The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. V. Rising X-Ray Emission from an Off-axis Jet

The Swift spacecraft (Gehrels et al. 2004) started observations of the optical counterpart of LIGO/Virgo GW170817 (Allam et al. 2017; Coulter et al. 2017a, 2017b; Soares-Santos et al. 2017; Yang et al. 2017) with the X-ray telescope (XRT; Burrows et al. 2005) on August 18, 03:33:33UT, 14.9 hr after the GW trigger. Swift-XRT observations span the time range 0.6–11.5 days since trigger, at which point the target entered into Sun constraint. Swift-XRT data have been analyzed using HEASOFT (v6.22) and corresponding calibration files, employing standard filtering criteria and following standard procedures (see Margutti et al. 2013 for details). No transient X-ray emission is detected at the location of the GW optical counterpart (Cenko et al. 2017; Evans et al. 2017a, 2017b), with typical count-rate limits of ∼ a few ${10}^{-3}\,{\rm{c}}\,{{\rm{s}}}^{-1}$. The neutral hydrogen column density in the direction of the transient is ${\mathrm{NH}}_{{mw}}=0.0784\times {10}^{22}\,{\mathrm{cm}}^{-2}$ (Kalberla et al. 2005). For a typical absorbed power-law spectrum with photon index ${\rm{\Gamma }}\sim 2$ and negligible intrinsic absorption (see below), the corresponding $3\sigma$ flux limit is $\sim {10}^{-13}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$ (unabsorbed, 0.3–10 keV), which is ${L}_{x}\lt$ a few ${10}^{40}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ at the distance of 39.5 Mpc. As we show in detail in Fong et al. (2017a), Swift-XRT observations constrain the X-ray emission associated with the optical counterpart of LIGO/Virgo GW170817 to be significantly fainter than cosmological short GRBs at the same epoch (Margutti et al. 2013; D'Avanzo et al. 2014; Fong et al. 2015).

We initiated deep X-ray follow-up of the optical transient with the Chandra X-ray Observatory (CXO) on 2017 August 19.71 UT, $\delta t\approx 2.3\,{\rm{d}}$ after the GW detection (observation ID 18955; PI: Fong; Program 18400052). Chandra ACIS-S data have been reduced with the CIAO software package (v4.9) and relative calibration files, applying standard ACIS data filtering. Using wavdetect we find no evidence for X-ray emission at the position of the optical transient (Margutti et al. 2017) and we infer a $3\sigma$ limit of $1.2\times {10}^{-4}\mathrm{cps}$ (0.5–8 keV energy range, total exposure time of 24.6 ks). For an assumed absorbed spectral power-law model with ${\rm{\Gamma }}=2$, negligible intrinsic absorption, and ${\mathrm{NH}}_{{mw}}=0.0784\times {10}^{22}\,{\mathrm{cm}}^{-2}$, the corresponding absorbed (unabsorbed) flux limit in the 0.3–10 keV energy range is ${F}_{{\rm{x}}}\lt 1.4\times {10}^{-15}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$ $({F}_{{\rm{x}}}\lt 1.7\times {10}^{-15}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$).¹⁵ The luminosity limit is ${L}_{{\rm{x}}}\lt 3.2\times {10}^{38}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ (0.3–10 keV), making the X-ray counterpart to GW170817 ≥ 1000 times fainter than on-axis short GRBs at the same epoch (Fong et al. 2017a).

We re-visited the location of the optical transient on September 1.64 UT (starting 15.1 days since the trigger) under a DDT program with shared data (observation ID 20728; data shared among Troja, Haggard, and Margutti; Program 18508587; official PI: Troja) with an effective exposure time of 46.7 ks. An X-ray source is blindly detected (Fong et al. 2017b) with high significance of $\sim 7.3\sigma$ at R.A. = 13^h09^m48076 and decl. = −23°22'5334 (J2000), see Figure 1, consistent with the optical transient and the findings by Troja et al. (2017a, 2017b).

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The source 0.5–8 keV count rate is $(3.8\pm 0.9)\times {10}^{-4}\,\mathrm{cps}$. The total number of 0.5–8 keV counts in the source region is 19. Based on Poissonian statistics, the probability to observe 0 events in 24.6 ks (as in our first observation), if the expected rate is 19 events in 46.7 ks, is ∼0.0045% (∼4 Gaussian σ equivalent). A similar result is obtained with a Binomial test ($P\sim 0.03$%, corresponding to ∼3.6 Gaussian σ). We can thus reject the hypothesis of a random fluctuation of a persistent X-ray source with high confidence, and we conclude that we detected rising X-ray emission in association with the optical counterpart to GW170817.

The limited statistics does not allow us to constrain the spectral model. We employ Cash statistics to fit the spectrum with an absorbed power-law spectral model with index Γ and perform a series of Markov Chain Monte Carlo simulations to constrain the spectral parameters. We find ${\rm{\Gamma }}={1.6}_{-0.1}^{+1.5}$ ($1\sigma$ c.l.) with no evidence for intrinsic neutral hydrogen absorption ${\mathrm{NH}}_{\mathrm{int}}\lt 3\times {10}^{22}\,{\mathrm{cm}}^{-2}$ $(3\sigma$ c.l.). For these parameters, the inferred 0.3–10 keV flux is $(3.0\mbox{--}5.6)\times {10}^{-15}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$ $(1\sigma$ c.l.). The corresponding unabsorbed flux is $(3.1\mbox{--}5.8)\,\times {10}^{-15}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$, and luminosity L_x is in the range $(5.9\mbox{--}11.1)\times {10}^{38}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ ($1\sigma$ c.l.).

Figure 2 shows our CXO light curve of the X-ray source associated with GW170817. In this figure, we add the X-ray measurement by Haggard et al. (2017a, 2017b) obtained 15.9 days after GW trigger (PI: Haggard, ID 18988) and rescaled to ${\rm{\Gamma }}=2$ in the 0.3–10 keV energy range, leading to ${F}_{x}\sim 4.5\times {10}^{-15}\,\mathrm{erg}\,{{\rm{s}}}^{-1}\,{\mathrm{cm}}^{-2}$. This flux is consistent with our observations obtained ∼24 hr before, with no statistically significant evidence for temporal variability of the source on this timescale. An estimate of the lower limit of the X-ray flux at $t\sim 10$ days, corresponding to the reported detection of X-ray emission with the CXO using an exposure time of 50 ks (Troja et al. 2017a, 2017b) is also shown to guide the eye.

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We first consider constraints on on-axis¹⁷ relativistic outflows (collimated or not collimated), under the assumption that the the blast wave has transferred to the interstellar medium (ISM) most of its energy by the time of our first CXO observation, and its hydrodynamics is thus well described by the Blandford-McKee (BM) self-similar deceleration solution (Blandford & McKee 1976). Electrons are accelerated at the shock front into a power-law distribution $N(\gamma )\propto {\gamma }^{-p}$ for $\gamma \geqslant {\gamma }_{\min }$ and cool through synchrotron emission and adiabatic losses.

In the standard synchrotron model (e.g., Granot & Sari 2002), the flux density ${F}_{\nu }\propto {n}^{1/2}{E}_{k,\mathrm{iso}}^{(3+p)/4}{\epsilon }_{e}^{p-1}{\epsilon }_{B}^{(1+p)/4}\,{t}^{(3-3p)/4}$ if the X-rays are on the ${\nu }^{(1-p)/2}$ spectral segment (i.e., ${\nu }_{x}\lt {\nu }_{c}$) and ${F}_{\nu }\propto {E}_{k,\mathrm{iso}}^{(2+p)/4}{\epsilon }_{e}^{p-1}{\epsilon }_{B}^{(p-2)/4}\,{t}^{(2-3p)/4}$ if the X-rays are on the ${\nu }^{-p/2}$ spectral segment (${\nu }_{x}\gt {\nu }_{c}$). ${\nu }_{c}$ is the synchrotron cooling frequency (e.g., Rybicki & Lightman 1979), ${\epsilon }_{e}$ and ${\epsilon }_{B}$ are the post-shock energy fractions in electrons and magnetic field, respectively, and n is the ISM density. We use a constant density medium as expected for a non-massive star progenitor.

Within this model, and for fiducial parameters ${\epsilon }_{e}=0.1$, ${\epsilon }_{B}=0.01$, and $p=2.4$ set by the median value of cosmological short GRBs (Fong et al. 2015), the deep CXO non-detection on day 2.34 constrains ${E}_{k,\mathrm{iso}}\leqslant {10}^{47}{n}_{0}^{-10/27}\,\mathrm{erg}$ for ${\nu }_{x}\lt {\nu }_{c}$ and ${E}_{k,\mathrm{iso}}\leqslant 4\times {10}^{46}\,\mathrm{erg}$ for ${\nu }_{x}\gt {\nu }_{c}$. n₀ is the circumburst density in units of ${\mathrm{cm}}^{-3}$. Consistent with the results from radio observations (Alexander et al. 2017a), this analysis points at low ${E}_{k,\mathrm{iso}}\leqslant {10}^{48}\,\mathrm{erg}$ for the range of densities $n\sim (3\mbox{--}15)\times {10}^{-3}\,{\mathrm{cm}}^{-3}$ associated with cosmological short GRBs, which are characterized by ${E}_{k,\mathrm{iso}}\sim (1\mbox{--}3)\times {10}^{51}\,\mathrm{erg}$ for the same microphysical parameters ${\epsilon }_{e}=0.1$ and ${\epsilon }_{B}=0.01$ (Fong et al. 2015). We note that this conclusion does not depend on the choice of p, with $p=2.1\mbox{--}2.4$ ($p\gt 2.4$ violates our radio limits). This solution is only valid during the relativistic phase at $t\lt {t}_{{NR}}$ (where ${t}_{{NR}}\sim 1100\,{({E}_{k,\mathrm{iso}}/{10}^{53}{n}_{0})}^{1/3}\,\mathrm{days}$; Piran 2004) and constrains the presence of an undetected, temporally decaying X-ray emission at $t\lt 2.34$ days, with properties that are clearly distinguished from cosmological short GRBs seen on-axis (Fong et al. 2017a).

A rising X-ray light curve can be the result of a delayed onset of the afterglow emission, as the blast wave decelerates into the environment and transfers energy to the circumburst medium. In this scenario, the initial Lorentz factor of the outflow is ${{\rm{\Gamma }}}_{0}\sim 8.0\,{E}_{k,\mathrm{iso},52}^{1/8}{n}_{0}^{-1/8}{t}_{\mathrm{pk},\mathrm{day}}^{-3/8}$, where ${t}_{\mathrm{pk},\mathrm{day}}$ is the peak time of the afterglow in days (Sari & Piran 1999). A distinguishing feature of the early afterglow emission is an initial very steep rise of the emission $\propto {t}^{2}$ or $\propto {t}^{11/3}$ (Sari & Piran 1999). The stable X-ray flux of the source at $t\sim 15\mbox{--}16$ days suggests that ${t}_{\mathrm{pk}}\sim 15\mbox{--}30$ days. Given the Fermi-GBM detection of a gamma-ray transient with fluence $F\sim 2.4\times {10}^{-7}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}$ (Goldstein et al. 2017), which gives ${E}_{k,\mathrm{iso}}\sim 5\times {10}^{47}\,\mathrm{erg}$ for a fiducial γ-ray efficiency ${\eta }_{\gamma }=0.1$, we infer a mildly relativistic ${{\rm{\Gamma }}}_{0}\sim 2$ for ${t}_{\mathrm{pk}}\sim 15\mbox{--}30$ days. After peak, when most of the fireball energy has been transferred to the ISM, the standard afterglow scalings apply. The latest CXO detection implies ${E}_{k,\mathrm{iso}}\sim {10}^{48}{n}_{0}^{-10/27}\,\mathrm{erg}$ if ${\nu }_{x}\lt {\nu }_{c}$, or ${E}_{k,\mathrm{iso}}\sim {10}^{48}\,\mathrm{erg}$ for ${\nu }_{x}\gt {\nu }_{c}$. Radio observations acquired around the same time (Alexander et al. 2017a) constrain $p\approx 2.2$. Mildly relativistic outflows with similar Γ and E_k that are found in shocks from supernovae (SN) with fast ejecta (i.e., relativistic SNe) are well described by $p\sim 3$ (e.g., Chevalier & Fransson 2006; Soderberg et al. 2010; Chakraborti et al. 2015). From a purely theoretical perspective, both analytical models and particle-in-cell (PIC) simulations confirm that $p=2.2$ is expected in the cases of ultra-relativistic shocks where particle acceleration is very efficient. We thus conclude that a late onset of a weak on-axis afterglow emission is unlikely to provide a satisfactory explanation of our observations across the electromagnetic spectrum, and we consider alternative explanations below.

A delayed onset of the X-ray emission can originate from the presence of an off-axis jet, originally pointed away from our line of sight. For a simple model of a point source at an angle ${\theta }_{\mathrm{obs}}$, moving at a Lorentz factor Γ, the peak in the light curve occurs when the beaming cone widens enough to engulf the line of sight, ${\rm{\Gamma }}({t}_{\mathrm{pk}})\sim 1/{\theta }_{\mathrm{obs}}$ (e.g., Granot et al. 2002). This is a purely dynamical effect that does not depend on the micropysical parameters ${\epsilon }_{e}$ and ${\epsilon }_{B}$ (which instead concur to determine the overall luminosity of the emission). From Granot & Sari (2002), the evolution of the Lorentz factor of a blast wave propagating into an ISM can be parameterized as ${\rm{\Gamma }}{(t)\sim 6.68({E}_{k,\mathrm{iso},52}/{n}_{0})}^{1/8}{t}_{\mathrm{days}}^{-3/8}$, which gives ${\theta }_{\mathrm{obs}}\,\sim 0.15{({E}_{k,\mathrm{iso},52}/{n}_{0})}^{-1/8}{t}_{\mathrm{pk},\mathrm{days}}^{3/8}$ or ${\theta }_{\mathrm{obs}}\sim 0.2\,{({E}_{k,50}/{n}_{0})}^{-1/6}{t}_{\mathrm{pk},\mathrm{days}}^{1/2}$. Before peak the off-axis model predicts a steep rise, with the flux scaling $\propto {t}^{2}$. As we argued above, the mild temporal evolution of the detected X-ray emission suggests a peak not too far from our last epoch of observation at ∼15 days. We find ${\theta }_{\mathrm{obs}}\sim {(15^\circ \mbox{--}30^\circ )({E}_{k,50}/{n}_{-3})}^{-1/6}\,\deg$ for ${t}_{\mathrm{pk}}=15\mbox{--}70$. If GW170817 harbored a relativistic off-axis jet with similar parameters to cosmological short GRBs (${E}_{k}\sim {10}^{49-50}\,\mathrm{erg}$ and $n\sim {10}^{-3}\,{\mathrm{cm}}^{-3}$; Fong et al. 2015), this simple analytical scaling suggests off-axis angles ${\theta }_{\mathrm{obs}}\sim 20^\circ \mbox{--}40^\circ$.

The actual values of the flux detected (and undetected) in the X-rays and radio pose additional constraints that break the model degeneracy in E_k and n as a function of ${\epsilon }_{e}$ and ${\epsilon }_{B}$. We employ realistic simulations of relativistic jets propagating into an ISM to fully capture the effects of lateral jet spreading with time, finite jet opening angle, and transition into the non-relativistic regime. To this aim, we run the publicly available code BOXFIT (v2; van Eerten et al. 2010; van Eerten & MacFadyen 2012), varying E_k, n, p, ${\epsilon }_{B}$, and ${\theta }_{j}$ (jet opening angle) and calculate the off-axis afterglow emission as observed from different lines of sight ${\theta }_{\mathrm{obs}}$, with ${\theta }_{\mathrm{obs}}$ varying from $5^\circ$ to $90^\circ$ (i.e., equatorial view). We explore a wide portion of parameter space corresponding to ${E}_{k}={10}^{48}\mbox{--}{10}^{51}\,\mathrm{erg}$, $n={10}^{-4}\mbox{--}1\,{\mathrm{cm}}^{-3}$, and ${\epsilon }_{B}={10}^{-4}\mbox{--}{10}^{-2}$. In our calculations, we assume the fiducial value ${\epsilon }_{e}=0.1$ (e.g., Sironi et al. 2015). For each parameter set, we consider two values for the power-law index of the electron distribution $p=2.4$ (median value from short GRBs afterglows from Fong et al. 2015) and $p=2.2$ (as expected from particle acceleration in the ultra-relativistic limit; Sironi et al. 2015), and we run each simulation for a collimated ${\theta }_{j}=5^\circ$ jet and a jet with ${\theta }_{j}=15^\circ$, representative of a less collimated outflow (Table 1). As a comparison, the measured ${\theta }_{j}$ in short GRBs range between $3^\circ$ and 10 °with notable lower limits ${\theta }_{j}\gt 15^\circ$ and ${\theta }_{j}\gt 25^\circ$ for GRBs 050709 and 050724A (Fong et al. 2015 and references therein).

Table 1.  BOXFIT Parameters

Parameter	Values Considered
Jet energy ${E}_{{\rm{k}}}$ (erg)	1048, 1049, 1050, 1051
Circum-merger density n (cm−3)	${10}^{-4},{10}^{-3},{10}^{-2},0.1,1$
Jet opening angle ${\theta }_{{\rm{j}}}$ (deg)	$5,15$
Observer angle ${\theta }_{\mathrm{obs}}$ (deg)	$0,5,10,20,30,45,60,75,90$
Fraction of post-shock energy in B ${\epsilon }_{B}$	${10}^{-4}$, ${10}^{-3}$, ${10}^{-2}$
Power-law index of electron distribution p	2.2, 2.4

Note. Simulations were run at fixed values ${\epsilon }_{e}=0.1$ in a constant density medium.

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The results from our simulations can be summarized as follows. (i) While we find a set of solutions with $p=2.4$ that can adequately fit the X-ray light curve, all of these simulations violate our radio limits as we detail in Alexander et al. (2017a). Models with $p\geqslant 2.4$ are ruled out, and we will not discuss these simulations further. (ii) Models that intercept the measured X-ray flux, but with ${t}_{\mathrm{pk}}\gg 15$ days, overpredict the radio emission, for which we have observations extending to $t\sim 40$ days (Alexander et al. 2017a). Jets with ${E}_{k}\gt {10}^{50}\,\mathrm{erg}$ belong to this category and are not favored. (iii) Most high-density environments with $n\sim 0.1\mbox{--}1\,{\mathrm{cm}}^{-3}$ cause an earlier deceleration of the jet. As a consequence, these models require ${\theta }_{\mathrm{obs}}$ between $40^\circ$ and $60^\circ$ to match the X-ray flux evolution (i.e., a range of ${\theta }_{\mathrm{obs}}$ not favored by the early blue colors of the kilonova; Cowperthwaite et al. 2017; Nicholl et al. 2017) and are not consistent with the radio limits. (iv) Low-energy jets with ${E}_{k}\sim {10}^{48}\,\mathrm{erg}$ also have shorter deceleration times and require ${\theta }_{\mathrm{obs}}\gt 45^\circ$ to explain the X-ray observations (and are consequently not favored by the kilonova colors). (v) Finally, wider jets have a larger allowed parameter space and are favored based on their broader light curves around peak time.

We identify a family of solutions that adequately reproduce the current data set across the spectrum (Figures 2–3). The successful models are characterized by an off-axis jet with ${10}^{49}\mathrm{erg}\lesssim {E}_{k}\lesssim {10}^{50}\,\mathrm{erg}$, ${\theta }_{j}=15^\circ$ viewed $\sim 20^\circ \mbox{--}40^\circ$ off-axis and propagating into an ISM with $n\sim {10}^{-4}\mbox{--}{10}^{-2}\,{\mathrm{cm}}^{-3}$, depending on the value of ${\epsilon }_{B}={10}^{-4}\mbox{--}{10}^{-2}$. The dependency of the best-fitting ${\theta }_{\mathrm{obs}}$ values on n and ${\epsilon }_{B}$ is illustrated in Figure 4. The successful models are portrayed in Figures 2–3. Collimated outflows with ${\theta }_{j}=5^\circ$ satisfy the observational constraints only for ${E}_{k}={10}^{49}\,\mathrm{erg}$, ${\epsilon }_{B}={10}^{-4}$, $n\sim {10}^{-3}\,{\mathrm{cm}}^{-3}$, and ${\theta }_{\mathrm{obs}}\sim 16^\circ$. From Figures 2–3 it is clear that the optical emission from the off-axis afterglow (green line in the right column plots) is always negligible compared to the contemporaneous kilonova emission. It is also worth noting that these models predict a radio flux density that is close to our flux limits (purple line and points), thus providing support to our tentative VLA detection at $t\sim 20$ days at the level of $\sim 20\,\mu \mathrm{Jy}$ (Alexander et al. 2017a). Our favored models are not in disagreement with the radio detection of a faint transient at the level of ${\rm{S}}/{\rm{N}}=5$ previously reported by Mooley et al. (2017) and Corsi et al. (2017) ∼15 days post-merger (Hallinan et al. 2017), and are fully consistent with our radio detection at 6 GHz at $t=39.4$ days, as detailed in Alexander et al. (2017a).

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Short GRBs are sometimes accompanied by late-time X-ray emission (e.g., Perley et al. 2009; Margutti et al. 2011; Fong et al. 2014), which may originate from a long-lived central engine, such as an accreting black hole (e.g., Perna et al. 2006) or a millisecond magnetar (e.g., Metzger et al. 2008).

GW170817 was accompanied by luminous optical and infrared emission, consistent with predictions for the kilonova emission originating from r-process radioactive heating of the merger ejecta (Chornock et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017). The observed X-ray transient is unlikely to originate from the central engine because the signal would be blocked by the photoelectric absorption in this same ejecta along the viewer's line of sight.

Given the estimated ejecta mass of $\gtrsim {10}^{-2}{M}_{\odot }$ and mean velocity ${v}_{\mathrm{ej}}\sim 0.1\mbox{--}0.2c$ (Chornock et al. 2017; Cowperthwaite et al. 2017; Nicholl et al. 2017), the optical depth through the ejecta of radius $R\sim {v}_{\mathrm{ej}}t$ and density $\rho \sim {M}_{\mathrm{ej}}/(4\pi {R}^{3}/3)$ is approximately given by

$$\begin{eqnarray}{\tau }_{{\rm{X}}} & \simeq & \rho R{\kappa }_{{\rm{X}}}\\ & \approx & 90\left(\displaystyle \frac{{\kappa }_{{\rm{X}}}}{1000\,{\mathrm{cm}}^{2}\,{{\rm{g}}}^{-1}}\right)\left(\displaystyle \frac{{M}_{\mathrm{ej}}}{{10}^{-2}{M}_{\odot }}\right){\left(\displaystyle \frac{{v}_{\mathrm{ej}}}{0.2c}\right)}^{-2}\\ & & \times {\left(\displaystyle \frac{t}{2\mathrm{week}}\right)}^{-2},\end{eqnarray} \tag{ 1 }$$

where ${\kappa }_{{\rm{X}}}\sim 1000$ cm² g⁻¹ is the expected bound-free opacity of neutral or singly ionized heavy r-process nuclei at X-ray energies ∼ a few keV (e.g., Metzger 2017). The fact that ${\tau }_{X}\gg 1$ suggests that any X-ray signal from the engine would be highly suppressed, by a factor ${e}^{-{\tau }_{{\rm{X}}}}\ll 1$. X-rays could escape at an earlier stage only if they were sufficiently powerful ${L}_{X}\gtrsim {10}^{43}\mbox{--}{10}^{44}$ erg s⁻¹ to photoionize the ejecta, as is clearly not satisfied by the observed source ${L}_{X}\lesssim {10}^{40}$ erg s⁻¹ (Metzger & Piro 2014).

Such a high optical depth is not necessarily expected for on-axis viewers more typical of gamma-ray bursts, especially at early times when the engine is most powerful, because the relativistic jet may clear a low-density funnel through the ejecta along the binary axis. As our orientation with GW170817 is unlikely to be so fortuitous, a central-engine origin of the X-ray emission is disfavored.