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Title: The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta

Abstract

We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $$\gtrsim 10^{48}$$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $$\gtrsim 20^{\circ}$$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $$\sim 10^{49}-10^{50}$$ erg that exploded in a uniform density environment with $$n\sim 10^{-4}-10^{-2}$$ cm$$^{-3}$$, viewed at an angle of $$\sim 20^{\circ}-40^{\circ}$$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $$\sim 5-10$$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.

Authors:
ORCiD logo; ORCiD logo; ORCiD logo; ORCiD logo; ; ORCiD logo; ; ; ; ; ; ; ; ; ; ; ; ; ; more »; ; ; « less
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1409350
Report Number(s):
arXiv:1710.05457; FERMILAB-PUB-17-474-A-AE-CD
Journal ID: ISSN 2041-8213; 1630797
DOE Contract Number:
AC02-07CH11359
Resource Type:
Journal Article
Resource Relation:
Journal Name: The Astrophysical Journal. Letters; Journal Volume: 848; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Alexander, K. D., Berger, E., Fong, W., Williams, P. K. G., Guidorzi, C., Margutti, R., Metzger, B. D., Annis, J., Blanchard, P. K., Brout, D., Brown, D. A., Chen, H. -Y., Chornock, R., Cowperthwaite, P. S., Drout, M., Eftekhari, T., Frieman, J., Holz, D. E., Nicholl, M., Rest, A., Sako, M., Soares-Santos, M., and Villar, V. A. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta. United States: N. p., 2017. Web. doi:10.3847/2041-8213/aa905d.
Alexander, K. D., Berger, E., Fong, W., Williams, P. K. G., Guidorzi, C., Margutti, R., Metzger, B. D., Annis, J., Blanchard, P. K., Brout, D., Brown, D. A., Chen, H. -Y., Chornock, R., Cowperthwaite, P. S., Drout, M., Eftekhari, T., Frieman, J., Holz, D. E., Nicholl, M., Rest, A., Sako, M., Soares-Santos, M., & Villar, V. A. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta. United States. doi:10.3847/2041-8213/aa905d.
Alexander, K. D., Berger, E., Fong, W., Williams, P. K. G., Guidorzi, C., Margutti, R., Metzger, B. D., Annis, J., Blanchard, P. K., Brout, D., Brown, D. A., Chen, H. -Y., Chornock, R., Cowperthwaite, P. S., Drout, M., Eftekhari, T., Frieman, J., Holz, D. E., Nicholl, M., Rest, A., Sako, M., Soares-Santos, M., and Villar, V. A. 2017. "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta". United States. doi:10.3847/2041-8213/aa905d. https://www.osti.gov/servlets/purl/1409350.
@article{osti_1409350,
title = {The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-time Emission from the Kilonova Ejecta},
author = {Alexander, K. D. and Berger, E. and Fong, W. and Williams, P. K. G. and Guidorzi, C. and Margutti, R. and Metzger, B. D. and Annis, J. and Blanchard, P. K. and Brout, D. and Brown, D. A. and Chen, H. -Y. and Chornock, R. and Cowperthwaite, P. S. and Drout, M. and Eftekhari, T. and Frieman, J. and Holz, D. E. and Nicholl, M. and Rest, A. and Sako, M. and Soares-Santos, M. and Villar, V. A.},
abstractNote = {We present Very Large Array (VLA) and Atacama Large Millimeter/sub-millimeter Array ALMA radio observations of GW\,170817, the first Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo gravitational wave (GW) event from a binary neutron star merger and the first GW event with an electromagnetic (EM) counterpart. Our data include the first observations following the discovery of the optical transient at both the centimeter ($13.7$ hours post merger) and millimeter ($2.41$ days post merger) bands. We detect faint emission at 6 GHz at 19.47 and 39.23 days after the merger, but not in an earlier observation at 2.46 d. We do not detect cm/mm emission at the position of the optical counterpart at frequencies of 10-97.5 GHz at times ranging from 0.6 to 30 days post merger, ruling out an on-axis short gamma-ray burst (SGRB) for energies $\gtrsim 10^{48}$ erg. For fiducial SGRB parameters, our limits require an observer viewer angle of $\gtrsim 20^{\circ}$. The radio and X-ray data can be jointly explained as the afterglow emission from an SGRB with a jet energy of $\sim 10^{49}-10^{50}$ erg that exploded in a uniform density environment with $n\sim 10^{-4}-10^{-2}$ cm$^{-3}$, viewed at an angle of $\sim 20^{\circ}-40^{\circ}$ from the jet axis. Using the results of our light curve and spectral modeling, in conjunction with the inference of the circumbinary density, we predict the emergence of late-time radio emission from the deceleration of the kilonova (KN) ejecta on a timescale of $\sim 5-10$ years that will remain detectable for decades with next-generation radio facilities, making GW\,170817 a compelling target for long-term radio monitoring.},
doi = {10.3847/2041-8213/aa905d},
journal = {The Astrophysical Journal. Letters},
number = 2,
volume = 848,
place = {United States},
year = 2017,
month =
}
  • We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component (more » $$T\approx5500$$ K at 1.5 days) that quickly reddens; spectra later than $$\gtrsim 4.5$$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $$\sim 7900$$ \AA\ at $$t\lesssim 4.5$$ days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $$A\lesssim 140$$. This indicates a sight-line within $$\theta_{\rm obs}\lesssim 45^{\circ}$$ of the orbital axis. Comparison to models suggests $$\sim0.03$$ M$$_\odot$$ of blue ejecta, with a velocity of $$\sim 0.3c$$. The required lanthanide fraction is $$\sim 10^{-4}$$, but this drops to $$<10^{-5}$$ in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $$\lesssim 12$$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.« less
  • We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart atmore » $0.47$ days to $18.5$$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy distribution (SED) inferred from this photometry at $$0.6$ days is well described by a blackbody model with $$T\approx 8300$$ K, a radius of $$R\approx 4.5\times 10^{14}$$ cm (corresponding to an expansion velocity of $$v\approx 0.3c$$), and a bolometric luminosity of $$L_{\rm bol}\approx 5\times10^{41}$$ erg s$$^{-1}$$. At $1.5$ days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set we find that models with heating from radioactive decay of $$^{56}$$Ni, or those with only a single component of opacity from $r$-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data, the resulting "blue" component has $$M_\mathrm{ej}^\mathrm{blue}\approx 0.01$$ M$$_\odot$$ and $$v_\mathrm{ej}^\mathrm{blue}\approx 0.3$$c, and the "red" component has $$M_\mathrm{ej}^\mathrm{red}\approx 0.04$$ M$$_\odot$$ and $$v_\mathrm{ej}^\mathrm{red}\approx 0.1$$c. These ejecta masses are broadly consistent with the estimated $r$-process production rate required to explain the Milky Way $r$-process abundances, providing the first evidence that BNS mergers can be a dominant site of $r$-process enrichment.« less
  • Here, we report the discovery of rising X-ray emission from the binary neutron star merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave (GW) source. Observations acquired with the Chandra X-ray Observatory ( CXO) atmore » $$t\approx 2.3$$ days post-merger reveal no significant emission, with $${L}_{x}\lesssim 3.2\times {10}^{38}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$$ (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching $${L}_{x}\approx 9\times {10}^{38}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$$ at $$\approx 15.1$$ days post-merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broadband X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy $${E}_{k}\sim {10}^{49-50}\,\mathrm{erg}$$, viewed off-axis with $${\theta }_{\mathrm{obs}}\sim 20^\circ \mbox{--}40^\circ $$. Our models favor a circumbinary density $$n\sim {10}^{-4}\mbox{--}{10}^{-2}\,{\mathrm{cm}}^{-3}$$, depending on the value of the microphysical parameter $${\epsilon }_{B}={10}^{-4}\mbox{--}{10}^{-2}$$. A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at $$t\gtrsim 100$$ days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on $${\theta }_{\mathrm{obs}}$$ are testable with GW information on GW170817 from advanced LIGO/Virgo on the binary inclination.« less
  • We present the properties of NGC 4993, the host galaxy of GW170817, the first gravitational wave (GW) event from the merger of a binary neutron star (BNS) system and the first with an electromagnetic (EM) counterpart. We use both archival photometry and new optical/near-IR imaging and spectroscopy, together with stellar population synthesis models to infer the global properties of the host galaxy. We infer a star formation history peaked atmore » $$\gtrsim 10$$ Gyr ago, with subsequent exponential decline leading to a low current star formation rate of 0.01 M$$_{\odot}$$ yr$$^{-1}$$, which we convert into a binary merger timescale probability distribution. We find a median merger timescale of $$11.2^{+0.7}_{-1.4}$$ Gyr, with a 90% confidence range of $6.8-13.6$ Gyr. This in turn indicates an initial binary separation of $$\approx 4.5$$ R$$_{\odot}$$, comparable to the inferred values for Galactic BNS systems. We also use new and archival $Hubble$ $Space$ $Telescope$ images to measure a projected offset of the optical counterpart of $2.1$ kpc (0.64$$r_{e}$$) from the center of NGC 4993 and to place a limit of $$M_{r} \gtrsim -7.2$$ mag on any pre-existing emission, which rules out the brighter half of the globular cluster luminosity function. Finally, the age and offset of the system indicates it experienced a modest natal kick with an upper limit of $$\sim 200$$ km s$$^{-1}$$. Future GW$-$EM observations of BNS mergers will enable measurement of their population delay time distribution, which will directly inform their viability as the dominant source of $r$-process enrichment in the Universe.« less