RADIATIVE TRANSFER SIMULATIONS OF NEUTRON STAR MERGER EJECTA

By Masaomi Tanaka, K. Hotokezaka. 2013

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Mergers of binary neutron stars (NSs) are among the most promising gravitational wave (GW) sources. Next generation GW detectors are expected to detect signals from NS mergers within about 200 Mpc. The detection of electromagnetic wave (EM) counterparts is crucial to understanding the nature of GW sources. Among the possible EM emission from the NS merger, emission powered by radioactive r-process nuclei is one of the best targets for follow-up observations. However, predictions so far have not taken into account detailed r-process element abundances in the ejecta. We perform for the first time radiative transfer simulations of the NS merger ejecta including all the r-process elements from Ga to U. We show that the opacity of the NS merger ejecta is about κ = 10 cm2 g−1, which is higher than that of Fe-rich Type Ia supernova ejecta by a factor of ∼100. As a result, the emission is fainter and lasts longer than previously expected. The spectra are almost featureless due to the high expansion velocity and bound–bound transitions of many different r-process elements. We demonstrate that the emission is brighter for a higher mass ratio of the two NSs and a softer equation of state adopted in the merger simulations. Because of the red color of the emission, follow-up observations in red optical and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc, the expected brightness of the emission is i = 22–25 AB mag, z = 21–23 AB mag, and 21–24 AB mag in the NIR JHK bands. Thus, observations with wide-field 4 m- and 8 m-class optical telescopes and wide-field NIR space telescopes are necessary. We also argue that the emission powered by radioactive energy can be detected in the afterglow of nearby short gamma-ray bursts.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ef23335a77966d9d6f4a4d1d00345260f207eddc

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Citations in the corpus

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South (2017, background): “The models of Tanaka & Hotokezaka (2013) have some roughly similar features, although their spectra are much broader and smoother than in our data and show a much larger drop in flux from 1.0 to 1.6 μm than the data, which may reflect limitations of the NIR line list used in that work.”
The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. II. UV, Optical, and Near-infrared Light Curves and Comparison to Kilonova Models (2017, background): “L 10 10p 40 41 erg s−1 over a longer timescale of ~t 1p week (a so-called “red” KN; Barnes & Kasen 2013; Kasen et al. 2013; Tanaka & Hotokezaka 2013).”
FERMI GBM OBSERVATIONS OF LIGO GRAVITATIONAL-WAVE EVENT GW150914 (2016, background): “…counterparts have been suggested, notably optical/infrared transients from the decay of r-process isotopes produced in the ejecta resulting from the binary merger, kilonovae, also known as macronovae (Li & Paczyński 1998; Metzger et al. 2010; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013).”
Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts (2018, background): “The optical opacity of lanthanides and actinides (A > 130, produced by the r-process) is such that the transient was expected to evolve from blue optical to near infrared within a few days (Kasen et al. 2013; Tanaka & Hotokezaka 2013; Barnes & Kasen 2013; Fontes et al. 2015), as observed.”
THE OBSERVABLE SIGNATURES OF GRB COCOONS (2016, background): “…they are comparable or even stronger than typical IR macronovae/kilonovae signals that are the hallmark of search for optical EM counterparts for gravitational wave events from mergers (Barnes & Kasen 2013; Tanaka & Hotokezaka 2013), (see Fernández & Metzger 2015; Tanaka 2016, for recent reviews).”
J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817 (2017, background): “…by guest on 26 July 2018 elements in the Universe as emission properties reflect the ejected mass and abundances of r-process elements (e.g., Kasen et al. 2013; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013; Tanaka et al. 2014; Metzger & Fernández 2014; Kasen et al. 2015).”
Impact of ejecta morphology and composition on the electromagnetic signatures of neutron star mergers (2017, methodology): “…we do not apply the expansion opacity formalism (Karp et al. 1977; Eastman & Pinto 1993), which employs the Sobolev approximation (Sobolev 1960) and is adopted in previous detailed macronova radiative transfer calculations (Barnes & Kasen 2013; Kasen et al. 2013; Tanaka & Hotokezaka 2013).”
PRODUCTION OF ALL THE r-PROCESS NUCLIDES IN THE DYNAMICAL EJECTA OF NEUTRON STAR MERGERS (2014, background): “…“kilonova” emission from the r-processed ejecta can be a promising electromagnetic counterpart to the gravitational-wave signal from a CBM event (Li & Paczyński 1998; Metzger et al. 2010; Goriely et al. 2011; Kasen et al. 2013; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013; Grossman et al. 2014).”
The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars (2017, background): “…of newly synthesized heavy elements in the KN/MN ejecta, particularly lanthanides and actinides, will render them faint in the optical, with emission instead appearing primarily in the near-infrared on timescales of several days (Barnes & Kasen 2013; Kasen et al. 2013; Tanaka & Hotokezaka 2013).”
The cocoon emission - an electromagnetic counterpart to gravitational waves from neutron star mergers (2017, background): “Two representative values that are discussed in the literature are κ = 10 cm2 gr−1 (Kasen et al. 2013; Tanaka & Hotokezaka 2013) for material containing Lanthanides and Actinides, and κ = 1 cm2 gr−1 for lighter heavy elements of mass numbers A < 130 (Martin et al. 2015).”
The Combined Ultraviolet, Optical, and Near-infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications (2017, background): “…UVOIR emission is due at least in part to a kilonova, a quasi-thermal transient powered by the radioactive decay of newly synthesized r-process nuclei and isotopes (Li & Paczyński 1998; Metzger et al. 2010; Roberts et al. 2011; Metzger & Berger 2012; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013).”
Neutrino-driven winds from neutron star merger remnants (2014, result): “The properties of this second detection are close to model predictions (Barnes & Kasen 2013; Kasen, Badnell & Barnes 2013; Tanaka & Hotokezaka 2013; Grossman et al. 2014; Rosswog et al. 2014a; Tanaka et al. 2014) for the so-called ‘macro-’ or ‘kilonovae’ (Li & Paczyński 1998; Kulkarni 2005;…”
BRIGHT “MERGER-NOVA” FROM THE REMNANT OF A NEUTRON STAR BINARY MERGER: A SIGNATURE OF A NEWLY BORN, MASSIVE, MILLISECOND MAGNETAR (2013, background): “…nuclear synthesis, radiative transfer, etc (e.g., Kulkarni 2005; Rosswog 2005; Metzger et al. 2010; Goriely et al. 2011; Roberts et al. 2011; Barnes & Kasen 2013; Bauswein et al. 2013; Grossman et al. 2013; Piran et al. 2013; Rosswog et al. 2013; Takami et al. 2013; Tanaka & Hotokezaka 2013).”
Kilonova from post-merger ejecta as an optical and near-infrared counterpart of GW170817 (2017, methodology): “Then, radioactive decays of r-process nuclei powers EM emission especially in the optical and near-infrared wavelengths (Li & Paczyński 1998; Kulkarni 2005; Metzger et al. 2010; Kasen et al. 2013; Barnes & Kasen 2013; Tanaka & Hotokezaka 2013).”
Multi-messenger observations of a binary neutron star merger (2017, methodology): “…kilonova; Li & Paczyński 1998; Kulkarni 2005; Rosswog 2005; Metzger et al. 2010; Roberts et al. 2011; Barnes & Kasen 2013; Kasen et al. 2013; Tanaka & Hotokezaka 2013; Grossman et al. 2014; Barnes et al. 2016; Tanaka 2016; Metzger 2017) due to radioactive decay of rapid neutron-capture…”
Kilonova light curves from the disc wind outflows of compact object mergers (2014, background): “…rise to an optical/infrared transient similar to, but dimmer and briefer than, an ordinary supernova (Li & Paczyński 1998; Kulkarni 2005; Metzger et al. 2010; Roberts et al. 2011; Barnes & Kasen 2013; Piran, Nakar & Rosswog 2013; Tanaka & Hotokezaka 2013; Grossman et al. 2014; Tanaka et al. 2014).”