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Blitzar

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In astronomy, blitzars are a hypothetical type of neutron star, specifically pulsars that can rapidly collapse into black holes if their spinning slows down. Heino Falcke and Luciano Rezzolla^[1] proposed these stars in 2013 as an explanation for fast radio bursts.^[2]

Overview

[edit]

These stars, if they exist, are thought to start from a neutron star with a mass that would cause it to collapse into a black hole if it were not rapidly spinning. Instead, the neutron star spins fast enough so that its centrifugal force overcomes gravity. This makes the neutron star a typical but doomed pulsar whose strong magnetic field radiates energy away and slows its spin.

Eventually the weakening centrifugal force is no longer able to halt the pulsar from collapsing into a black hole. At that moment, part of the pulsar's magnetic field outside the black hole is suddenly cut off from its vanished source. This magnetic energy is instantly transformed into a burst of wide spectrum radio energy.^[5] As of January 2015, seven^[6] radio events detected so far might represent such possible collapses; they are projected to occur every 10 seconds within the observable universe.^[5] Because the magnetic field had previously cleared the surrounding space of gas and dust, there is no nearby material that will fall into the new black hole. Thus there is no burst of X-rays or gamma rays that usually happens when other black holes form.^[5]

If blitzars exist, they may offer a new way to observe details of black hole formation.^[7]

References

[edit]

^ "Afscheidsgroet van een stervende ster" (Press release) (in Dutch). Nijmegen, NL: Radboud University. 4 July 2013. Archived from the original on 10 January 2020. Retrieved 26 July 2015.
^ Heino Falcke; Luciano Rezzolla (2014). "Fast radio bursts: The last sign of supramassive neutron stars". Astronomy & Astrophysics. 562: A137. arXiv:1307.1409. Bibcode:2014A&A...562A.137F. doi:10.1051/0004-6361/201321996. S2CID 32284857.
^ "Mysterious radio flashes may be farewell greetings from massive stars collapsing into black holes". ScienceDaily (Press release). July 2013.
^ "Cosmic radio bursts point to cataclysmic origin". ScienceDaily (Press release). July 2013.
^ ^a ^b ^c Thornton, D.; Stappers, B.; Bailes, M.; Barsdell, B.; Bates, S.; Bhat, N.D.R.; et al. (5 July 2013). "A population of fast radio bursts at cosmological distances". Science. 341 (6141): 53–56. arXiv:1307.1628. Bibcode:2013Sci...341...53T. doi:10.1126/science.1236789. PMID 23828936. S2CID 206548502.
For a popular summary see^[3]^[4]
^ "Extremely short, sharp flash of radio waves from unknown source in the universe, caught as it was happening". ScienceDaily (Press release). 19 January 2015. Retrieved 3 March 2016.
^ Falcke, Heino; Rezzolla, Luciano. "Blitzars: Fast radio bursts from supramassive rotating neutron stars". Astronomy. Nijmegen, NL: Radboud University. Retrieved 8 July 2013.

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A blitzar is a hypothetical astrophysical event in which a supramassive rotating neutron star, supported against gravitational collapse by its rapid spin, succumbs to magnetic braking, leading to a sudden implosion into a black hole and the emission of a bright, millisecond-long radio burst from the snapping of its twisted magnetic field lines.^[1] Proposed in 2013 by astronomers Heino Falcke and Luciano Rezzolla, the blitzar model emerged as a potential explanation for fast radio bursts (FRBs), enigmatic millisecond-duration radio signals detected from cosmological distances with no known repeating counterparts at the time.^[1] These bursts are theorized to arise when the neutron star's rotation slows sufficiently to destabilize it, causing the stellar surface to vanish behind an event horizon while the magnetosphere's reconfiguration accelerates electrons to produce coherent radio emission.^[1] The term "blitzar" combines "blitz" (for the burst's speed) and "black hole" (for the endpoint), highlighting the event's rapid and terminal nature.^[2] Blitzars could serve as unique probes of isolated stellar-mass black hole formation, particularly at high redshifts (z > 0.7), complementing detections via gravitational waves from merging binaries.^[1] While the model aligns with the non-repeating, high-dispersion properties of many FRBs, ongoing observations of repeating bursts and diverse host environments have diversified FRB origin theories, though blitzars remain a viable mechanism for one-off events from supramassive neutron stars born in core-collapse supernovae.^[3]

Definition and Characteristics

Core Concept

A blitzar is a hypothetical astrophysical event in which a supramassive rotating neutron star, often characterized as a pulsar with a mass exceeding the Tolman–Oppenheimer–Volkoff (TOV) limit for non-rotating neutron stars—typically around 2.1 solar masses—but stabilized temporarily by rapid rotation, suddenly collapses into a black hole once its spin slows sufficiently.^[4] This collapse is triggered by mechanisms such as magnetic braking, rendering the star unstable as its rotational support diminishes, leading to a transformation on timescales shorter than 1 millisecond.^[4] The concept was proposed in 2013 by astrophysicists Luciano Rezzolla and Heino Falcke as a potential explanation for fast radio bursts (FRBs), positing that the sudden reconfiguration of the star's magnetic field during the collapse could produce a bright, coherent radio emission observable across cosmological distances.^[4] Unlike stable neutron stars, which remain supported below the TOV limit, blitzars represent an extreme, transient phase where masses approaching 2–3 solar masses push the boundaries of general relativity, highlighting their precarious equilibrium between stellar remnant and compact object formation.^[4] The term "blitzar" is a portmanteau coined by Rezzolla and Falcke, blending "blitz" (German for lightning, evoking the sudden burst) with "star" (referring to the neutron star).^[4]

Physical Properties

Blitzars involve supramassive neutron stars with masses greater than approximately 2.1 solar masses, exceeding the maximum mass for non-rotating neutron stars (around 2.0–2.2 solar masses depending on the equation of state of dense matter, constrained by recent observations), but relying on rotation for stability against immediate collapse into a black hole. Recent observations constrain the non-rotating TOV limit to approximately 2.0–2.2 M⊙, making supramassive configurations up to ~2.3 M⊙ feasible in some models.^[1]^[5] Their rapid rotation, with initial spin periods around 3.8 ms for near-Keplerian configurations (typically 1 to 10 milliseconds), provides centrifugal support that prevents gravitational instability.^[1] These stars possess strong magnetic fields, typically on the order of

10^{12}

G (up to

10^{13}

G in some models), which facilitate pulsar-like emission and drive the gradual spin-down via magnetic dipole radiation. These fields must be rapidly dissipated during the eventual collapse triggered by spin reduction.^[1] With radii of about 10 to 15 km, similar to those of standard neutron stars, these stars achieve extreme densities exceeding nuclear saturation density (

\rho \approx 2.8 \times 10^{14}

g cm

^{-3}

), often reaching central rest-mass densities several times higher in their cores.^[5] Their stability critically depends on the neutron star equation of state (EOS); softer EOS permit higher maximum masses for rotating configurations (up to

\sim

2.5

M_\odot

in some models), allowing supramassive states to persist longer before the spin decreases below the critical threshold.^[1]

Theoretical Formation and Collapse

Spin-Down Mechanism

The spin-down of a supramassive neutron star, which supports its mass exceeding the non-rotating Tolman-Oppenheimer-Volkoff limit through rapid rotation, is primarily driven by magnetic dipole radiation. In this process, the rotating magnetic field of the neutron star emits electromagnetic waves, leading to a loss of rotational energy. The power radiated via this mechanism is given by

P = \frac{2}{3} \frac{B^2 R^6 \Omega^4 \sin^2 \alpha}{c^3},

where

B

is the surface magnetic field strength,

R

is the neutron star radius,

\Omega

is the angular velocity,

\alpha

is the inclination angle between the magnetic and rotation axes, and

c

is the speed of light.^[6] This energy loss causes the spin period to increase gradually, reducing the centrifugal support against gravitational collapse. Additional energy dissipation can occur through gravitational wave emission if the neutron star exhibits asymmetries, such as non-axisymmetric mass distributions or instabilities like r-modes, which further accelerate the spin-down. The characteristic spin-down timescale for such neutron stars, starting from millisecond periods, typically ranges from

10^4

10^6

years, depending on the initial magnetic field strength (around

10^{12}

–

10^{14}

G) and rotation rate.^[6] Collapse becomes inevitable when the angular velocity

\Omega

falls below a critical value where the maximum stable mass for that spin,

M_{\max}(\Omega)

, is less than the star's actual mass; this often approaches the Keplerian limit

\Omega_K = \sqrt{GM/R^3}

ΩK=GM/R3

, at which point rotational kinetic energy can no longer balance gravitational binding.^[6] In binary systems, accretion from a companion star can increase the neutron star's mass over time, hastening the approach to this instability threshold without providing additional rotational support. This gradual evolution culminates in a rapid collapse to a black hole once the spin slows sufficiently.

Collapse Dynamics

The collapse of a supramassive neutron star, or blitzar, into a black hole is triggered by the gradual loss of centrifugal support through spin-down mechanisms, resulting in a sudden implosion on millisecond timescales once the star's rotation rate falls below the critical threshold for stability.^[7] This rapid phase begins with the free-fall timescale of approximately 0.04 milliseconds, culminating in the total collapse duration of 0.15–0.35 milliseconds.^[4] During the dynamics of collapse, matter infalls under extreme gravitational forces, governed by general relativistic hydrodynamic equations that extend the static Tolman-Oppenheimer-Volkoff framework to account for dynamic instability and rotation. The event horizon forms in less than 1 millisecond as the core density surpasses the critical value, with the resulting black hole exhibiting an event horizon radius of approximately 3–5 km for a typical progenitor mass of 1.5–2 solar masses.^[8] Accompanying this infall, the star's anchored magnetic field lines snap violently, generating a near-light-speed shock wave that propagates outward and facilitates energy expulsion.^[4] The energy budget of the collapse is dominated by the reconfiguration of the magnetic field, with gravitational waves and neutrinos contributing minimally to make the event nearly silent in those channels—gravitational wave emission is below detectable thresholds due to the axisymmetric nature of the infall, and post-cooling neutrino losses are negligible at ~10^{50} erg from prior phases.^[4] Magnetic field reconnection releases approximately 10^{40}–10^{42} erg in radio waves over the brief duration, scaling with the square of the poloidal magnetic field strength (typically 10^{12}–10^{13} G). The resultant black hole is a spinning Kerr black hole with a dimensionless spin parameter a ≈ 0.2–0.54, determined by the angular momentum retained from the pre-collapse rotation.^[4] Hypothetical transients during collapse may include brief gamma-ray or X-ray flashes arising from shock heating and pair production in strong magnetic fields, though these are expected to be weak or suppressed for standard field strengths, distinguishing them from broader supernova emissions.

Observational Connections

Link to Fast Radio Bursts

The blitzar model proposes that the sudden collapse of a supramassive neutron star leads to the detachment of its magnetosphere, generating a strong magnetic shock that coherently accelerates electron-positron pairs in the surrounding plasma. This process produces a coherent radio pulse through curvature radiation from the accelerated particles, serving as the final electromagnetic signature before black hole formation.^[7] The emission arises on the dynamical timescale of the collapse, with the shock wave propagating outward and disrupting the corotating magnetosphere anchored to the star.^[7] These bursts exhibit durations of approximately 1 ms, fluences on the order of 10 Jy ms at gigahertz frequencies (such as 1.4 GHz), and peak luminosities reaching up to

10^{42}

erg/s, derived from a small fraction (efficiency

\eta_e \lesssim 0.3\%

) of the magnetospheric energy release, which is powered by the star's initial magnetic field strength of around

10^{12}

G.^[7] The characteristic frequencies fall in the 1-2 GHz range, consistent with observed fast radio bursts (FRBs), due to the plasma frequency and curvature radiation properties in the post-collapse environment.^[7] High dispersion measures (DM) of 100-1000 pc cm

^{-3}

further indicate extragalactic origins at redshifts

z \approx 0.3-1

, aligning with the distances expected for such collapsing systems in distant galaxies.^[7] This mechanism distinguishes blitzars from alternative FRB progenitors, such as magnetar giant flares or cosmic string cusps, by predicting non-repeating, one-off events intrinsically linked to the irreversible formation of a stellar-mass black hole, rather than recurrent magnetospheric activity or topological defects in spacetime.^[7] Unlike magnetar flares, which often involve incoherent thermal or synchrotron emission over longer timescales and may associate with gamma-ray bursts, blitzar emission relies on the unique coherent dynamics of the collapse-induced shock.^[7]

Potential Detections

The blitzar hypothesis was initially motivated by the discovery of non-repeating fast radio bursts (FRBs) between 2007 and 2013, with the first detection reported in 2007 using the Parkes radio telescope, followed by additional events observed by Arecibo and other facilities, prompting theoretical models linking these transients to neutron star collapses.^[7] Among potential candidates, FRB 110523, detected in 2011 by the Green Bank Telescope, stands out due to measurements of high Faraday rotation indicating a strong magnetic field near the source, consistent with the magnetized environment expected in a blitzar collapse at a redshift of approximately z ≈ 0.5.^[9] In contrast, repeating FRBs like FRB 121102, first identified in 2012 and localized in 2017, do not align well with the single-event nature of the blitzar model, though they highlight the diversity of FRB populations.^[10] Advancements in real-time detection capabilities have enhanced prospects for identifying blitzar-like events, exemplified by the rapid localization of FRB 20201124A in 2020 using the Australian Square Kilometre Array Pathfinder (ASKAP) and subsequent follow-up with the Karl G. Jansky Very Large Array (VLA), though its repeating nature precludes a blitzar confirmation. Key telescopes contributing to FRB searches include the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which has detected hundreds of FRBs since 2018, primarily non-localized non-repeaters suitable for blitzar scrutiny; Arecibo, which identified early repeaters before its 2020 collapse; and ASKAP, enabling precise localizations for counterpart searches. However, no direct evidence of black hole formation, such as gravitational waves or X-ray afterglows, has been confirmed post-FRB detection. Efforts to verify blitzars through multi-wavelength counterparts have yielded non-detections, including the absence of gamma-ray emission associated with localized FRBs in searches using Fermi Large Area Telescope data from 2018 to 2022, setting upper limits on gamma-ray fluences below ~10^{-7} erg cm^{-2} for short timescales (e.g., 10 s) for typical FRB events.^[11] Similarly, optical and infrared follow-ups with telescopes like the VLT and Keck have found no transients linked to non-repeating FRBs, with upper limits on optical magnitudes around 22-24, constraining models requiring luminous electromagnetic counterparts.^[12] These limits, combined with low upper bounds on repeat rates for apparently non-repeating sources (less than 1% for fields observed over months), support the blitzar scenario's prediction of one-off emissions but also underscore the challenges in distinguishing it from other progenitors.^[12] As of 2025, ongoing surveys with the MeerKAT telescope in South Africa have localized several FRBs to host galaxies, such as FRB 20210405I in 2021 and more recent events like FRB 20240304B at high redshift (z ≈ 2.15) in 2024, often without detectable neutron star remnants or persistent radio sources, which challenges repeating-FRB models but leaves room for blitzars among non-repeaters.^[13]^[14] These observations, part of broader efforts by CHIME and the Five-hundred-meter Aperture Spherical radio Telescope (FAST), continue to refine search strategies, focusing on dispersion measures and polarization to identify high-magnetization signatures indicative of blitzar candidates.

Astrophysical Implications

Role in Stellar Evolution

Blitzars, defined as supramassive rotating neutron stars, represent a transient phase in the evolution of compact objects, bridging the gap between stable neutron stars and stellar-mass black holes. These objects form during the core-collapse supernovae of massive progenitor stars with initial masses typically in the range of 8–25 solar masses, where the explosion dynamics and rapid rotation inherited from the progenitor allow the remnant to achieve a gravitational mass exceeding the Tolman–Oppenheimer–Volkoff limit for non-rotating configurations (approximately 2–2.2 solar masses, depending on the equation of state), yet remain temporarily stable due to centrifugal support.^[7]^[15] Following their birth as young, rapidly spinning pulsars, blitzars evolve through a spin-down process driven primarily by magnetic braking, gradually losing angular momentum over timescales ranging from thousands to millions of years. In binary systems or dense stellar fields, accretion of material from companions can further increase their mass, pushing them toward hypermassive or unstable configurations that accelerate the onset of collapse. This evolutionary pathway highlights how blitzars integrate into broader stellar lifecycles, transforming post-supernova remnants into black holes and thereby shaping the endpoint distributions of massive star evolution.^[7] Estimates indicate that blitzars arise from a small fraction of core-collapse events, potentially around 3% of those originating from massive stars, making them rare but significant contributors to the neutron star population. In high-density environments such as globular clusters or galactic centers, where dynamical interactions and accretion opportunities are enhanced, the formation probability may be elevated, though overall occurrences remain limited to a minority of neutron stars in these settings.^[7] As intermediaries, blitzars connect the neutron star realm to black holes by ultimately collapsing under their own gravity once rotational support diminishes, producing stellar-mass black holes at the low-mass end of the spectrum (around 2–3 solar masses). This process contributes to the black hole mass function, particularly in populating the elusive gap below 5 solar masses, and may influence the rates of compact object mergers observed by gravitational-wave detectors like LIGO and Virgo, where such lightweight black holes could participate in binary systems with neutron stars or other black holes.^[7]

Research Challenges

One major challenge in identifying blitzars lies in distinguishing their associated fast radio bursts (FRBs) from those produced by other phenomena, such as repeating bursts from magnetars, as both can exhibit similar dispersion measures and durations without clear multi-wavelength counterparts.^[4] Confirming a blitzar origin requires precise localization and follow-up observations across radio, optical, and X-ray bands to detect signatures like supernova remnants or absence of persistent emission, but current telescopes often lack the sensitivity for such faint, transient events at cosmological distances.^[4] Theoretical models of blitzars are heavily dependent on the neutron star equation of state (EOS) at extreme densities, which remains untested experimentally and leads to uncertainties in the maximum stable mass for non-rotating neutron stars (typically estimated at 2.1–2.5 M⊙).^[4] Simulations of the collapse process demand advanced general relativistic hydrodynamics to accurately model the spin-down via magnetic braking and the subsequent reconfiguration of the magnetic field, but variations in initial magnetic field strengths (10^{12}–10^{15} G) and braking timescales (on the order of kiloyears) introduce significant predictive ambiguities.^[4] Observationally, blitzar events are rare, with estimated rates of approximately 6 \times 10^{-5} per year per galaxy based on the fraction of core-collapse supernovae producing supramassive neutron stars (~3%), compounded by their faintness at high redshifts (z > 0.5), making detection challenging with existing arrays.^[4] Next-generation facilities like the Square Kilometre Array (SKA) are expected to increase FRB detection rates by orders of magnitude, enabling better statistical samples to test blitzar predictions against star formation rates.^[7] Alternative explanations for non-repeating FRBs, including young magnetar flares or neutron star–asteroid collisions, compete with the blitzar model and highlight the need for improved localization to rule out local interlopers or identify host galaxies.^[4] For instance, magnetar models can produce similar bright, millisecond bursts without invoking collapse, necessitating polarization measurements of FRBs to probe magnetic field geometries unique to blitzars.^[10] Future progress may come from gravitational wave detectors like LIGO/Virgo, which could capture signals from nearby neutron star collapses, though rates for blitzars are low, approximately 10^{-4}–10^{-5} per year in the Milky Way. Additionally, detailed studies of FRB polarization and spectral properties could reveal evidence of magnetic reconnection during collapse, providing a pathway to confirm blitzars as black hole birth signals. In January 2025, the detection of FRB 20241220A from a galaxy that ceased star formation about 6 billion years ago was suggested to be consistent with a blitzar origin.^[16]

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