5 research outputs found
A Unified Picture of Short and Long Gamma-ray Bursts from Compact Binary Mergers
The recent detections of the -s long -ray bursts (GRBs)
211211A and 230307A followed by softer temporally extended emission (EE) and
kilonovae, point to a new GRB class. Using state-of-the-art first-principles
simulations, we introduce a unifying theoretical framework that connects binary
neutron star (BNS) and black hole-NS (BH-NS) merger populations with the
fundamental physics governing compact-binary GRBs (cbGRBs). For binaries with
large total masses , the compact remnant
created by the merger promptly collapses into a BH, surrounded by an accretion
disk. The duration of the magnetically arrested disk (MAD) phase sets the
duration of the roughly constant power cbGRB and could be influenced by the
disk mass, : long cbGRBs such as 211211A are produced by massive disks
(), which form for large binary mass ratio
in BNS or in BH-NS mergers. Once the disk becomes
MAD, the jet power drops with the mass accretion rate as ,
establishing the EE decay. Two scenarios are plausible for short cbGRBs. They
can be powered by BHs with less massive disks, which form for other values.
Alternatively, for binaries with , mergers
should go through a hypermassive NS (HMNS) phase, as inferred for GW170817.
Magnetized outflows from such HMNSs, which typically live for , offer an alternative progenitor for short cbGRBs. The first scenario is
challenged by the bimodal distribution of cbGRB durations and the fact that the
Galactic BNS population peaks at sufficiently low masses that most mergers
should go through a HMNS phase. HMNS-powered jets also more readily account for
other light curve features, from precursor flares to EE characteristics
Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole-Neutron Star Mergers
We present the first numerical simulations that track the evolution of a
black hole-neutron star (BH-NS) merger from pre-merger to
. The disk that forms after a merger of mass ratio
ejects massive disk winds (). We introduce
various post-merger magnetic configurations, and find that initial poloidal
fields lead to jet launching shortly after the merger. The jet maintains a
constant power due to the constancy of the large-scale BH magnetic flux, until
the disk becomes magnetically arrested (MAD), where the jet power falls off as
. All jets inevitably exhibit either excessive luminosity due
to rapid MAD activation when accretion rate is high, or excessive duration due
to delayed MAD activation, compared to typical short gamma-ray burst (sGRBs).
This provides a natural explanation to long sGRBs such as GRB 211211A, but also
raises a fundamental challenge to our understanding of jet formation in binary
mergers. One possible implication being the necessity of higher binary mass
ratios or moderate BH spins to launch typical sGRB jets. For post-merger disks
with a toroidal magnetic field, dynamo processes delay jet launching such that
the jets break out of the disk winds after several seconds. We show for the
first time that sGRB jets with initial magnetization retain
significant magnetization () at , emphasizing
the importance of magnetic processes in the prompt emission. The jet-wind
interaction leads to a power-law angular energy distribution by inflating an
energetic cocoon, whose emission is studied in a companion paper.Comment: For movies of the simulations, see https://oregottlieb.com/bhns.htm
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Hours-long Near-UV/Optical Emission from Mildly Relativistic Outflows in Black Hole–Neutron Star Mergers
Abstract:
The ongoing LIGO–Virgo–KAGRA observing run O4 provides an opportunity to discover new multimessenger events, including binary neutron star (BNS) mergers such as GW170817 and the highly anticipated first detection of a multimessenger black hole–neutron star (BH–NS) merger. While BNS mergers were predicted to exhibit early optical emission from mildly relativistic outflows, it has remained uncertain whether the BH–NS merger ejecta provides the conditions for similar signals to emerge. We present the first modeling of early near-ultraviolet/optical emission from mildly relativistic outflows in BH–NS mergers. Adopting optimal binary properties, a mass ratio of q = 2, and a rapidly rotating BH, we utilize numerical relativity and general relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary’s evolution from premerger to homologous expansion. We use an M1 neutrino transport GRMHD simulation to self-consistently estimate the opacity distribution in the outflows and find a bright near-ultraviolet/optical signal that emerges due to jet-powered cocoon cooling emission, outshining the kilonova emission at early time. The signal peaks at an absolute magnitude of ∼−15 a few hours after the merger, longer than previous estimates, which did not consider the first principles–based jet launching. By late 2024, the Rubin Observatory will have the capability to track the entire signal evolution or detect its peak up to distances of ≳1 Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within ∼200 Mpc. The BH–NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals
Hourslong Near-UV/Optical Emission from Mildly Relativistic Outflows in Black Hole-Neutron Star Mergers
The ongoing LIGO-Virgo-KAGRA observing run O4 provides an opportunity to
discover new multi-messenger events, including binary neutron star (BNS)
mergers such as GW170817, and the highly anticipated first detection of a
multi-messenger black hole-neutron star (BH-NS) merger. While BNS mergers were
predicted to exhibit early optical emission from mildly-relativistic outflows,
it has remained uncertain whether the BH-NS merger ejecta provides the
conditions for similar signals to emerge. We present the first modeling of
early near-ultraviolet/optical emission from mildly-relativistic outflows in
BH-NS mergers. Adopting optimal binary properties, mass ratio of and
rapidly rotating BH, we utilize numerical-relativity and general-relativistic
magnetohydrodynamic (GRMHD) simulations to follow the binary's evolution from
the pre-merger to homologous expansion. We use an M1 neutrino transport GRMHD
simulation to self-consistently estimate the opacity distribution in the
outflows, and find a bright near-ultraviolet/optical signal that emerges due to
jet-powered cocoon cooling emission, outshining the kilonova emission at early
time. The signal peaks at an absolute magnitude of to a few hours
after the merger, longer than previous estimates, which did not consider the
first-principles-based jet launching. By late 2024, the Rubin Observatory will
have the capability to track the entire signal evolution, or detect its peak up
to distances Gpc. In 2026, ULTRASAT will conduct all-sky surveys
within minutes, detecting some of these events within Mpc. BH-NS
mergers with higher mass ratios or lower BH spins would produce shorter and
fainter signals.Comment: For movies of the simulations, see https://oregottlieb.com/bhns.htm
Recommended from our members
Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole–Neutron Star Mergers
Abstract:
We present the first numerical simulations that track the evolution of a black hole–neutron star (BH–NS) merger from premerger to r ≳ 1011 cm. The disk that forms after a merger of mass ratio q = 2 ejects massive disk winds (3–5 × 10−2
M
⊙). We introduce various postmerger magnetic configurations and find that initial poloidal fields lead to jet launching shortly after the merger. The jet maintains a constant power due to the constancy of the large-scale BH magnetic flux until the disk becomes magnetically arrested (MAD), where the jet power falls off as L
j
∼ t
−2. All jets inevitably exhibit either excessive luminosity due to rapid MAD activation when the accretion rate is high or excessive duration due to delayed MAD activation compared to typical short gamma-ray bursts (sGRBs). This provides a natural explanation for long sGRBs such as GRB 211211A but also raises a fundamental challenge to our understanding of jet formation in binary mergers. One possible implication is the necessity of higher binary mass ratios or moderate BH spins to launch typical sGRB jets. For postmerger disks with a toroidal magnetic field, dynamo processes delay jet launching such that the jets break out of the disk winds after several seconds. We show for the first time that sGRB jets with initial magnetization σ
0 > 100 retain significant magnetization (σ ≫ 1) at r > 1010 cm, emphasizing the importance of magnetic processes in the prompt emission. The jet–wind interaction leads to a power-law angular energy distribution by inflating an energetic cocoon whose emission is studied in a companion paper