The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet (1710.05431v1)

Abstract: We report the discovery of rising X-ray emission from the binary neutron star (BNS) merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave source. Observations acquired with the Chandra X-ray Observatory (CXO) at t~2.3 days post merger reveal no significant emission, with L_x<=3.2x10^38 erg/s (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L_x\sim 9x10^39 erg/s at ~15.1 days post merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broad-band X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E_k~10^49-10^50 erg, viewed off-axis with theta_obs~ 20-40 deg. Our models favor a circumbinary density n~ 0.0001-0.01 cm-3, depending on the value of the microphysical parameter epsilon_B=10^{-4}-10^{-2}. A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at $t\gtrsim 100$ days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on theta_obs are testable with gravitational wave information on GW170817 from Advanced LIGO/Virgo on the binary inclination.

• The paper presents Chandra X-ray observations showing emission brightening over two weeks, providing the first X-ray detection from a gravitational wave source.
• Combining X-ray data with broad-band observations and simulations, the rising emission is interpreted as coming from an off-axis relativistic jet.
• These findings confirm the presence of relativistic ejecta in neutron star mergers and enhance understanding of off-axis jets and multi-messenger astrophysics.

A Detailed Analysis of Rising X-ray Emission from GW170817

The paper by Margutti et al. offers an in-depth examination of the electromagnetic aftermath of the binary neutron star merger event GW170817, notably focusing on the rising X-ray emission captured by the Chandra X-ray Observatory. This study marks the first observation of X-ray emission from a gravitational-wave source, providing crucial insights into the dynamics of such cataclysmic events.

Observational Findings

The research primarily revolves around two significant phases of observation. Initial observations with Chandra at around 2.3 days post-merger showed no significant X-ray emissions, placing an upper limit on the X-ray luminosity. However, continued monitoring revealed an X-ray source brightening over time, achieving an isotropic-equivalent luminosity of approximately $9 \times 10^{39} \, \rm{erg \, s^{-1}}$ by about 15.1 days after the merger. This evolution is interpreted in relation to relativistic outflows, suggesting emission from an off-axis jet with kinetic energy ranging between $10^{49}$ and $10^{50} \, \rm{erg}$, viewed from an angle of about 20-40 degrees relative to the jet axis.

Theoretical Context and Interpretations

The broad-band X-ray to radio observations constrain the physical interpretations of the source dynamics. The paper argues against a central-engine origin for the X-ray emission, favoring instead the scenario of an off-axis jet. The inferred circumbinary density, ranging from $10^{-4}$ to $10^{-2} \, \rm{cm^{-3}}$, further refines the model. The study employs relativistic jet simulations, notably using the BOXFIT code, to model the afterglow emission from different jet configurations.

Consistent with the multi-messenger astrophysics approach, these results suggest viewing angles and kinetic energies that align with properties derived from gravitational wave analyses. The X-ray findings also complement radio observations, providing a cohesive picture of the off-axis dynamics anticipated in neutron star mergers.

Implications and Future Prospects

The confirmation of X-ray emissions from a gravitational wave event opens up new possibilities for constraining the physics of neutron star mergers, such as those producing short gamma-ray bursts (SGRBs). The detection confirms the relativistic nature of the ejected material in such mergers and demonstrates the capability of current observational facilities to distinguish between different emission models.

This research underscores the critical role of sophisticated modeling and multi-wavelength observations in understanding these events. Future observations, particularly at later times when the emission profiles further evolve, will help resolve model degeneracies and test hypotheses about jet dynamics and energy distribution. The anticipated correlation of X-ray emission patterns with gravitational wave data from detectors like LIGO/Virgo will enrich our understanding of neutron star merger kinematics.

In conclusion, Margutti et al. provide a pivotal contribution to our comprehension of electromagnetic counterparts to gravitational-wave sources, guiding future observational strategies and theoretical models in the domain of astrophysics. The findings illustrate the synergy between electromagnetic and gravitational wave observations, charting new territory in the study of multi-messenger phenomena.