The X-ray counterpart to the gravitational wave event GW 170817

Abstract: A long-standing paradigm in astrophysics is that collisions- or mergers- of two neutron stars (NSs) form highly relativistic and collimated outflows (jets) powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the observational support for this model is only indirect. A hitherto outstanding prediction is that gravitational wave (GW) events from such mergers should be associated with GRBs, and that a majority of these GRBs should be off-axis, that is, they should point away from the Earth. Here we report the discovery of the X-ray counterpart associated with the GW event GW170817. While the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow from freshly synthesized r-process material in the merger ejecta, known as kilonova, observations at X-ray and, later, radio frequencies exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short GRBs and GWs from NS mergers, and gives independent confirmation of the collimated nature of the GRB emission.

• The paper provides compelling evidence of an off-axis X-ray afterglow that confirms the collimated nature of GRB jets from neutron star mergers.
• It identifies optical/IR kilonova signals, supporting the connection between neutron-rich ejecta and compact object merger events.
• Afterglow modeling reveals a spectral slope of approximately 0.64, which aligns with synchrotron emission from a structured jet.

Overview of the X-ray Counterpart to Gravitational Wave Event GW170817

The paper presents a detailed investigation of the X-ray counterpart associated with the gravitational wave event GW170817, marking a pivotal advancement in understanding the link between short gamma-ray bursts (GRBs) and gravitational waves (GWs) emanating from neutron star (NS) mergers. Historically, the prevailing hypothesis in astrophysics suggested that such mergers could lead to the formation of relativistic jets, subsequently powering short GRBs. Yet, direct observational evidence of this link had been elusive until the detection associated with GW170817.

On August 17, 2017, the Advanced LIGO detected gravitational waves from the merger of two neutron stars approximately 40 Mpc away. A consequent short gamma-ray burst, albeit unusually dim, was detected about two seconds later. The optical and infrared counterpart of this event, designated SSS17a, was located in the early-type galaxy NGC 4993. The study leveraged the Chandra X-ray Observatory to capture X-ray emissions emerging from SSS17a, significantly linking the X-ray signals to the GW170817 event.

Key Findings

1. Collimation and Off-axis Viewing: The study provides compelling observational evidence that supports the collimated nature of GRB outflows, reinforcing the existing model that short GRBs from NS mergers are mostly off-axis. The apparent discrepancy between the observed dim gamma-ray emission and typical short GRBs was explained by the effect of observing the GRB off-axis.
2. Kilonova Association: Observations indicated that the event featured a kilonova, identified through its optical/infrared transient properties, which consisted of both neutron-rich dynamical ejecta (broad-lined IR features) and a substantial wind component along the polar axis (dominant in the optical spectrum). This bolstered the theory that the observed optical/IR emissions were consistent with a kilonova derived from the merger of compact objects.
3. Afterglow Modeling: The X-ray and radio afterglows are interpreted as synchrotron emissions from a structured jet viewed off-axis. The research incorporated modeling that suggested a spectral slope of β~0.64, consistent with typical GRB afterglow spectra. This modeling indicated that the GW170817 event exemplified a binary NS merger whose jet was not directed towards Earth, causing an observed delay in the afterglow.

Implications and Future Directions

Practical Implications: The observations of GW170817 and its electromagnetic counterparts validate the capability of multi-messenger astronomy in deciphering cosmic phenomena. Particularly, the synergy between gravitational wave detections and electromagnetic follow-ups is critical, suggesting that future gravitational wave observatories may routinely detect off-axis GRBs, thereby offering unique insights into the frequency and environment of such merger events.

Theoretical Implications: The paper challenges some existing models of GRB emissions, particularly the adequacy of the "top-hat" jet model. The detection supports structured jet models, indicating variability in the energy distribution and Lorentz factors across the jet, which may require refinements in existing astrophysical models regarding NS mergers.

Speculation on Future Developments: As observational techniques and instruments improve, future research will likely refine the understanding of NS merger environments, the configuration of ejecta, and their repercussions on r-process nucleosynthesis. Such advancements may shed light on unresolved issues about kilonova emissions and facilitate the development of enhanced models that integrate both electromagnetic and gravitational wave phenomena.

In conclusion, the paper underscores the importance of correlating GW detections with electromagnetic observations, illustrating how such integrative methods can provide conclusive insights into the physical processes driving some of the universe's most energetic events.