Combined XMM-Newton and Chandra observations of the X-ray light curve plateau in GW170817

ATel #11924; Marion Burnichon, Hannah Dykaar, Melania Nynka, John J. Ruan, Daryl Haggard (McGill/MSI), Phil Evans (Leicester)
on 9 Aug 2018; 03:45 UT
Credential Certification: John Ruan (jjr2125@gmail.com)

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We report previously-unpublished Director's Discretionary Time XMM-Newton observations of neutron star merger GW170817 (PI: Schartel). This XMM-Newton observation at 162 days post-merger confirms previous Chandra observations of a plateau in the X-ray light curve of GW170817's non-thermal synchrotron afterglow, before its current fading (e.g., Nynka et al., 2018, Alexander et al. 2018).

XMM-Newton obtained a 111.5 ks observation of GW170817 (obsID: 0811212701) on 26 January 2018 UT, approximately 162 days post-merger. We extract and fit the X-ray spectrum assuming an absorbed power-law spectral model with fixed NH = 7.5e20 cm^-2. We measure an absorbed flux of f(0.3-8 keV) = 1.7 (+0.6/-0.5) x10^-14 erg/s/cm^2 (all uncertainties are 90% c.l.) and photon index of Gamma = 1.61 (+0.26/-0.24), which corresponds to an unabsorbed luminosity of L(0.3-10 keV) = 4.6 (+1.5/-1.5) x10^39 erg/s assuming a luminosity distance of 42.5 Mpc.

Our results from this XMM-Newton observation of GW170817 at 162 days are consistent with our Chandra observations at 159.7 days, which revealed an absorbed flux of f(0.3-8 keV) = 2.06 (+0.34/-0.30) x10^-14 erg/s/cm^2 and Gamma = 1.58 (+0.23/-0.22) (Nynka et al., 2018). A joint spectral fit of both the XMM-Newton observation at 162 days and Chandra observations at 159.7 days gives Gamma = 1.60 (+0.17/-0.17).

Previously, XMM-Newton also obtained a 44.2ks DDT observation of GW170817 at 135 days post-merger (PI: Schartel, obsID: 0811210101). From this data, D'Avanzo et al. (2018) measured an unabsorbed flux of f(0.3-10 keV) = 2.1 (+1.1/-0.8) x10^-14 erg/s/cm^2 and Gamma = 1.7 (+0.5/-0.4), which corresponds to an absorbed flux of f(0.3-8 keV) = 1.7 (+1.1/-0.8) x10^-14 erg/s/cm^2. This XMM-Newton flux measurement at 135 days was surprisingly fainter than our previous Chandra measurement at 109.2 days (Ruan et al., 2018), as well our later Chandra measurement at 159.7 days (Nynka et al., 2018), although the uncertainties are large. We provide a independent reanalysis of this XMM-Newton data at 135 days, and find a unabsorbed flux of f(0.3-10 keV) = 2.4 (+1.1/-0.9) x10^-14 erg/s/cm^2 (equivalent to an absorbed flux of 1.9 (+1.7/-0.9) x10^-14 erg/s/cm^2 in the 3-8 keV band) and Gamma = 1.7 (+0.8/-0.6), slightly brighter than previously reported and more consistent with our Chandra observations.

Below, we provide a light curve table of all currently-available Chandra and XMM-Newton observations of GW170817. We note that all flux and luminosity measurements are from our own independent analysis of the data (reported here and in Nynka et al., 2018), except for the Chandra observations at 2.3 days, which are rescaled from Margutti et al. (2017).

Days, Telescope, Flux*, Luminosity**
2.3, Chandra, <0.13, <3.2
9.2, Chandra, 0.34 (+0.15/-0.11), 9.2 (+4.6/-4.9)
15.6, Chandra, 0.36 (+0.17/-0.12), 10.8 (+5.2/-2.6)
109.2, Chandra, 1.88 (+0.38/-0.28), 51.0 (+8.2/-9.3)
135, XMM-Newton, 1.9 (+1.7/-0.9), 52.0 (+24/-19)
159.7, Chandra, 2.06 (+0.34/-0.30), 55.3 (+12.9/-8.9)
162, XMM-Newton, 1.7 (+0.6/-0.5), 46 (+15/-15)
260.0, Chandra, 1.09 (+0.24/-0.20), 29.6 (+7.1/-6.5)

*Flux units: [10^-14 erg/s/cm^2], 0.3 - 8 keV absorbed flux
**Luminosity units: [10^38 erg/s], 0.3 - 10 keV unabsorbed luminosity, assuming DL = 42.5 Mpc
(all uncertainties are 90% c.l.)