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Title: A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a

Abstract

The merging neutron star gravitational-wave event GW170817 has been observed throughout the entire electromagnetic spectrum from radio waves to γ -rays. The resulting energetics, variability, and light curves are shown to be consistent with GW170817 originating from the merger of two neutron stars, in all likelihood followed by the prompt gravitational collapse of the massive remnant. The available γ -ray, X-ray, and radio data provide a clear probe for the nature of the relativistic ejecta and the non-thermal processes occurring within, while the ultraviolet, optical, and infrared emission are shown to probe material torn during the merger and subsequently heated by the decay of freshly synthesized r -process material. The simplest hypothesis, that the non-thermal emission is due to a low-luminosity short γ -ray burst (sGRB), seems to agree with the present data. While low-luminosity sGRBs might be common, we show here that the collective prompt and multi-wavelength observations are also consistent with a typical, powerful sGRB seen off-axis. Detailed follow-up observations are thus essential before we can place stringent constraints on the nature of the relativistic ejecta in GW170817.

Authors:
; ; ; ; ; ; ; ;  [1];  [2];  [3]; ; ; ; ;  [4];  [5]
  1. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
  2. Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (United States)
  3. Instituto de Astronomía, Universidad Nacional Autónoma de México, Circuito Exterior, C.U., A. Postal 70-264, 04510 Cd. de México, México (Mexico)
  4. The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
  5. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
Publication Date:
OSTI Identifier:
22654365
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 848; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COSMIC GAMMA BURSTS; DECAY; EMISSION; GAMMA RADIATION; GRAVITATIONAL COLLAPSE; GRAVITATIONAL WAVES; LIMITING VALUES; LUMINOSITY; NEUTRON STARS; R PROCESS; RADIOWAVE RADIATION; RELATIVISTIC RANGE; SIMULATION; SPECTRA; ULTRAVIOLET RADIATION; VISIBLE RADIATION; WAVELENGTHS; X RADIATION

Citation Formats

Murguia-Berthier, A., Ramirez-Ruiz, E., Kilpatrick, C. D., Foley, R. J., Coulter, D. A., Pan, Y.-C., Prochaska, J. X., Rojas-Bravo, C., Siebert, M. R., Kasen, D., Lee, W. H., Piro, A. L., Drout, M. R., Madore, B. F., Shappee, B. J., Simon, J. D., and Rest, A. A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA91B3.
Murguia-Berthier, A., Ramirez-Ruiz, E., Kilpatrick, C. D., Foley, R. J., Coulter, D. A., Pan, Y.-C., Prochaska, J. X., Rojas-Bravo, C., Siebert, M. R., Kasen, D., Lee, W. H., Piro, A. L., Drout, M. R., Madore, B. F., Shappee, B. J., Simon, J. D., & Rest, A. A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a. United States. doi:10.3847/2041-8213/AA91B3.
Murguia-Berthier, A., Ramirez-Ruiz, E., Kilpatrick, C. D., Foley, R. J., Coulter, D. A., Pan, Y.-C., Prochaska, J. X., Rojas-Bravo, C., Siebert, M. R., Kasen, D., Lee, W. H., Piro, A. L., Drout, M. R., Madore, B. F., Shappee, B. J., Simon, J. D., and Rest, A. 2017. "A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a". United States. doi:10.3847/2041-8213/AA91B3.
@article{osti_22654365,
title = {A Neutron Star Binary Merger Model for GW170817/GRB 170817A/SSS17a},
author = {Murguia-Berthier, A. and Ramirez-Ruiz, E. and Kilpatrick, C. D. and Foley, R. J. and Coulter, D. A. and Pan, Y.-C. and Prochaska, J. X. and Rojas-Bravo, C. and Siebert, M. R. and Kasen, D. and Lee, W. H. and Piro, A. L. and Drout, M. R. and Madore, B. F. and Shappee, B. J. and Simon, J. D. and Rest, A.},
abstractNote = {The merging neutron star gravitational-wave event GW170817 has been observed throughout the entire electromagnetic spectrum from radio waves to γ -rays. The resulting energetics, variability, and light curves are shown to be consistent with GW170817 originating from the merger of two neutron stars, in all likelihood followed by the prompt gravitational collapse of the massive remnant. The available γ -ray, X-ray, and radio data provide a clear probe for the nature of the relativistic ejecta and the non-thermal processes occurring within, while the ultraviolet, optical, and infrared emission are shown to probe material torn during the merger and subsequently heated by the decay of freshly synthesized r -process material. The simplest hypothesis, that the non-thermal emission is due to a low-luminosity short γ -ray burst (sGRB), seems to agree with the present data. While low-luminosity sGRBs might be common, we show here that the collective prompt and multi-wavelength observations are also consistent with a typical, powerful sGRB seen off-axis. Detailed follow-up observations are thus essential before we can place stringent constraints on the nature of the relativistic ejecta in GW170817.},
doi = {10.3847/2041-8213/AA91B3},
journal = {Astrophysical Journal Letters},
number = 2,
volume = 848,
place = {United States},
year = 2017,
month =
}
  • 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
  • Here, 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 » $$\gtrsim 10\,\mathrm{Gyr}$$ 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 $${11.2}_{-1.4}^{+0.7}$$ Gyr, with a 90% confidence range of $$6.8\mbox{–}13.6\,\mathrm{Gyr}$$. 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 (0.64r 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 ~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
  • Here, we present a comprehensive comparison of the properties of the radio through X-ray counterpart of GW170817 and the properties of short-duration gamma-ray bursts (GRBs). For this effort, we utilize a sample of 36 short GRBs spanning a redshift range ofmore » $$z \approx 0.12-2.6$$ discovered over 2004-2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is $$\approx 3000$$ times less than the median value of on-axis short GRB X-ray afterglows, and $$\gtrsim10^{4}$$ times less than that for detected short GRB radio afterglows. Moreover, the allowed jet energies and particle densities inferred from the radio and X-ray counterparts to GW170817 and on-axis short GRB afterglows are remarkably similar, suggesting that viewing angle effects are the dominant, and perhaps only, difference in their observed radio and X-ray behavior. From comparison to previous claimed kilonovae following short GRBs, we find that the optical and near-IR counterpart to GW170817 is comparatively under-luminous by a factor of $$\approx 3-5$$, indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on $$\lesssim 1$$ day timescales also rules out a "blue" kilonova of comparable optical isotropic-equivalent luminosity in one previous short GRB. Finally, we investigate the host galaxy of GW170817, NGC4993, in the context of short GRB host galaxy stellar population properties. We find that NGC4993 is superlative in terms of its large luminosity, old stellar population age, and low star formation rate compared to previous short GRB hosts. Additional events within the Advanced LIGO/VIRGO volume will be crucial in delineating the properties of the host galaxies of NS-NS mergers, and connecting them to their cosmological counterparts.« 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