The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum

Abstract

We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at t < 160 d after merger. These observations show that GW170817 has been steadily brightening with time and might have now reached its peak, and constrain the emission process as non-thermal synchrotron emission where the cooling frequency ν c is above the X-ray band and the synchrotron frequency ν m is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons N(γ) ∝ γ −p accelerated by a shock launched by a NS-NS merger to date. We find p = 2.17 ± 0.01, which indicates that radiation from ejecta with Γ ∼ 3 − 10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the nonthermal emission from GW170817 is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at t ≤ 200 days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.

Continued Brightening of the Afterglow of GW170817/GRB 170817A as Being Due to a Delayed Energy Injection

“…In our calculations, the initial value of the bulk Lorentz factor is taken as Γ 0 =100, a typical value for GRB outflow (e.g., Racusin et al 2011). We fix the power-law index of the electron spectrum as p=2.2, which has been derived from the spectrum of the brightening afterglow of GRB 170817A by some researchers (see Dobie et al 2018;D'Avanzo et al 2018;Margutti et al 2018;Ruan et al 2018 for details). According to the studies on the environment of short GRBs, the circumburst density is generally low, typically in the range of ∼10 −3 -10 −2 cm −3 (Fong et al 2015;Hallinan et al 2017).…”

“…The X-ray data points are taken from Ruan et al (2018), Lazzati et al (2017c), andD'Avanzo et al (2018). The optical (in HST optical bands of F814W and F606W) data points are taken from Lyman et al (2018) and Margutti et al (2018). Note that the 1 keV upper limit shown as the black triangle is taken from Margutti et al (2017).…”

“…The fall-back accretion process was first discussed by Colgate (1971). It is originally assumed to be Kim et al (2017), Mooley et al (2018), and Margutti et al (2018). The X-ray data points are taken from Ruan et al (2018), Lazzati et al (2017c), andD'Avanzo et al (2018).…”

“…The observed long-term brightening of the multiband afterglow disfavors the simple off-axis top-hat jet model (Lazzati et al 2017c;Lyman et al 2018;Margutti et al 2018). It has been proposed that the structured jet model or the cocoon model could match the late-time behavior of GRB 170817A (Gottlieb et al 2017;Granot et al 2017;Lamb & Kobayashi 2017;Lazzati et al 2017c;Wang & Huang 2017;Lyman et al 2018;Mooley et al 2018;.…”

The Merger of Two Compact Stars: A Tool for Dense Matter Nuclear Physics

“…The second one assumes instead a standard short GRB emission, that was observed off-axis [60]. While at the moment, about a hundred days after the event, both possibilities can explain the data, the analysis of the future time-evolution of the synchrotron emission will ultimately be able to distinguish between these two scenarios, telling therefore if GRB170817A was a standard short GRB seen off-axis or if it belongs to a new class of phenomena [61].…”