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Title: The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale

Abstract

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 at $$\gtrsim 10\,\mathrm{Gyr}$$ 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.

Authors:
 [1]; ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1];  [2];  [1];  [3];  [4];  [1];  [1];  [5];  [6];  [7];  [3];  [1];  [8];  [3] more »;  [9];  [10];  [6];  [11] « less
  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Northwestern Univ., Evanston, IL (United States)
  3. Univ. of Chicago, Chicago, IL (United States)
  4. Ohio Univ., Athens, OH (United States)
  5. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
  6. Univ. of Pennsylvania, Philadelphia, PA (United States)
  7. Syracuse Univ., Syracuse, NY (United States)
  8. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); The Univ. of Chicago, Chicago, IL (United States)
  9. Columbia Univ., New York, NY (United States)
  10. Space Telescope Science Institute, Baltimore, MD (United States); The Johns Hopkins Univ., Baltimore, MD (United States)
  11. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Brandeis Univ., Waltham, MA (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation (NA-20)
OSTI Identifier:
1417825
Report Number(s):
LA-UR-17-28901
Journal ID: ISSN 2041-8213
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal. Letters
Additional Journal Information:
Journal Volume: 848; Journal Issue: 2; Journal ID: ISSN 2041-8213
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; galaxies: individual (NGC 4993); gravitational waves; stars: neutron

Citation Formats

Blanchard, P. K., Berger, E., Fong, W., Nicholl, M., Leja, J., Conroy, C., Alexander, K. D., Margutti, R., Williams, P. K. G., Doctor, Z., Chornock, R., Villar, V. A., Cowperthwaite, P. S., Annis, J., Brout, D., Brown, D. A., Chen, H. -Y., Eftekhari, T., Frieman, J. A., Holz, D. E., Metzger, B. D., Rest, A., Sako, M., and Soares-Santos, M. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale. United States: N. p., 2017. Web. doi:10.3847/2041-8213/aa9055.
Blanchard, P. K., Berger, E., Fong, W., Nicholl, M., Leja, J., Conroy, C., Alexander, K. D., Margutti, R., Williams, P. K. G., Doctor, Z., Chornock, R., Villar, V. A., Cowperthwaite, P. S., Annis, J., Brout, D., Brown, D. A., Chen, H. -Y., Eftekhari, T., Frieman, J. A., Holz, D. E., Metzger, B. D., Rest, A., Sako, M., & Soares-Santos, M. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale. United States. doi:10.3847/2041-8213/aa9055.
Blanchard, P. K., Berger, E., Fong, W., Nicholl, M., Leja, J., Conroy, C., Alexander, K. D., Margutti, R., Williams, P. K. G., Doctor, Z., Chornock, R., Villar, V. A., Cowperthwaite, P. S., Annis, J., Brout, D., Brown, D. A., Chen, H. -Y., Eftekhari, T., Frieman, J. A., Holz, D. E., Metzger, B. D., Rest, A., Sako, M., and Soares-Santos, M. 2017. "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale". United States. doi:10.3847/2041-8213/aa9055.
@article{osti_1417825,
title = {The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VII. Properties of the Host Galaxy and Constraints on the Merger Timescale},
author = {Blanchard, P. K. and Berger, E. and Fong, W. and Nicholl, M. and Leja, J. and Conroy, C. and Alexander, K. D. and Margutti, R. and Williams, P. K. G. and Doctor, Z. and Chornock, R. and Villar, V. A. and Cowperthwaite, P. S. and Annis, J. and Brout, D. and Brown, D. A. and Chen, H. -Y. and Eftekhari, T. and Frieman, J. A. and Holz, D. E. and Metzger, B. D. and Rest, A. and Sako, M. and Soares-Santos, M.},
abstractNote = {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 at $\gtrsim 10\,\mathrm{Gyr}$ 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.},
doi = {10.3847/2041-8213/aa9055},
journal = {The Astrophysical Journal. Letters},
number = 2,
volume = 848,
place = {United States},
year = 2017,
month =
}

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  • 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
  • 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 (more » $13.7$ 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.« less
  • Here, we present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational-wave emission, GW170817. Our observations commenced 10.5 hr post-merger, as soon as the localization region became accessible from Chile. We imaged 70 deg 2 in the i and z bands, covering 93% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hr post-merger we detected a bright optical transient locatedmore » $$10\buildrel{\prime\prime}\over{.} 6$$ from the nucleus of NGC 4993 at redshift z = 0.0098, consistent (for $${H}_{0}=70$$ km s –1 Mpc –1) with the distance of 40 ± 8 Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes of $i=17.3$ and $z=17.4$, and thus an absolute magnitude of $${M}_{i}=-15.7$$, in the luminosity range expected for a kilonova. We identified 1500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC 4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5% confidence level. We therefore conclude that the optical counterpart we have identified near NGC 4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.« less
    Cited by 6
  • We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational wave emission, GW170817. Our observations commenced 10.5 hours post-merger, as soon as the localization region became accessible from Chile. We imaged 70 degmore » $^2$ in the $i$ and $z$$ bands, covering 93\% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detected a bright optical transient located $$10.6''$$ from the nucleus of NGC\,4993 at redshift $$z=0.0098$, consistent (for $$H_0 = 70$$\, km s$$^{-1}$$ Mpc$$^{-1}$$) with the distance of $$40 \pm 8$$\, Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes $$i\approx 17.30$$ and $$z\approx 17.45$$, and thus an absolute magnitude of $$M_i = -15.7$$, in the luminosity range expected for a kilonova. We identified 1,500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC\,4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5\% confidence level. We therefore conclude that the optical counterpart we have identified near NGC\,4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves, and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.« less
    Cited by 6