GW170817

R. Ricci (INRiM/INAF-IRA), B. O' Connor (GWU/UMD), E. Troja (UTV) on
behalf of  a larger collaboration

We report on the Karl G. Jansky Very Large Array latest observations
of GW170817 as part of our ongoing X-ray and radio monitoring. The
observations were performed in S-band between May 2nd and May 11th
2022 in eight execution blocks for a total observing time of 16 hours.
The data were calibrated, concatenated and imaged in CASA. A
preliminary analysis of the cleaned map reveals no source at the
target position.
We derive a 3-sigma flux density upper limit of 6 microJy at 3 GHz,
consistent with the behavior observed at X-ray energies (O'Connor et
al., GCN 32065
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 ).

Further joint Chandra/VLA observations are planned in 2023.

We thank the VLA staff for executing these observations.

B. O'Connor (GWU/UMD), E. Troja (Tor Vergata),
on behalf of a larger collaboration:

The Chandra X-ray Observatory (CXO) carried out observations of GW170817
starting at 1671 d post-merger (ObsIDs: 25733, 25734; PI: Troja) for 59.4
ks,
an additional epoch was performed starting at 1734 d (ObsID: 25527) for
36.6 ks.
The data were re-processed and the astrometry corrected following Troja et
al.
2020, MNRAS, 4, 5643. In the 0.5-8 keV energy range we measure 2 counts
within
a 1" aperture centered on the X-ray position. A background contribution
of about 0.85 counts was estimated from adjacent source-free regions.

Following Kraft, Burrows, and Nousek (1991), we derive a 95% upper limit of
<5.5 source counts, yielding a rate of <6E-5 cts/s in the combined 95.8 ks.
This corresponds to a 0.3-10 keV flux limit of 1.3E-15 erg/cm2/s for a
power-law with photon index of 1.585. This limit is comparable to the value
measured at 1234 d (1.6E-15 erg/cm2/s; Troja et al. 2022, MNRAS, 510, 1902),
and does not show evidence for a rebrightening of the X-ray emission.

By combining this new epoch with the CXO observations acquired in
December 2021 at 1575 d (Hajela et al. 2021, GCN 31231
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 ), we obtain
a weak (3-sigma) detection with an average count-rate of 3E-05 cts/s,
and a 0.3-10 keV flux of ~0.6E-15 erg/cm2/s.

An updated light curve can be found at this link:
https://nora.users.roma2.infn.it/GW170817_May2022.jpg

We thank the Chandra staff for scheduling these observations.

A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander
(Northwestern U.), J. S. Bright (UC Berkeley), T. Laskar (Radboud U.), E.
Berger (Harvard U.) report:

Chandra X-ray Observatory (CXO) observed GW170817 at t ~ 1575 days (~ 4.3
years) after the binary neutron star merger in four distinct exposures
(ObsID 23869, 26223, 24336, 24337; PI Margutti; program 22510329). We
reported the preliminary results from the first epoch (ObsID 23869) in GCN
31187
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  (Hajela et al.). Here, we report results from all four observations
acquired for a total exposure time of ~ 98ks.

We merged all four observations after reprocessing and correcting for the
absolute astrometry (as described in Hajela et al. 2021), and found 5 total
photons in 0.5-8 keV energy range in an 1��� region at the position of
GW170817. This corresponds to a net 0.5-8 keV count-rate of ~4.3e-5 c/s,
which translates to a detection of X-ray emission at the location of
GW170817 with a statistical significance of  ~3.2 sigma (Gaussian
equivalent). We joint-fitted all the four observations with an absorbed
power law model with photon index fixed to ~ 1.6, no intrinsic absorption
(NH,int = 0 cm-2 as found in e.g. Hajela et al. 2019), and Galactic neutral
hydrogen column density NH,gal = 7.8e+20 cm-2  (Kalberla et al., 2005). The
derived unabsorbed flux is ~1.3e-15 erg/cm2/s (0.3-10 keV).

From a preliminary analysis we find that this measurement is in slight
excess to, but statistically consistent with, the expectations from the
off-axis jets models of Hajela et al., 2021. However, we also note that,
when compared to the Chandra observations of GW170817 obtained in March
2020 (t=939.31 days since merger) and Dec 2020+Jan2021 (t=1234.11 days
since merger), and using a Binomial test for variability in the count phase
space, we find no statistically significant evidence of fading of the X-ray
source associated with GW170817 in the last ~650 days.

We thank the entire Chandra team for scheduling and executing these
observations.

A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander
(Northwestern U.), T. Laskar (Radboud U.), R. Chornock (UC Berkeley), E.
Berger (Harvard), V. Ashley Villar (PSU)  report:

We have acquired X-ray observations of GW170817 with the Chandra X-ray
Observatory (CXO) at t ~ 1573 days (~ 4.3 years) after the binary neutron
star merger. We report here the preliminary results from the first epoch
(ObsID 23869; PI Margutti; program 22510329), for a total exposure time of
28.5 ks (out of a total of 100 ks requested; we will report on the results
from the complete data set in the next few days).

After aligning the images to a common astrometric solution as described in
Hajela et al., 2021, we find evidence for X-ray emission at the location of
GW170817 with a statistical significance of  3.8 sigma (Gaussian
equivalent) corresponding to a net count-rate of  (1.31 +/- 0.72)e-04 c/s
(0.5 - 8 keV). Assuming a simple absorbed power-law spectral model with a
photon index Gamma ~ 1.6, no intrinsic absorption (NH,int = 0 cm-2 as found
in e.g. Hajela et al. 2019) and Galactic neutral hydrogen column density
NH,gal = 7.8e+20 cm-2  (Kalberla et al., 2005), we derive an unabsorbed
flux of ~ 1.7e-15 erg/cm2/s (0.3-10 keV), corresponding to a luminosity of
~3.4e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018).

Based on the extrapolation of the jet afterglow models calculated with
JetFit (gammaB=12) and the universal post jet break models with start time
tstart=196 days as calculated in Hajela et al., 2021, and the observed
background, the expected number of photons within a 1��� region at the
location of GW170817  is ~0.55 (0.5-8 keV, exposure time of 28.5 ks). The
observed number of counts after Xspec filtering corresponds to a ~10%
chance fluctuation probability for universal post jet break models (~12%
for JetFit). For this choice of tstart and gammaB,  the total probability
of observing at least as many counts as those collected for GW170817 at
t>900 days based on jet afterglow models as a result of a chance
fluctuation is ~6e-5 (~ 4.0 sigma Gaussian equivalent, for the universal
post jet break model) and ~3e-5 (~ 4.2 sigma Gaussian equivalent for
JetFit). The remaining ~75 ks of planned CXO observations will further
constrain the statistical evidence of an excess of X-ray emission in
GW170817 with respect to jet afterglow models as reported in Hajela et al.,
2021.

We thank the entire Chandra team for scheduling and executing these
observations.

A. Hajela, R. Margutti, J. Bright, K. D. Alexander, W. Fong (Northwestern
U.), E. Berger (Harvard), R. Chornock, D. L. Coppejans, P. Blanchard
(Northwestern U.), V. A. Villar (Columbia U.), T. Eftekhari (Harvard U), A.
Kathirgamaraju (UC Berkeley), D. Giannios (Purdue U.), T. Laskar (Bath), J.
Zrake (Clemson), A. MacFadyen (NYU), K. Paterson (Northwestern U.) report:

We acquired 3 distinct exposures (ObsID 23870, 24923, and 24924) of
GW170817 with the Chandra X-ray Observatory (CXO) between January 18 and
January 27, 2021, under our program 22510329 (PI Margutti), for a total
exposure time of ~98 ks. We initiated these observations after we inferred
a potential excess of X-ray emission compared to our off-axis jet model in
our previous round of observations (ObsID 22677, 24887, 24888, and 24889;
results reported in Hajela et al., GCN 29055
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 ; also see Troja et al. GCN
27411
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 ). The 3 new observations combined with our previous round span
1209-1258 days post-merger and have a mean epoch of 1234 days.

We merge all 7 observations and find significant X-ray emission at the
location of GW170817 at the level of ~5 sigma (Gaussian equivalent). From a
joint spectral fit with a simple power-law spectral model using a photon
index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g.
Hajela et al. 2019), and Galactic neutral hydrogen column density NH,gal =
7.8e+20 cm-2 (Kalberla et al., 2005), we estimate an unabsorbed flux of
2.4(+0.53 -0.82) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties),
corresponding to a luminosity of 4.8 (+1.1 -1.6) e+38 erg/s at the distance
of 40.7 Mpc (Cantiello et al., 2018). Based on the expectations from the
best-fitting off-axis jet model from Hajela+2019,  the chance probability
to obtain at ~1234 days after the merger an X-ray count-rate at least as
large as the measured count-rate is P=0.0014 (corresponding to 3.2 sigma,
Gaussian equivalent). Radio observations to date have not shown the same
excess (Alexander et al. GCN 29053
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 ), but more detailed analysis is in
progress.

We thank the entire Chandra team for scheduling and executing these
observations.

C. D. Kilpatrick, W. Fong, A. Hajela, K. D. Alexander (Northwestern), E. Berger (Harvard), P. K. Blanchard, R. Chornock, R. Margutti, K. Paterson, J. Rastinejad (Northwestern) report:

We report on new Hubble Space Telescope (HST) observations of the near-infrared counterpart to GW170817 at a mid-time of 4.98 Jan 2021 UT, 1236.45 days from merger (GO-15886, PI Fong).  These observations consist of 3 orbits observed in WFC3/IR F140W for a total of 7.8 ks on-source exposure time.

Compared to previous F606W HST imaging of the optical and near-infrared afterglow to GW170817 (Lyman et al. 2018, Nature Astronomy, 2, 751; Lamb et al. 2019, ApJL, 870, L15; Fong et al. 2019, ApJL, 883, L1), we do not visually detect a counterpart at the site of GW170817 in the latest observations.  We injected artificial sources at the afterglow site and recover a 3-sigma limiting magnitude of >26.3 mag (AB and without correcting for Galactic extinction).

Based on recent Chandra detections of the X-ray counterpart to GW170817 at 1209-1213 days after merger (Hajela et al., GCN 29041
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 , GCN 29019
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 ; Troja et al., GCN 29038
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 ) as well as radio non-detections with the VLA at 1216 days post-merger (Alexander et al., GCN 29053
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 ), we consider whether this limit is constraining in the context of the detection discussed in Hajela et al., GCN 29055
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 .  Assuming the counterpart had an unabsorbed flux of 3.46e-15 erg/cm2/s (0.3-10 keV) at that time and a Gamma~1.6 photon index, we infer that the source would have f_F140W = 1.5e-8 Jy or m_F140W = 28.5 mag (AB; unabsorbed) or 28.6 mag (AB; accounting for Galactic extinction) in the latest observations.  Similarly, the radio observations yielded a 3-sigma limiting flux of 13 muJy at a central frequency of 3 GHz, which imply that m_F140W > 28 mag for the same photon index, significantly fainter than our limit.

Thus the latest observations are not constraining for this model.  However, additional HST observations are planned in F814W, F110W, and F160W between now and 2021 March 1 with comparable exposure times in each filter, enabling deeper constraints on the optical and near-infrared counterpart to GW170817 over a broader range of its spectral energy distribution.

We thank the STScI staff for facilitating these observations, especially our program coordinator Shelly Meyett.

A. Hajela, R. Margutti, K. D. Alexander, W. Fong (Northwestern U.), E.
Berger (Harvard), T. Laskar (Bath), T. Eftekhari (Harvard), D. Giannios
(Purdue U.), P. K. Blanchard, R. Chornock, D. L. Coppejans (Northwestern
U.),  V. A. Villar (Columbia U.),  P. Cowperthwaite (Carnegie), L. Sironi
(Columbia), B. Margalit (Berkeley) report:

We report on the complete Chandra data set of observations of GW170817
acquired at t=1209-1213 days since merger (IDs 22677, 24887, 24888, and
24889; PI Margutti; program 21510449) for a total exposure time of ~91 ks.
Preliminary results from a subset of these observations have been reported
by Hajela et al., GCN 29041
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 , GCN 29019
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 ; Troja et al., GCN 29038
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 .

We confirm the presence of significant X-ray emission from GW170817
(significance of detection > 5 sigma, Gaussian equivalent) from the merged
observations, with a source count rate of (0.9 +/- 0.3) e-4 cps (0.5-8
keV). From a joint spectral fit with a simple power-law spectral model
using a photon index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2
(as found in e.g. Hajela et al. 2019), and Galactic neutral hydrogen column
density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005), we estimate an
unabsorbed flux of 3.46(+1.06 -1.31) e-15 erg/cm2/s (0.3-10 keV, 1-sigma
uncertainties), corresponding to a luminosity of 6.9 (+2.1 -2.6) e+38 erg/s
at the distance of 40.7 Mpc (Cantiello et al., 2018). The observed emission
is a factor of ~2 brighter than the first X-ray non-detection of GW170817
at ~2 days after merger.

As previously reported in Hajela et al., GCN 29041
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 , the inferred flux is in
excess to the extrapolation of the off-axis jet model that best fits the
broad-band afterglow observations at t<580 days (e. g., Hajela et al. 2019)
and might represent the emergence of a new component of emission. An
updated plot of the X-ray evolution of GW170817 can be found here:
https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home


We thank the entire Chandra team for scheduling and executing these
observations.

We thank the entire Chandra team for scheduling and executing these
observations.

K. D. Alexander, A. Hajela, R. Margutti, J. Bright (Northwestern U.), T.
Eftekhari (Harvard U.), A. Kathirgamaraju (UC Berkeley), and E. Berger
(Harvard U.) report on behalf of a larger collaboration:

We observed GW170817 with the NSF���s Karl G. Jansky Very Large Array (the
VLA) at t =1216 days (~3.3 years) after the binary neutron star merger
(joint program SL0449, PI: Margutti). In our preliminary analysis, no radio
emission is detected at the location of GW170817; we infer a 3-sigma upper
limit of ~13 microJy at a central frequency of 3 GHz. Assuming the X-ray
flux density recently reported by Hajela et al. (GCN 29041
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 ) and a simple
power-law spectrum, this is fully consistent with the F_nu ~ nu^-0.575
spectrum observed throughout GW170817���s earlier evolution (e.g. Hajela et
al. 2019).

We thank the VLA staff for scheduling and executing these observations.

A. Hajela, R. Margutti, K. D. Alexander,  W. Fong  (Northwestern U.), T.
Laskar (Bath), D. L. Coppejans (Northwestern U.), V. A. Villar (Columbia
U.), D. Giannios (Purdue U.), T. Eftekhari (Harvard U.), L. Sironi
(Columbia), B. Margalit (Berkeley) report:

We observed GW170817 with the Chandra X-ray Observatory (CXO) at t ~
1209-1212 days (~3.3 years) after the binary neutron star merger. We report
here the preliminary results from the first three epochs acquired so far,
for a total exposure time of ~ 64 ks (out of a total of 100 ks requested;
we will report on the results from the complete data set in the next few
days). The observations were taken in three distinct exposures of ~30, ~18,
and ~17 ks, corresponding to ObsID 22677, 24888, and 24889, respectively (PI
Margutti; program 21510449). Preliminary results on these data have been
reported in Troja et al. (GCN 29038
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 ).


We find evidence for statistically significant X-ray emission at the
location of GW170817 in the merged file with a statistical significance of
>5 sigma (Gaussian equivalent) corresponding to a net count-rate of
(1.2+\- 0.4)e-04 (0.5 - 8 keV). We perform a joint spectral fit of all
three epochs with a simple power-law model using a photon index Gamma ~
1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al.
2019) and Galactic neutral hydrogen column density NH,gal = 7.8e+20
cm-2  (Kalberla
et al., 2005). With these spectral parameters we derive an unabsorbed flux
of 4.4 (+1.78, -1.50) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties),
corresponding to a luminosity of 8.7 (+3.5, -2.9)e+38 erg/s at the distance
of 40.7 Mpc (Cantiello et al., 2018).


These observations are consistent with a constant X-ray flux at t> 580
days, which deviates from the predictions of the off-axis jet afterglow
model (e.g. Hajela et al. 2019). The derived flux at 3.3 yrs after the
merger is in excess to the structured jet model that best described all the
observations up to t ~ 743 days after the merger and might represent the
first evidence for the emergence of the kilonova afterglow (
https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home
).


We thank the entire Chandra team for scheduling and executing these
observations.

E. Troja (UMD/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD),
B. O' Connor (UMD/GWU), H. van Eerten (U. Bath),
T. Sakamoto (AGU) report:

We analyzed the latest Chandra observations of GW170817
carried out between Dec 09 and Dec 13, 2020 for a total
exposure of  about 65 ks (out of 100 ks approved for
program 21510449; PI: Margutti)
At the transient position we detect a total of 9 counts
within a circular region of 1 arcsec radius, corresponding
to a count rate (1.5 +/- 0.5)E-04 cts/s (0.5-8.0 keV).
This is slightly higher than the value measured at 940 days
(0.8 +/- 0.3)E-04 cts/s  (Troja et al., 2020, MNRAS, 498, 5643)
and consistent within the errors. By using a power-law spectrum
with photon index Gamma=1.57, we derive an X-ray flux of
(2.6 +/- 0.9)E-15 erg/cm2/s.

Further analysis is on-going.

A. Hajela, R. Margutti, K. D. Alexander (Northwestern U.), T. Laskar (U. of
Bath), D. Giannios (Purdue U.), V. A. Villar (Columbia U.) report:


We observed GW170817 with the Chandra X-ray Observatory (CXO) at t =1209
days (~3.3 years) after the binary neutron star merger. Here we report the
preliminary results from the analysis of the first epoch of data acquired
so far, for a total exposure time of 29.7  ks (out of a total of ~100 ks to
be completed over the next few days) corresponding to ObsID 22677 (PI
Margutti; program 21510449).

No X-ray emission is detected at the location of GW170817 and we infer a
3-sigma 0.5-8 keV count-rate upper limit of 1.0e-4 cts/s. Assuming a simple
power-law spectral model with a photon index Gamma =1.57 and no intrinsic
absorption NHint = 0 cm-2 (as found from previous CXO observations of
GW170817, e.g., Hajela et al. 2019), the corresponding 3-sigma absorbed
flux limit is Fx<1.6e-15 erg/cm2/s (0.3-10 keV), which is Lx< 3.5e+38 erg/s
at the distance of 40.7 Mpc (Cantiello et al., 2018).  The Galactic neutral
hydrogen column density in the direction of GW170817 is NH,gal = 7.8e+20
cm-2  (Kalberla et al., 2005).  As a comparison, the off-axis jet models
that best fit the earlier multi-band evolution of the afterglow predict a
luminosity of  Lx~1.5e+38 erg/s (0.3-10 keV) at the present epoch, which is
below the sensitivity of current CXO observations.  Further Chandra
observations planned in the next few days will probe deeper fluxes.

We thank the entire Chandra team for scheduling and executing these
observations.

A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D.
Alexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley),  D.
Giannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson,
M. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G.
Hosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams
(Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P.
S. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D.
Metzger, L. Sironi (Columbia U.), D. Radice (PSU) report:

We carried out Chandra X-ray observations to follow the evolution of
GW170817 at t ~ 935-941 days after the neutron star merger, for a total
exposure time of ~96 ks. The observations were taken in 4 distinct
exposures of ~ 24ks, ~16ks, ~19ks, and ~36 ks, respectively  (ObsID 21323,
23183, 23184 and 23185; PI Margutti; program 20500299, for which we waived
proprietary data rights). The observations were taken on March 09, 2020
through March 15, 2020.  We reported the results from our first observation
(ObsID 21323) in Hajela et al. (GCN 27357
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 ). Here we report on the results
from our entire data set of 96ks, using the most recent data products
distributed after a data-processing issue affecting ObsID 23185 was
resolved by the CXC-DS's SAP team. An earlier report can be found in Troja
et al., GCN 27411
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 .

X-ray emission is detected at the location of GW170817 with a significance
of ~5 sigma with a total of ~7 source photons above background in the
merged file, which translates to a 0.5-8 keV count-rate of (7.2 +/- 2.7)e-5
cts/s. We perform a joint spectral analysis of all the observations using a
simple power-law spectral model with photon index Gamma ~ 1.6 and NHint = 0
cm-2, as found from the homogeneous modeling of all previous Chandra
observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H). We freeze the
galactic NH = 7.8e+20 cm-2  (Kalberla et al., 2005) and derive an
unabsorbed flux of (2.3 +/- 0.9)e-15 erg/cm2/s  (0.3-10 keV energy range)
with 1-sigma uncertainties. The flux measurement derived here is slightly
in excess to, but consistent with the predictions from updated off-axis jet
models that best fitted the multi-wavelength afterglow until t~743 days. A
similar conclusion was initially reported by Hajela et al., GCN 27357
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 , and
later by Troja et al., GCN 27411
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 . An updated light-curve plot is linked
here:
https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home

These observations might represent the last X-ray detection of the jet
afterglow of GW170817. Future Chandra observations are planned to be
acquired in December 2020 as part of our program.

We thank the entire Chandra team for making these observations possible.

E. Troja (UMD/NASA/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), H. van Eerten
(Bath U.), B. Zhang (UNLV) ob behalf of a larger collaboration

The Chandra X-ray Observatory re-observed the field of GW170817 between
March 9th and March 15th, 2020,and performed four short exposures
observations (ObsIDs: 21323, 23183, 23184, 23185)
for a total of 96 ks, as part of its on-going monitoring program. At the
afterglow position, an X-ray source is detected with significance >4.5
sigma
at an average count rate of (8+/-0.4)e-05 cts/s in the 0.5-8.0 keV energy
band. By using an absorbed power-law model with absorption column fixed at
the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.585, as
derived from our broadband analysis, we estimate an unabsorbed X-ray flux
of (1.1 +/- 0.6)E-15 erg/cm2/s in the 0.3-10 keV energy band. The quoted
error is at the 68% confidence level.
This new measurement is higher than earlier predictions of a structured
jet, based on a full year of data (Troja E., et al., 2019, MNRAS, 489,
1919),
yet still consistent with this model within the uncertainties.

Additional components of emission, such as continued energy injection from
a long-lived neutron star (Piro L., et al., 2019, MNRAS, 483, 1912) or
afterglow from the sub-relativistic ejecta (Kathirgamaraju A., Giannios D.,
Beniamini P., 2019, MNRAS, 487, 3914), cannot be excluded at this time.

Further analysis is on-going.

A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D.
Alexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley),  D.
Giannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson,
M. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G.
Hosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams
(Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P.
S. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D.
Metzger, L. Sironi (Columbia U.), D. Radice (Princeton) report:

We initiated a new round of observations of the NS merger GW170817 with the
Chandra X-ray Observatory (CXO) to continue following its evolution at ~
2.5 years after the merger. Four observations will be acquired between
March 9 and March 15, for a total exposure time of 100 ks. Here we report
on the first observation (ObsID 21323, PI Margutti; program 20500299),
which was acquired on March 09, 2020 at 22:04:48 UT (~935 days since
merger) with an exposure time of 24.29 ks.

X-ray emission is detected at the location of GW170817 with a significance
of ~ 3-sigma (Gaussian equivalent) and a 0.5-8 keV count-rate of  (1.2 +/-
0.7)e-04 c/s, and a total of ~ 2.8 net photons. Assuming a simple power-law
spectrum with photon index Gamma=1.6 and no intrinsic absorption, as found
in previous Chandra observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H),
and following the same spectra-extraction procedure as in Hajela et al.
2019, the count-rate above translates into an unabsorbed flux of  ~ (3.2
+/- 1.9)e-15 erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column
density in the direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et
al. 2005). This X-ray flux measurement is nominally above, yet
statistically consistent with the off-axis jet model predictions given the
large flux uncertainties. Further CXO observations comprising the remaining
exposure time of ~75 ks are scheduled in the next few days and will help
quantify the statistical evidence for the possible departure from the
off-axis jet model (which might represent the onset of the kilonova
afterglow).

An updated plot with the most recent results can be found at this link:
https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home


We thank the entire Chandra team for making these observations possible.

A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U Bath),  D.
Coppejans, G. Terreran, W. Fong, K. D. Alexander, A. Baldeschi, K.
Paterson (Northwestern
U.), E. Berger, P. K. Blanchard, T. Eftekhari, G. Hosseinzadeh, S. Gomez,
V. A. Villar, P. K. G. Williams (Harvard U), M. Nicholl (U Edinburgh), R.
Chornock (Ohio U), P. S. Cowperthwaite (Carnegie Observatories), D.
Giannios (Purdue U.), A. MacFadyen (NYU), A. Kathirgamaraju (UC Berkeley)
report:

We report on Chandra observations of GW170817 acquired ~2 yrs after the
binary neutron star merger. The Chandra X-ray Observatory started observing
GW170817 August 27, 2019 at 14:16:40 UT (t ~ 740 d after merger) for an
exposure time of 40.0 ks (PI Troja; program 20500691, observation ID 21372).
Another two epochs of Chandra observations were acquired on August 29, 2019
at 13:46:01 UT (ID 22736) and on August 30, 2019 at 11:46:44 UT (ID 22737)
for an exposure time of 33.6 ks and 25.3 ks, respectively.

A preliminary reduction shows that GW170817 is detected in the merged
exposure with a ~3 sigma significance and a net count-rate of (1.03 +\-
0.33)e-4 c/s (0.5-8 keV). Assuming negligible intrinsic absorption and a
Galactic neutral hydrogen column density N_h = 7.8E+20cm-2 (Kalberla et
al., 2005), we infer a best-fit photon index Gamma=1.0+/-1.3 and an
unabsorbed flux of  ~3e-15 erg/s/cm2 (0.3-10 keV). The photon index is
loosely constrained by the observations and we therefore adopt the best-fit
photon index from the entire Chandra data set acquired in the first two
years of observations of GW170817 (Gamma=1.6) for the spectral calibration.
We find an unabsorbed flux of (2.9 +\- 1.0)e-15 erg/s/cm2. These results
are consistent within the uncertainties with the extrapolation of the
off-axis structured jet model by Wu & MacFadyen (ApJ, 869, 55) at the
current epoch.

We thank the entire Chandra team for making these observations possible.

A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), E.
Berger, P. Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans
(Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh  (Harvard), G.
Terreran (Northwestern/CIERA), A. Villar (Harvard) report:

���We initiated a series of  Chandra X-ray observations of GW170817 at t~581
days since merger, for a total exposure time of 100 ks. The results from
the first observation with the exposure time of ~36 ks is reported in
Hajela et al. (GCN 23987
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 ).

Here, we report on the remaining ~64ks of Chandra observations of GW170817
acquired starting on 22 March 2019 at 06:17 UT (obsID 22157, exposure time
of ~38 ks) and 23 March 2019 at 23:52 UT (obsID 22158, exposure time of ~25
ks, PIs Margutti and Fong). X-ray emission from GW170817 is detected with a
significance of >=3 sigma in each of the observations, with a total of ~6
and ~3 photons from the source in obsID 22157 and obsID 22158, respectively
(0.5-8 keV energy range).

Assuming a simple power-law spectral model with photon index Gamma = 1.6
and NHint = 0, as found in previous Chandra observations (e.g., Margutti et
al., 2018; Alexander et al., 2018), we find an unabsorbed flux of (2.7 +\-
1.2)e-15 erg/s/cm2 and (6.3+\- 3.5)e-15 erg/s/cm2 for ObsID 22157 and
22158, respectively (0.3-10 keV). The Galactic neutral hydrogen column
density in the direction of the transient is NH = 7.8e+20cm-2 (Kalberla et
al., 2005).

From a joint spectral fit of the three epochs of data (obsID 21322, 22157,
and 22158) we find that the photon index is loosely constrained to
Gamma=1.6 +/- 0.8 and there is no evidence for intrinsic absorption.  For
Gamma=1.6, and NHint=0, the joint fit of the entire 100ks of Chandra
observations indicates an unabsorbed flux of (3.6 +/- 0.9)e-15 erg/s/cm2 (t~582
days since merger, 0.3-10 keV). We confirm that this flux measurement is
consistent with the expectations from the fast decay of the emission from
the jet (e.g., Margutti et al., 2018; Alexander et al., 2018; Wu et al.,
2018).  All fluxes reported are in the 0.3-10 keV energy range with 1-sigma
uncertainties.

We thank the entire Chandra team for making these observations possible.���

A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), E.
Berger, P. Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans
(Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh  (Harvard), G.
Terreran (Northwestern/CIERA), A. Villar (Harvard) report:

���We report the results from the first 36ks of Chandra observations of
GW170817 (another ~ 64ks will be acquired in the next few days). Chandra
started to observe GW170817 on 21 March 2019 at 07:44 UT (obsID 21322,
Co-PIs Margutti and Fong; program 20500299). X-ray emission from GW170817
is detected at the level of ~3 sigma  (assuming pure Poissonian
statistics), with a 0.5-8 keV count-rate of  (1.5 +\-0.7)e-4 c/s, and a
total of ~5 net photons. Assuming a simple power-law spectrum with photon
index Gamma=1.6 and no intrinsic absorption, as found in previous Chandra
observations (e.g., Margutti et al., 2018; Alexander et al., 2018), the
count-rate above translates into an unabsorbed flux of  (3.9+/-1.8)e-15
erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column density in the
direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et al., 2005).
This flux measurement is consistent with the expectations from the fast
decay of the emission from the jet (e.g., Margutti et al., 2018; Alexander
et al., 2018; Wu et al., 2018). Further Chandra observations are ongoing.

We thank the entire Chandra team for making these observations possible.���

A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), P.
Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans
(Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh  (Harvard), G.
Terreran (Northwestern/CIERA), A. Villar (Harvard) report:

���We report the results from the first 36ks of Chandra observations of
GW170817 (another ~ 64ks will be acquired in the next few days). Chandra
started to observe GW170817 on 21 March 2019 at 07:44 UT (obsID 21322,
Co-PIs Margutti and Fong; program 20500299). X-ray emission from GW170817
is detected at the level of ~3 sigma  (assuming pure Poissonian
statistics), with a 0.5-8 keV count-rate of  (1.5 +\-0.7)e-4 c/s, and a
total of ~5 net photons. Assuming a simple power-law spectrum with photon
index Gamma=1.6 and no intrinsic absorption, as found in previous Chandra
observations (e.g., Margutti et al., 2018; Alexander et al., 2018), the
count-rate above translates into an unabsorbed flux of  (2.3+\- 1.1)e-15
erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column density in the
direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et al., 2005).
This flux measurement is consistent with the expectations from the fast
decay of the emission from the jet (e.g., Margutti et al., 2018; Alexander
et al., 2018; Wu et al., 2018). Further Chandra observations are ongoing.

We thank the entire Chandra team for making these observations possible.���

Daryl Haggard, Melania Nynka, and J. J. Ruan (McGill/MSI) write:

We correct a typo in GCN 23137
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 : The 0.5-8 keV count rate measured in the 
10 August 2018 Chandra observation of GW170817 should have read 4.9 
(+0.9/-0.9) e-04 ct/s. All fluxes, photon indices, luminosities, light 
curve slopes, etc., reported in GCN 23137
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  are correct, and were derived 
from this correct count rate.

GCN OPS NOTE(28aug18):  Per author's request, the Subject-line
was corrected with the proper referencing Subject-line.]

Daryl Haggard, Melania Nynka, and J. J. Ruan (McGill/MSI) report:

We analyze new Chandra X-ray observations of neutron star merger 
GW170817 at 358.6 days post-merger, which now reveals fading at a t^-1.6 
rate.

Chandra obtained a 67.16 ks observation of GW170817 (ObsID: 21371, PI: 
Troja) on 10 August 2018, at 358.6 days post-merger. GW170817 is still 
clearly detected, and we measure a 0.5-8 keV count rate of 4.9 
(+0.9/-0.9) cts/s. 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) = 6.3 (+3.8/-2.1) x10^-15 
erg/s/cm^2 (90% c.l.) and photon index of Gamma = 1.6 (+1.3/-0.9), which 
corresponds to an unabsorbed luminosity of L(0.3-10 keV) = 1.7 
(+1.1/-0.5) x10^39 erg/s assuming a luminosity distance of 42.5 Mpc.

The previous Chandra observation at 260.0 days showed an absorbed flux 
of f(0.3-8 keV) = 1.09 (+0.24/-0.20) x10^-14 erg/s/cm^2 (Nynka et al. 
2018). A power-law fit of this previous 260.0 day flux to the new 358.6 
day flux reveals fading at a t^-1.6 rate. This is steeper than the 
t^-1.3 fading observed between 159.7 and 260.0 days post-merger, and 
thus the X-ray afterglow light curve is now fading more rapidly.

E. Troja (UMD/GSFC), L. Piro (IAPS/INAF), G. Ryan (UMD) report on
behalf of a larger collaboration:


The Chandra X-ray Observatory re-observed the field of GW170817
starting on May 3rd, 2018 as part of its on-going monitoring
program (PI: Wilkes). Observations were split into two exposures
of 50.7 ks and 46 ks, respectively.

The X-ray afterglow is significantly detected at an average level
of (8.5 +/- 1.4)E-04 cts/s, well below the value (14.8 +/- 1.1)E-04 cts/s
measured�� at ~160 d after the merger (Troja & Piro, GCN Circ. 22374
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 ;
Troja et al. 2018). The spectrum is well described by an absorbed
power-law model with photon index 1.5 +/- 0.2, consistent with the
value of 1.575 derived at earlier times from the broadband spectrum.
By using this value we derive an unabsorbed X-ray flux of
(1.4 +/- 0.2)E-14 erg/cm2/s at 260 days after the merger.

A comparison with the afterglow models derived in Troja et al. (2018)
is shown at this link:
https://www.astro.umd.edu/~gsryan/GW170817/xray.png

This new X-ray observation supports earlier claims (D'Avanzo et al. 2018;
Dobie et al. 2018) that the afterglow entered its post-peak decay phase.
However, no clear distinction between the competing models is possible.
Further observations are critical to probe the structure of the
relativistic outflow.

A. Hajela (Northwestern/CIERA), K. D. Alexander, T. Eftekhari
 (Harvard/CfA), R. Margutti, W. Fong (Northwestern/CIERA), E. Berger
(Harvard/CfA) report

"The Chandra X-ray Observatory started another round of follow-up
observations of GW170817. The first observation (ID 21080) was taken on May
03, 2018 at 10:41:26 UT (t ~ 259 d after merger) for a total exposure time
of 50.8 ks (PI Wilkes; program 19408644).   We find that an X-ray source is
detected at the location of GW170817 with a count-rate of (7.7 +/- 1.3)e-4
cts/s (0.5 - 8 keV) and 13.8-sigma significance. We modeled the spectrum
with an absorbed simple power-law and find a best-fitting photon index,
 Gamma = 1.4 +/- 0.3 and no evidence for intrinsic absorption. The galactic
neutral hydrogen column density in the direction of the transient is, N_H =
7.8E+20cm-2 (Kalberla et al., 2005). For these parameters, the unabsorbed
flux is (1.3 +/- 0.3)e-14 ergs cm-2 s-1 (0.3 - 10 keV).

A second observation (ID 21090) was acquired on May 05, 2018 at 01:25:30 UT
(~ 261 days after merger) for a total exposure time of 46.0 ks. GW170817 is
detected with 14.8-sigma significance and a count-rate of (8.3 +/-  1.4)e-4
cts/s (0.5 - 8 keV). The best-fitting photon-index is Gamma = 1.3 +/- 0.3.
The corresponding unabsorbed flux is (1.5 +0.5/-0.3) e-14 ergs cm-2 s-1
(0.3 - 10 keV).

The best fitting photon index from a joint spectral fit is Gamma = 1.43 +/-
0.15, consistent with the photon index measured at ~160 days. We conclude
that there is no evidence for the passage of the synchrotron cooling
frequency through the Chandra band.

The total 0.5-8 keV count-rate inferred from the two observations is
(8.0+/-0.9)e-4 cts/s.  The X-ray source shows significant fading compared
to the previous epoch at ~160 days (inferred rate of (1.50 +\-0.12)e-3
cts/s, Margutti et al., 2018). Applying a simple binomial test, we find
that the detected fading is significant at a level > 4 sigma. We can thus
reject the hypothesis of a random fluctuation of the X-ray count-rate, and
conclude that these Chandra observations provide the first statistically
significant evidence for a decaying X-ray emission from GW170817.

We thank the entire Chandra team for making these observations possible."