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Faint rapid red transients from Neutron star -- CO white-dwarf mergers

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Yossef Zenati, Alexey Bobrick, Hagai B. Perets

Solar and Stellar Astrophysics · Submitted 28 Aug 2019 · 52 references

Pith's verdict: First real radiative-transfer predictions for NS-WD mergers, with an honest but easily-quoted caveat that the peak brightness is an upper limit.

T0 · AI review

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arXiv ID1908.10866 (version 3)
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CitationAlexey Bobrick, Hagai B. Perets, Yossef Zenati. "Faint rapid red transients from Neutron star -- CO white-dwarf mergers." arXiv:1908.10866 (2019). https://pith.science/paper/G4I3A3XT
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The paper

The paper claims

When a neutron star shreds and accretes a carbon-oxygen white dwarf, the merger should appear as a faint, fast-fading red transient far dimmer than any type-Ia supernova.

The reading

This paper argues that neutron star-carbon-oxygen white dwarf mergers are a previously unrecognized source of observable explosive transients: faint, rapidly evolving reddened transients (RRTs). Combining 2D and 3D hydrodynamics with nuclear post-processing and radiative-transfer models, it predicts absolute B-band peaks of roughly -11 to -13 and R/I-band peaks of roughly -13 to -15, with fast declines (B fading 3-4 magnitudes in 15 days). Those numbers place RRTs far below ordinary and peculiar type-Ia supernovae, which argues that they form a distinct class. The same models suggest a secondary, months-long red/infrared afterglow from slower disk-wind ejecta, and rate estimates put 10-70 RRT detections per year within LSST's reach.

What carries the argument

The prediction is carried by a three-stage modeling pipeline: 2D FLASH adaptive-mesh simulations of the debris disk and 3D SPH (smoothed particle hydrodynamics) simulations of the full merger; post-processing with a 125-isotope MESA network to obtain nuclear yields; and mapping the resulting ejecta onto the 1D velocity grid of the SuperNu radiative-transfer code, which solves special-relativistic transport with implicit Monte Carlo and discrete-diffusion Monte Carlo methods. The physically load-bearing quantity is the tiny $^{56}$Ni yield, which sets the luminosity scale; because gravitational energy, not nuclear burning, drives the outflows, the transient light curve is essentially the radioactive decay curve of a small nickel mass.

Extended reading notes

Core claim

In all five simulated merger models, the disruption of a CO white dwarf by a neutron star yields only a few times $10^{-3}\,M_\odot$ of $^{56}$Ni, because most of the shredded star remains gravitationally bound and only a fast, unbound fraction expands homologously and radiates. That small nickel mass powers a transient peaking at $B$ about $-10.9$ to $-12.8$, $R$ about $-13.2$ to $-14.7$, and $I$ about $-13.9$ to $-15$, declining by 3-4 magnitudes in $B$ and 1.8-2.3 magnitudes in $I$ within 15 days. The spectra are red, show strong silicon lines, and contain no hydrogen or helium, making RRTs fainter and redder than rapidly evolving transients such as SN2010x and SN2005ek. Applying LSST's single-visit limits to a population-synthesis merger rate, the paper predicts 10-70 detections per year out to roughly 250 Mpc, with ZTF expected to see only 0.05-0.3 per year.

Load-bearing premise

The predicted brightness depends on a substantial part of the shredded white dwarf being thrown outward at high speed; the simulations show most material remains bound, so if that fast component is small the flashes would be far fainter or absent.

Editorial extensions

If this is right

  • LSST should catch RRTs in R/I bands at 10-70 per year out to about 250 Mpc if the fast-ejecta fraction is as large as the FLASH models suggest.
  • Shallower surveys should mostly miss them: the expected ZTF rate is only 0.05-0.3 per year, matching the current absence of detections.
  • Each event may be followed by a months-long, 1-2 magnitude dimmer red/infrared afterglow from slower disk-wind ejecta.
  • RRTs should preferentially occur in late-type host galaxies, since the merger delay-time distribution peaks at hundreds of Myr to about a Gyr.
  • An RRT accompanied by an ultra-long, faint gamma-ray burst would be a nearly unambiguous fingerprint of a neutron star-white dwarf merger.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If RRTs are real, they fill a gap in the transient luminosity function between classical supernovae and the faintest known explosions, implying a larger population of sub-solar-nickel thermonuclear events than currently catalogued.
  • The predicted rate is testable in the first few LSST seasons; a null result would not rule out the merger channel but would push the fast-ejecta fraction down, making the transients fainter and rarer.
  • Existing ZTF and archival survey data could be re-searched for the specific fingerprint of very red, fast, silicon-rich transients with no H/He, which would distinguish RRTs from other faint fast events.
  • The same pipeline could be extended to black hole-white dwarf mergers, where the deeper gravitational potential would alter disk temperatures and likely reduce nickel yields further.
Original abstract

Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. We use 3D hydrodynamical (SPH) simulations, as well as 2D hydrodynamical-thermonuclear simulations (using the FLASH AMR code) to model the disruption of CO-WDs by NSs, which produce faint transient events. We post-process the simulations using a large nuclear network and make use of the SuperNu radiation-transfer code to predict the observational signatures and detailed properties of these transients. We calculate the light-curves (LC) and spectra for five models of NS - CO-WD mergers. The small yields of Ni56 (few x 0.001Msun) results in faint, rapidly-evolving reddened transients (RRTs) with B (R) - peak magnitudes of ~ -12 (-13) to ~ -13 (-15), much shorter and fainter than both regular and faint/peculiar type-Ia SNe. We show that the spectra of RRTs share some similarities with rapidly - evolving transients such as SN2010x, though RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of ~ 10 - 70 yr^-1, through observations in the R/I bands.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 5,895 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.26 · human participants 0.01 · controlled comparison 0.26 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.89 · mentions code or data availability 0.03.

The abstract, sentence by sentence

arXiv listing abstract v3. Results and the main claim are underlined; .

Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. We use 3D hydrodynamical (SPH) simulations, as well as 2D hydrodynamical-thermonuclear simulations (using the FLASH AMR code) to model the disruption of CO-WDs by NSs, which produce faint transient events. We post-process the simulations using a large nuclear network and make use of the SuperNu radiation-transfer code to predict the observational signatures and detailed properties of these transients. We calculate the light-curves (LC) and spectra for five models of NS - CO-WD mergers. The small yields of Ni56 (few x 0.001Msun) results in faint, rapidly-evolving reddened transients (RRTs) with B (R) - peak magnitudes of ~ -12 (-13) to ~ -13 (-15), much shorter and fainter than both regular and faint/peculiar type-Ia SNe. We show that the spectra of RRTs share some similarities with rapidly - evolving transients such as SN2010x, though RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of ~ 10 - 70 yr^-1, through observations in the R/I bands.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. background 1.00 0.09 0.10 hedged possibility 1.00
s1 We use 3D hydrodynamical (SPH) simulations, as well as 2D hydrodynamical-thermonuclear simulations (using the FLASH AMR code) to model the disruption of CO-WDs by NSs, which produce faint transient events. method 0.98 0.96 0.07 asserted 0.99
s2 We post-process the simulations using a large nuclear network and make use of the SuperNu radiation-transfer code to predict the observational signatures and detailed properties of these transients. method 0.99 0.94 0.03 asserted 0.96
s3 We calculate the light-curves (LC) and spectra for five models of NS - CO-WD mergers. method 0.76 0.96 0.05 asserted 0.99
s4 The small yields of Ni56 (few x 0.001Msun) results in faint, rapidly-evolving reddened transients (RRTs) with B (R) - peak magnitudes of ~ -12 (-13) to ~ -13 (-15), much shorter and fainter than both regular and faint/peculiar type-Ia SNe. result 0.99 0.90 0.70 asserted 1.00
s5 We show that the spectra of RRTs share some similarities with rapidly - evolving transients such as SN2010x, though RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. result 0.91 0.92 0.12 asserted 1.00
s6 We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of ~ 10 - 70 yr^-1, through observations in the R/I bands. result 0.68 0.89 0.19 hedged possibility 0.81

What kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
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not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Snapshots for density distribution from the SPH simulation of model D. The left column shows the density in the horizontal x-y plane, and the right column shows the density distribution in the vertical plane xcorot-z, corotating with the bulk of the white dwarf mass and centered on the NS. The snapshots are taken at 34.4 s, 111.7 s and 303.6 s from the beginning of the simulation. The simulation starts with the donor relaxed into the state when the mass-transfer stream has already developed. As the mass-transfer rate grows over time, the donor gets increasingly deformed by the tidal…
Figure 2. The evolution of the WD debris for model E based on FLASH simulations. The panels show the colour coded density distribution and velocity fields (black arrows) throughout the simulation. The velocity scale is 4500 km · sec−1 . Magenta lines show the surfaces of constant temperature. MNRAS 000, 1–11 (2019)
Figure 3. The bolometric lightcurves. Top: The bolometric lightcurves for all models based on the FLASH and SPH sim￾ulations. The subplot shows the light curves for all the models based on the FLASH simulations. Bottom: Comparison of the bolometric light curves from the SPH and the FLASH models for models D and E. 3 RESULTS 3.1 Merger dynamics We infer the three-dimensional picture of the merger and its long-term behaviour from our SPH simulations (see
Figure 4. The UBVRI light curve (LC) for model D (left) and model E (right). The top panels show the results from the ra￾diative transfer processing of the SPH simulations. The bottom panels show for the same models from based 2D FLASH simula￾tions. SPH modelling show that the mergers produce only up to ∼ 10−3 − 10−2 M of intermediate or even iron group ele￾ments. In particular, the amount of 56Ni is always at most 10−2 M . The bulk of the nuclear material is produced in the developed disc stages, well represented both by SPH and FLASH. The total released nuclear energy, as reported from the…
Figure 6. Light curve (top panel), BVRI bands (middle panel) and spectra (bottom panel) for the largest possible secondary transient for model D, in which all the material is ejected as a slow wind v = 400 km/s, and a 1-σ spread of 200 km/s. The upper panel: the light curve zoom on the first 240 days, the inset figure shows the light curve for up to 372 days. The middle panel: lightcurves in the B, V, R, I bands. U band is not shown since its luminosity is below mag −8. The lower panel: the spectra at −1, 50, 100, 150 days after peak. The secondary red transient is expected to have a wide…
Figure 7. Nuclear yields from the FLASH and the SPH simulations (star and box symbols, correspondingly), with the bars showing the difference between the SPH and the FLASH yields. The yields for 17 elements resulting from nuclear post-processing of models D, E and F show an overall good agreement: the abundances of most alpha-elements, including 56Ni are within 1 dex from the two codes, and the overall abundance patterns are similar. 4 DISCUSSION AND SUMMARY In our study, we explored the observable manifestation of transients from NS-WD mergers. We have made use of AMR and SPH hydrodynamical…
Figure 8. Comparison of the R-band lightcurves of SN2010x and SN2005ek with the modelled lightcurves based on the FLASH runs. While sharing similar durations, the rapid red transients (RRTs) resulting form NS-CO WD mergers show much fainter peak luminosities. type Ia SNe such as Ca-rich 2005E-like SNe (Perets et al. 2010) or SN 2008ha-like SNe (Foley et al. 2009) are still much brighter in the blue bands, are not as red and decline much slower than our predicted RRTs. Transients from NS-WD mergers may, therefore, repre￾sent a completely different class of SNe that might be ob￾served mostly in…

Showing 7 of the paper's 8 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1908.10866 by the authors.

References
How the literature uses it

5 arXiv papers cite this in 5 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 1 sentence without a confident label

Contests it 1 paper
Cites it as support 1 paper
  • Much of the escaping material is made up of unburned matter entrained into the outflow, so C/O WD disruptions are unlikely to produce CaSTs due to the high O abundances in the ejecta [ref] .

    The Progenitors of Calcium-Strong Transients · Tidal disruptions of white dwarfs by neutron stars · 0.97

Cites it as background 2 papers

What it cites

Uses data or values from 2 papers
  • By using the galactic merger rate of $1-2·10^-4 yr^-1$ from our population synthesis studies of NS-WD mergers [ref] , the galactic blue luminosity of $1.7·10^10L_B,⊙$ [ref] and the blue luminosity for the local universe $1.98·10^8 L_B,⊙/Mpc^3$ [ref] , we find that LSST will detect between $10$ and $70$ transients per year within its field of view.

    The demographics of neutron star - white dwarf mergers: rates, delay-time distributions and progenitors · Rate estimates · 1.00

  • With the $5$-$σ$ magnitude limits for single-visit detections of $24.0$ and $24.7$ in I and R bands, respectively [ref] , LSST will be able to detect such transients within $15$ days past the peak up to a distance of $250 Mpc$.

    LSST Science Book, Version 2.0 · Rate estimates · 0.98

Cites as support 1 paper
Cites as background 1 paper

Discussion

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