A "kilonova" associated with short-duration gamma-ray burst 130603B (1306.4971v2)

Abstract: Short-duration gamma-ray bursts (SGRBs) are intense flashes of cosmic gamma-rays, lasting less than ~2 s, whose origin is one of the great unsolved questions of astrophysics today. While the favoured hypothesis for their production, a relativistic jet created by the merger of two compact stellar objects (specifically, two neutron stars, NS-NS, or a neutron star and a black hole, NS-BH), is supported by indirect evidence such as their host galaxy properties, unambiguous confirmation of the model is still lacking. Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint transient in the days following the burst, a so-called "kilonova". Indeed, it is speculated that this mechanism may be the predominant source of stable r-process elements in the Universe. Recent calculations suggest much of the kilonova energy should appear in the near-infrared (nIR) due to the high optical opacity created by these heavy r-process elements. Here we report strong evidence for such an event accompanying SGRB 130603B. If this simplest interpretation of the data is correct, it provides (i) support for the compact object merger hypothesis of SGRBs, (ii) confirmation that such mergers are likely sites of significant r-process production and (iii) quite possibly an alternative, un-beamed electromagnetic signature of the most promising sources for direct detection of gravitational waves.

• The paper demonstrates that nIR observations by HST revealed a transient source consistent with a kilonova distinct from standard afterglow emissions.
• Spectral and photometric analyses indicate that the observed nIR light curves and energy distribution align with theoretical predictions for r-process nucleosynthesis.
• The study underscores the role of kilonovae as isotropic electromagnetic counterparts that enhance the detection of gravitational-wave events from compact binary mergers.

Insightful Overview of a "Kilonova" Associated with Short-Duration γ-Ray Burst 130603B

The paper presents compelling evidence for the occurrence of a kilonova associated with the short-duration γ-ray burst (SGRB) 130603B. SGRBs, characterized by intense bursts of γ-rays lasting less than two seconds, have long puzzled astrophysicists regarding their origins. The prevailing hypothesis suggests that these bursts result from relativistic jets produced by the merger of compact binaries such as neutron star-neutron star (NS-NS) or neutron star-black hole (NS-BH) systems. Support for this hypothesis has been largely circumstantial, relying heavily on characteristics of the host galaxies and a lack of correlation with massive star formations. This paper provides a more direct evidence in support of this hypothesis by reporting the detection of a kilonova—a radioactive decay-driven transient following the SGRB—in the aftermath of SGRB 130603B.

Key Findings

1. Kilonova Detection via HST Observations: The Hubble Space Telescope (HST) was used to image the site of the burst at two epochs: approximately nine days and thirty days post-burst. The first epoch exhibited clear evidence of a transient source in the near-infrared (nIR) band, which diminished by the second epoch, consistent with theoretical kilonova predictions. A detailed analysis suggests this nIR source cannot be attributed to typical afterglow emission, implying it principally originates from kilonova ejecta.
2. Spectral and Photometric Analysis: The spectral energy distribution (SED) indicates that the late-time nIR enhancement observed aligns closely with recent theoretical kilonova models. These models predict a significant nIR component due to the high optical opacity caused by heavy r-process nuclei generated during the merger, effectively supporting the notion that such mergers are substantial sites for the production of these elements.
3. Consistency with Theoretical Models: The observed nIR light curves, peaking in brightness several days following the burst, concur with theoretical predictions of kilonova behavior predicated on r-process nucleosynthesis. This congruity underlines the importance of kilonovae as electromagnetic counterparts to compact binary mergers and as potential signatures for gravitational-wave sources.

Implications and Future Directions

The implications of this paper are threefold: First, it offers substantial credence to the compact binary merger hypothesis as a progenitor of SGRBs. Second, it confirms these mergers as likely production sites for r-process elements, which are crucial to understanding astrophysical nucleosynthesis. Third, it identifies kilonova emissions as an alternative, isotropic electromagnetic signal that could significantly enhance efforts to detect gravitational-wave events associated with compact binary coalescences. Given the future advancements in gravitational wave observatories such as Advanced LIGO and Advanced Virgo, accurate identification and localization of such electromagnetic counterparts will be vital for constraining the astrophysical processes involved.

The findings of this research underscore the potential of kilonovae to offer new insights into the nature of compact object mergers and their resultant emissions. Future work should focus on expanding observational campaigns to capture more candidate kilonovae across varied host environments and redshifts. Such endeavors would refine our understanding of their frequency and the diversity of their progenitor mergers, thereby enhancing the synergy between electromagnetic and gravitational-wave astronomy.