Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

Hajela, A; Margutti, R; Bright, JS; Alexander, KD; Metzger, BD; Nedora, V; Kathirgamaraju, A; Margalit, B; Radice, D; Guidorzi, C; Berger, E; MacFadyen, A; Giannios, D; Chornock, R; Heywood, I; Sironi, L; Gottlieb, O; Coppejans, D; Laskar, T; Cendes, Y; Duran, RB; Eftekhari, T; Fong, W; McDowell, A; Nicholl, M

Journal article

Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

Abstract:: The binary neutron-star (BNS) merger GW170817 is the first celestial object from which both gravitational waves (GWs) and light have been detected enabling critical insight on the pre-merger (GWs) and post-merger (light) physical properties of these phenomena. For the first $\sim 3$ years after the merger the detected radio and X-ray radiation has been dominated by emission from a structured relativistic jet initially pointing $\sim 15-25$ degrees away from our line of sight and propagating into a low-density medium. Here we report on observational evidence for the emergence of a new X-ray emission component at $\delta t>900$ days after the merger. The new component has luminosity $L_x \approx 5\times 10^{38}\rm{erg s^{-1}}$ at 1234 days, and represents a $\sim 3.5\sigma$ - $4.3\sigma$ excess compared to the expectations from the off-axis jet model that best fits the multi-wavelength afterglow of GW170817 at earlier times. A lack of detectable radio emission at 3 GHz around the same time suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with synchrotron emission from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context our simulations show that the X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. However, radiation from accretion processes on the compact-object remnant represents a viable alternative to the kilonova afterglow. Neither a kilonova afterglow nor accretion-powered emission have been observed before.Comment: 66 pages, 12 figures, Submitte

Publication status:: Published

Peer review status:: Peer reviewed

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APA Style

Hajela, A., Margutti, R., Bright, J. S., Alexander, K. D., Metzger, B. D., Nedora, V., Kathirgamaraju, A., Margalit, B., Radice, D., Guidorzi, C., Berger, E., MacFadyen, A., Giannios, D., Chornock, R., Heywood, I., Sironi, L., Gottlieb, O., Coppejans, D., Laskar, T., … Nicholl, M. (2022). Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component. The Astrophysical Journal Letters, 927(1), L17–L17.

MLA Style

Hajela, A, et al. “Evidence for X-Ray Emission in Excess to the Jet-Afterglow Decay 3.5 Yr after the Binary Neutron Star Merger GW 170817: A New Emission Component.” The Astrophysical Journal Letters, vol. 927, no. 1, 2022, pp. L17–L17.

Chicago Style

Hajela, A, R Margutti, JS Bright, et al. 2022. “Evidence for X-Ray Emission in Excess to the Jet-Afterglow Decay 3.5 Yr after the Binary Neutron Star Merger GW 170817: A New Emission Component.” The Astrophysical Journal Letters 927 (1): L17–L17.
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