The discovery of the electromagnetic counterpart of GW170817: kilonova AT 2017gfo/DLT17ck (1710.05854v1)

Abstract: During the second observing run of the Laser Interferometer gravitational- wave Observatory (LIGO) and Virgo Interferometer, a gravitational-wave signal consistent with a binary neutron star coalescence was detected on 2017 August 17th (GW170817), quickly followed by a coincident short gamma-ray burst trigger by the Fermi satellite. The Distance Less Than 40 (DLT40) Mpc supernova search performed pointed follow-up observations of a sample of galaxies regularly monitored by the survey which fell within the combined LIGO+Virgo localization region, and the larger Fermi gamma ray burst error box. Here we report the discovery of a new optical transient (DLT17ck, also known as SSS17a; it has also been registered as AT 2017gfo) spatially and temporally coincident with GW170817. The photometric and spectroscopic evolution of DLT17ck are unique, with an absolute peak magnitude of Mr = -15.8 \pm 0.1 and an r-band decline rate of 1.1mag/d. This fast evolution is generically consistent with kilonova models, which have been predicted as the optical counterpart to binary neutron star coalescences. Analysis of archival DLT40 data do not show any sign of transient activity at the location of DLT17ck down to r~19 mag in the time period between 8 months and 21 days prior to GW170817. This discovery represents the beginning of a new era for multi-messenger astronomy opening a new path to study and understand binary neutron star coalescences, short gamma-ray bursts and their optical counterparts.

• The paper establishes the discovery of the kilonova AT 2017gfo as the electromagnetic counterpart to GW170817.
• It employs the DLT40 survey, noting an absolute peak magnitude of –15.8 and a rapid decline rate of 1.1 mag/day that validate kilonova models.
• The study reinforces r-process nucleosynthesis models by constraining ejecta mass to 3×10⁻³–10⁻² M⊙ and boosts prospects for multi-messenger astronomy.

The Discovery of the Electromagnetic Counterpart of GW170817: Kilonova AT 2017gfo/DLT17ck

The detection of gravitational waves (GWs) has ushered in a transformative period for astrophysics, particularly with the Advanced LIGO and Virgo collaborations. Among the noteworthy events captured in this context is the observation of a gravitational-wave signal, GW170817, attributed to the merger of a binary neutron star (BNS) system. Complementing the GW detection was the simultaneous observation of a short gamma-ray burst (sGRB) by the Fermi satellite. This paper by Valenti et al. delineates the discovery of an electromagnetic (EM) counterpart to GW170817, an optical transient identified as kilonova AT 2017gfo/DLT17ck.

Observational Framework and Methodology

During the second observing phase of LIGO and Virgo, a robust protocol for follow-up observations was in place. The DLT40 Mpc supernova search leveraged existing survey data to prioritize galaxies within the LIGO+Virgo and Fermi error localization regions. The systematic follow-up led to the detection of DLT17ck. An observed absolute peak magnitude of $M_{r}=-15.8 \pm 0.1$ and a rapid $r$-band decline rate of $1.1\,\rm{mag}/\rm{d}$ were among the distinct characteristics of DLT17ck, aligning well with theoretical kilonova profiles arising from BNS mergers.

Spectroscopic and Photometric Insights

DLT17ck's light curve evolution, marked by its rapid decline, is consistent with kilonova models powered by the radioactive decay of r-process nuclei. This scenario assumes a small ejected mass with substantial neutron-rich material. Observational data confirmed the transient nature of DLT17ck with no detectable emission down to $r$ ∼19 mag in prior archival data, thereby affirming its association with GW170817.

Implications and Theoretical Considerations

The singular photometric and spectroscopic characteristics of DLT17ck substantiate theoretical predictions for kilonova emission from BNS mergers. The data aligns with scenarios involving low-mass ejecta and moderately high velocities, constraining the ejected mass to roughly $3 \times 10^{-3} - 10^{-2} M_{\odot}$. These observations are pivotal for validating r-process nucleosynthesis as the driving force behind the observed EM counterparts to GW events.

Future Trajectory and Theoretical Implications

The discovery of DLT17ck provides a compelling case paper in the nascent era of multi-messenger astronomy. This event not only illustrates the power of synchronized GW and EM observations but also charts new pathways for understanding the dynamics, composition, and rates of BNS mergers. Moving forward, enhanced sensitivity of GW observatories should extend the detection horizon further into the universe, allowing for more comprehensive studies and improved constraints on the rates and characteristics of kilonovae. The interplay of future observational data and model advancement will be crucial in refining the theoretical frameworks surrounding GW and EM correlations.

In sum, this paper contributes significantly to our growing comprehension of BNS mergers and kilonovae, spearheading further investigations into the interplay between gravitational-wave astronomy and traditional EM astrophysics. As the field advances, the synthesis of observational data with robust theoretical models will undoubtedly enhance our understanding of these cosmic phenomena.