decline rate and significance of the excess. As found in previous studies, the late-time optical/X-ray afterglow emission of GRB 060614 can be interpreted within the fireball forward shock model 20,21 . Motivated by such a fact, we assume that the I, R and V light curves follow the same power-law decline. In our fit there are four free parameters, three are related to the initial flux/magnitude in these three bands and the last is the decline rate needed in further analysis. We fitted all the VLT data (combined I, R and V band together) during the first 15 days (after which there are just upper limits) to determine these four parameters as well as their errors. The best-fit decline is found to be pt À 2.3±0.03 , well consistent with that obtained in optical to X-ray bands in previous studies 3, 20,21 . As a result of the propagation of uncertainties, the errors of the best-fit light curves are consequently inferred (the shadow regions in the residual plot of Fig. 2 represent the 1s errors of the best-fit light curves). Please note that in Fig. 2 the VLT V/I band emission have been calibrated to HST F606W/F814W filters with proper k-corrections. The flux separation between the HST F814W-band data and the fitted curve at tB13.6 days is F excess ¼ 0.182 mJy. The flux error of the F814W-band emission at tB13.6 days is dF obs E0.024 mJy. The flux error of the best fitted F814W-band light curve at tB13.6 days is dF fit E0.012 mJy. The significance of the excess component is estimated by R ¼ F excess = ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dF 2 obs þ dF 2 fit p $ 6. We therefore suggest that the excess component identified in this work is statistically significant at a confidence level of B6s.