High Energy Astrophysical Phenomena 20
☆ Stable Collisionless Tori Around Kerr Black Holes
In low luminosity active galactic nuclei like M87$^*$ and Sgr A$^*$, the
accretion flow in the vicinity of the black hole is in the collisionless
regime, meaning that the collisional mean free path of charged particles is
much larger than the dynamic length scales. To properly model the particle
energization and emission from the collisionless accretion flow, a promising
approach is to employ the global general relativistic particle-in-cell
simulations -- a newly developed, fully kinetic, first-principles method.
However, it has been challenging to set up an initial condition that involves
collisionless gas with finite angular momentum. We present, for the first time,
a class of analytic kinetic equilibria of collisionless tori around a Kerr
black hole. We have successfully implemented the collisionless tori in our
GPU-based GRPIC code framework Aperture, and found them to be stable for
hundreds to thousands of dynamical times in 2D axisymmetric simulations when
there is no initial seed magnetic field. These kinetic equilibria serve as
ideal starting points for future studies of the physics of collisionless
accretion and jet launching.
comment: 13 page, 3 figures
☆ Multi-Energy and Multi-Sample Searches for Neutrinos from GW Events
The IceCube Neutrino Observatory at the South Pole detects neutrinos of
astrophysical origin via their interactions with ice. The main array is
optimized for the detection of neutrinos with energies above 1 TeV. A much
smaller infill array, known as IceCube DeepCore, extends the sensitivity down
to a few GeV. Neutrinos observed in both parts of the detector are used for
astrophysical-source searches with multiple messengers. We present two analyses
that follow up archival gravitational wave (GW) events from runs O1 through O3
of LIGO/Virgo/KAGRA. The first analysis uses two neutrino datasets: one with
high-energy tracks and another consisting of low-energy tracks and cascades.
These two neutrino datasets were previously used independently to follow-up GW
events. In the analysis presented here, a combined likelihood search is
performed using both datasets to search for neutrinos coincident with the GW
events across a wide energy range, from a few GeV to several PeV. The second
analysis, for the first time, uses a neutrino-induced cascade sample with
events of energy above ~1 TeV for searches of coincident neutrino-GW emission.
We present results from both analyses and discuss prospects for conducting
these analyses in real time.
comment: Presented at the 39th International Cosmic Ray Conference (ICRC2025)
☆ GRB minimum variability timescales with Fermi/GBM
R. Maccary, C. Guidorzi, A. E. Camisasca, M. Maistrello, S. Kobayashi, L. Amati, L. Bazzanini, M. Bulla, L. Ferro, F. Frontera, A. Tsvetkova
Context. Gamma-ray bursts (GRBs) have traditionally been classified by
duration into long (LGRBs) and short (SGRBs), with the former believed to
originate from massive star collapses and the latter from compact binary
mergers. However, events such as the SGRB 200826A (coming from a collapsar) and
the LGRBs 211211A and 230307A (associated with a merger) suggest that
duration-based classification could be sometimes misleading. Recently, the
minimum variability timescale (MVT) has emerged as a key metric for classifying
GRBs.
Aims. We calculate the MVT, defined as the full width at half maximum (FWHM)
of the narrowest pulse in the light curve, using an independent dataset from
Fermi/GBM and we compare our results with other MVT definitions. We update the
MVT-T90 plane and analyse peculiar events like long-duration merger candidates
211211A, 230307A, and other short GRBs with extended emission (SEE-GRBs). We
also examine extragalactic magnetar giant flares (MGFs) and explore possible
new correlations with peak energy.
Methods. We used the MEPSA algorithm to identify the shortest pulse in each
GRB light curve and measure its FWHM. We calculated the MVT for around 3700
GRBs, 177 of which with spectroscopically known redshift.
Results. SEE-GRBs and SGRBs share similar MVTs (from few tens to a few
hundreds of ms), indicating a common progenitor, while extragalactic MGFs
exhibit even shorter values (from few ms to few tens of ms). Our MVT estimation
method consistently yields higher values than another existing technique, the
latter aligning with the pulse rise time. For LGRBs, we confirmed the
correlations of MVT with peak luminosity and Lorentz factor.
Conclusions. We confirmed that, although MVT alone cannot determine the GRB
progenitor, it is a valuable tool when combined with other indicators, helping
to flag long-duration mergers and distinguish MGFs from typical SGRBs.
comment: Accepted by A&A, 14 pages, 18 figures. Table 1 data will be available
at the CDS
☆ Where are Gaia's small black holes?
Gaia has recently revealed a population of over 20 compact objects in wide
astrometric binaries, while LIGO-Virgo-KAGRA (LVK) have observed around 100
compact object binaries as gravitational-wave (GW) mergers. Despite belonging
to different systems, the compact objects discovered by both Gaia and the LVK
follow a multimodal mass distribution, with a global maximum at neutron star
(NS) masses ($\sim 1$-$2\,M_\odot$) and a secondary local maximum at black hole
(BH) masses $\sim10\,M_\odot$. However, the relative dearth of objects, or
``mass gap," between these modes is more pronounced among the wide binaries
observed by Gaia compared to the GW population, with $9^{+10}_{-6}\%$ of GW
component masses falling between $2.5$--$5\,M_\odot$ compared to $\lesssim5\%$
of Gaia compact objects. We explore whether this discrepancy can be explained
by the natal kicks received by low-mass BHs. GW progenitor binaries may be more
likely to survive natal kicks, because the newborn BH has a more massive
companion and/or is in a tighter binary than Gaia progenitor binaries. We
compare the survival probabilities of Gaia and GW progenitor binaries as a
function of natal kick strength and pre-supernova binary parameters, and map
out the parameter space and kick strength required to disrupt the progenitor
binaries leading to low-mass BHs in Gaia systems more frequently than those in
GW systems.
comment: 20 pages, 4 figures
☆ Carpet-3 300 TeV Photon Event as an Evidence for Lorentz Violation
The detection by the Carpet-3 Group of a 300 TeV photon, observed 4536
seconds after the prompt emission of the historic gamma-ray burst GRB 221009A,
provides unprecedented opportunities to test Lorentz invariance violation (LV)
at energy scales approaching the Planck regime. By analyzing the temporal and
spatial properties of this ultra-high-energy photon in conjunction with
lower-energy photons from other bursts and the same burst, we demonstrate
consistency with subluminal LV scenarios characterized by an energy scale \(
E_{\rm LV} \sim 3 \times 10^{17} \, \rm{GeV} \). This work bridges multi-year
LV studies using GeV-TeV photons and establishes GRB 221009A as a pivotal
laboratory for quantum spacetime phenomenology.
comment: 6 pages, 2 figures
☆ Spontaneous charge separation in accelerating relativistic plasmas
The Stewart-Tolman effect posits that accelerating conductors exhibit both
charge separation and rest-frame electric fields (``inertia of charge''), while
the Ehrenfest-Tolman effect states that acceleration induces temperature
gradients (``inertia of heat''). We study the interplay of these effects in
thermodynamic equilibrium. Specifically, we derive from first principles a
partial differential equation governing the electrothermal stratification of a
fully ionized plasma in equilibrium under irrotational relativistic
accelerations in curved spacetime. We then solve it in two settings: a plasma
enclosed in a uniformly accelerated box, and a plasma shell suspended above a
black hole horizon. The resulting electric fields are found not to depend on
the electric conductivity of the medium.
comment: 12 pages, 4 captioned figures, comments welcome!
☆ Calibration and Performance Validation of the SST-1M Telescopes Using Crab Nebula Observations
SST-1M is a prototype single mirror Small Sized Cherenkov Telescope designed
for very high energy (VHE) gamma-ray astronomy. With a 4 meter primary mirror
and a 5.6 meter focal length, it provides a wide 9 degree optical field of
view, optimized for detecting VHE gamma-rays from 1 TeV to several hundred TeV.
Its focal plane is equipped with DigiCam, a fully digital trigger and readout
camera made of 1296 silicon photomultiplier (SiPM) pixels. The use of SiPM
sensors enables observation under high night sky background (NSB) conditions,
significantly enhancing the instrument's duty cycle and allowing observations
under moonlight.
Currently, two SST-1M telescopes are deployed at the Ond\v rejov Observatory
in the Czech Republic, operating in stereo, at 510 m altitude, to observe
astrophysical sources. This contribution presents the SiPM calibration
procedure and performance validation of the instrument, based on updated
results from Crab Nebula observations.
☆ From collider to cosmic rays: Pythia 8/Angantyr for air shower simulations in CORSIKA 8
The simulation of extensive air showers is pivotal for advancing our
understanding of high-energy cosmic ray interactions in Earth's atmosphere. The
CORSIKA 8 framework is being developed as a modern, flexible, and efficient
tool for simulating these interactions with a variety of high-energy hadronic
models. We present the ongoing implementation and validation of Pythia
8/Angantyr within CORSIKA 8. Pythia 8, successfully used in collider physics,
provides a detailed and well-tested treatment of hadronic interactions, while
the Angantyr model extends its capabilities to describe heavy-ion collisions in
a consistent manner. With the inclusion of Pythia 8, the CORSIKA 8 suite now
enables further tuning possibilities, improving the exploration of hadronic
interactions in air showers.
In this contribution, we compare the capability of Pythia 8/Angantyr to
reproduce fundamental observables of high-energy particle collisions $-$
inelastic cross-sections and multiplicities $-$ to that of several established
high-energy interaction models in air shower simulations. We further compare
the predictions for key air shower properties, including longitudinal shower
development and muon content, for iron-induced shower.
comment: 10 pages, 7 figures, 39th International Cosmic Ray Conference (ICRC
2025) proceeding
☆ Constraining Super-Heavy Dark Matter with the KM3-230213A Neutrino Event
Recently, the KM3NeT collaboration detected an astrophysical neutrino event,
KM3-230213A, with an energy of approximately $220~\rm PeV$, providing
unprecedented insights into the ultra-high-energy Universe. In this study, we
introduce a novel likelihood framework designed to leverage this event to
constrain the properties of super-heavy dark matter (SHDM) decay. Our approach
systematically integrates multi-messenger constraints from galactic and
extragalactic neutrino flux measurements by IceCube, the absence of comparable
neutrino events at IceCube and Auger observatories, and the latest gamma-ray
experiment upper limits. Our findings impose the most stringent constraints to
date, placing a lower bound on the SHDM lifetime at $\gtrsim 5\cdot
10^{29}-10^{30} \rm s$. Importantly, we identify, for the first time, the
significant potential of galactic neutrino flux measurements in advancing dark
matter research. Future investigations targeting astrophysical neutrinos
originating from the Galactic Center at energies above $10~\rm PeV$ will be
crucial, not only for understanding the origin of the cosmic-ray knee but also
for exploring the possible contributions of super-heavy dark matter to our
Universe.
comment: 7 pages included the appendix. 4 Figures. Comments are welcome
☆ Evidence for optical pulsations from a redback millisecond pulsar
A. Papitto, F. Ambrosino, M. Burgay, R. La Placa, C. J. Clark, C. Ballocco, G. Illiano, C. Malacaria, A. Miraval Zanon, A. Possenti, L. Stella, A. Ghedina, M Cecconi, F. Leone, M. Gonzalez, H. Perez Ventura, M. Hernandez Diaz, J. San Juan, H. Stoev
Recent detections of optical pulsations from both a transitional and an
accreting millisecond pulsar have revealed unexpectedly bright signals,
suggesting that the presence of an accretion disk enhances the efficiency of
optical emission, possibly via synchrotron radiation from accelerated
particles. In this work, we present optical observations of the redback
millisecond pulsar PSR J2339-0533, obtained with the SiFAP2 photometer mounted
on the Telescopio Nazionale Galileo. Data accumulated during the campaign with
the longest exposure time (12 hr) suggest that its $\sim$18 mag optical
counterpart exhibits pulsations at the neutron star's spin frequency. This
candidate signal was identified by folding the optical time series using the
pulsar ephemeris derived from nearly simultaneous observations with the 64-m
Murriyang (Parkes) radio telescope. The detection significance of the candidate
optical signal identified in those data lies between 2.9 and 3.5 $\sigma$,
depending on the statistical test employed. The pulsed signal has a duty cycle
of $\approx 1/32$, and the de-reddened pulsed magnitude in the V band is $(26.0
\pm 0.6)$ mag. At a distance of 1.7 kpc, this corresponds to a conversion
efficiency of $\sim 3 \times 10^{-6}$ of the pulsar's spin-down power into
pulsed optical luminosity, comparable to values observed in young, isolated
pulsars like the Crab, but 50-100 times lower than in disk-accreting
millisecond pulsars. If confirmed, these findings suggest that optical
pulsations arise independently of an accretion disk and support the notion that
such disks boost the optical emission efficiency.
comment: submitted to A&A Letters, 5 pages, 6 figures
☆ CORSIKA 8: A modern and universal framework for particle cascade simulations
CORSIKA 8 represents a significant update in the simulation of particle
showers, building on the well-established foundation of CORSIKA 7. It has been
entirely rewritten as a modular and modern C++ framework, addressing the
limitations of its predecessor to provide a flexible platform designed to
satisfy current and novel use cases. This allows for application beyond pure
air-shower scenarios such as cross-media particle cascades and an advanced
calculation of the radio emission. A first official "physics-complete" version
has already been released that supports the treatment of hadronic interactions
with Sibyll 2.3d, QGSJet-II.04, and EPOS-LHC and the treatment of the
electromagnetic cascade with PROPOSAL 7.6.2. In this presentation, we will
discuss the design principles, current functionality, and validation efforts of
CORSIKA 8, emphasizing its potential applications for future experiments.
comment: Presented at 7th International Symposium on Ultra High Energy Cosmic
Rays (UHECR2024)
☆ Challenging a binary neutron star merger interpretation of GW230529
GW230529_181500 represented the first gravitational-wave detection with one
of the component objects' mass inferred to lie in the previously hypothesized
mass gap between the heaviest neutron stars and the lightest observed black
holes. Given the expected maximum mass values for neutron stars, this object
was identified as a black hole, and, with the secondary component being a
neutron star, the detection was classified as a neutron star-black hole merger.
However, due to the low signal-to-noise ratio and the known waveform degeneracy
between the spin and mass ratio in the employed gravitational-wave models,
GW230529_181500 could also be interpreted as a merger of two heavy ($\gtrsim 2
\mathrm{M}_\odot$) neutron stars with high spins. We investigate the
distinguishability of these scenarios by performing parameter estimation on
simulated signals obtained from numerical-relativity waveforms for both neutron
star-black hole and binary neutron star systems, with parameters consistent
with GW230529_181500, and comparing them to the analysis of the real event
data. We find that GW230529_181500 is more likely to have originated from a
neutron star-black hole merger, though the possibility of a binary neutron star
origin can not be ruled out. Moreover, we use the simulation data to estimate
the signatures of potential electromagnetic counterparts emitted by the
systems. We find them to be too dim to be located by current wide-field surveys
if only the dynamical ejecta is considered, and detectable by the Vera C. Rubin
Observatory during the first two days after merger if one accounts for
additional disk wind ejecta.
comment: 20 pages, 9 figures
☆ Pushchino multibeam pulsar search. VII. The results of the timing of 12 slow pulsars
We have performed timing of a number of known slow pulsars with poorly known
coordinates and parameters of their intrinsic rotation. We used data from the
archive of round-the-clock monitoring observations on the third (stationary)
beam pattern of the Large Phased Array radio telescope (LPA LPI) at a frequency
of 111 MHz, which has an unsatisfactory connection of the local quartz time
standards to the reference scale (UTC). To compensate for the resulting errors,
we applied an algorithm previously developed by us, which uses Pulsar Timescale
as an intermediate reference scale to compute corrections to the pulses Times
of Arrival (TOAs) measured by the local clocks and to switch to UTC. Analyzing
a ten-year observational data set we substantially refined the rotational and
astrometric parameters of 12 pulsars. The spin frequencies $\nu$ and their
first derivatives $\dot\nu$ were determined with accuracies of $10^{-10}$ Hz
and $10^{-19}$ s$^{-2}$, respectively, which is 5-6 orders of magnitude better
than the values quoted in the catalogue. The coordinates are determined with
accuracies ranging from units to tens of arcseconds.
comment: 15 pages, 3 figures, submitted to Astronomy Reports, translated by AI
with correction of scientific lexis
☆ Possible detection of HFQPOs associated with 'unknown' variability class of GRS 1915+105
We present a comprehensive spectro-temporal analysis of GRS $1915+105$
observed with AstroSat during June, $2017$. A detailed study of the temporal
properties reveals the appearance of an `unknown' variability class ($\tau$)
during $\rho \rightarrow \kappa$ class transition of the source. This new
`unknown' class ($\tau$) is characterized by the irregular repetition of low
count `dips' along with the adjacent `flare' like features in between two
successive steady count rate durations, resulting in uniform `$C$' shaped
distribution in the color-color diagram. A detailed comparative study of the
variability properties between the $\tau$ class and other known variability
classes of GRS $1915+105$ indicates it as a distinct variability class of the
source. Further, we find evidence of the presence of possible HFQPO features at
$\sim 71$ Hz with quality factor $\sim 13$, rms amplitude $\sim 4.69\%$, and
significance $3\sigma$, respectively. In addition, a harmonic-like feature at
$\sim 152$ Hz is also seen with quality factor $\sim 21$, rms amplitude $\sim
5.75\%$ and significance $\sim 4.7\sigma$. The energy-dependent power spectral
study reveals that the fundamental HFQPO and its harmonic are present in $3-15$
keV and $3-6$ keV energy ranges, respectively. Moreover, the wide-band
($0.7-50$ keV) spectral modelling comprising of thermal Comptonization
component indicates the presence of a cool ($kT_{\rm e}\sim 1.7$ keV) and
optically thick (optical depth $\sim 14$) Comptonizing `corona', which seems to
be responsible in regulating the HFQPO features in GRS $1915$+$105$. Finally,
we find the bolometric luminosity ($L_{\rm bol}$) to be about $42\% L_{\rm
Edd}$ within $1-100$ keV, indicating the sub-Eddington accretion regime of the
source.
comment: 14 pages, 9 figures, 2 tables, Comments welcome
☆ Accretion is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway
GW231123 represents the most massive binary-black-hole merger detected to
date, lying firmly within, or even above, the pair-instability mass gap. The
component spins are both exceptionally high ($a_1 = 0.90^{+0.10}_{-0.19}$, $a_2
= 0.80^{+0.20}_{-0.51}$), which is difficult to explain with repeated mergers.
Here we show that the black hole spin vectors are closely aligned with each
other while significantly tilted relative to the binary's orbital angular
momentum, pointing to a common accretion-driven origin. We examine
astrophysical formation channels capable of producing near-equal, high-mass,
and mutually aligned spins consistent with GW231123 -- particularly binaries
embedded in AGN disks and Pop~III remnants, which grew via coherent misaligned
gas accretion. We further argue that other high-mass, high-spin events, e.g.,
GW190521 may share a similar evolutionary pathway. These findings underscore
the critical role of sustained, coherent accretion in shaping the most extreme
black hole binaries.
comment: 7 pages, 1 figure
☆ Optical emission from luminous and very soft X-ray sources in nearby galaxies: testing the scenario of edge-on supercritical accretion systems
Supercritical accretion onto compact objects is expected to drive optically
thick winds, resulting in observed X-ray emission as a function of viewing
angle. However, their optical emission, either from the outer accretion disk or
companion surface tends to be nearly isotropic. Based on a sample of luminous
and very soft X-ray sources that are argued to be supercritical accretion
systems viewed close to edge-on, we identified the optical counterparts for
some of them and compared the optical properties with those of ultraluminous
X-ray sources (ULXs), which are supposed to be supercritical accretion systems
viewed close to face-on. The optical luminosity is found in a wide range, with
the absolute visual magnitude ranging from dimmer than -1.2 in some sources to
about -7 in one case. Most sources show a power-law like spectrum while four of
them display a blackbody shape. One of them shows an optical spectrum
resembling a B type main sequence, suggesting that it may be a Be white dwarf
system. Strong variability in flux at timescales as short as 10 days are
revealed, indicating that some of these sources are powered by accretion onto
compact objects. These suggest that the luminous and very soft X-ray sources in
nearby galaxies have a diverse population, and some of them are indeed
consistent with emission from supercritical accretion, with consistent optical
magnitudes and colors. Future optical spectroscopic observations are needed to
further constrain their natures.
comment: Accepted for publication in ApJ
☆ Transition to Petschek Reconnection in Subrelativistic Pair Plasmas: Implications for Particle Acceleration
While relativistic magnetic reconnection in pair plasmas has emerged in
recent years as a candidate for the origin of radiation from extreme
astrophysical environments, the corresponding subrelativistic pair plasma
regime has remained less explored, leaving open the question of how
relativistic physics affects reconnection. In this paper, we investigate the
differences between these regimes by contrasting 2D particle-in-cell
simulations of reconnection in pair plasmas with relativistic magnetization
($\sigma \gg 1$) and subrelativistic magnetization ($\sigma < 1$). By utilizing
unprecedentedly large domain sizes and outflow boundary conditions, we
demonstrate that lowering the magnetization results in a change in the
reconnection geometry from a plasmoid chain to a Petschek geometry, where
laminar exhausts bounded by slow-mode shocks emanate from a single diffusion
region. We attribute this change to the reduced plasmoid production rate in the
low-$\sigma$ case: when the secondary tearing rate is sufficiently low,
plasmoids are too few in number to prevent the system from relaxing into a
stable Petschek configuration. This geometric change also affects particle
energization: we show that while high-$\sigma$ plasmoid chains generate
power-law energy spectra, low-$\sigma$ Petschek exhausts merely heat incoming
plasma and yield negligible nonthermal acceleration. These results have
implications for predicting the global current sheet geometry and the resulting
energy spectrum in a variety of systems.
comment: 14 pages, 12 figures, submitted to ApJ. Comments welcome
♻ ☆ Analysis and simulations of binary black hole merger spins -- the question of spin-axis tossing at black hole formation
Hans C. G. Larsen, Casper C. Pedersen, Thomas M. Tauris, Ali Sepas, Claudia Larsen, Christophe A. N. Biscio
The origin of binary black hole (BH) mergers remains a topic of active
debate, with effective spins (chi_eff) measured by the LIGO-Virgo-KAGRA (LVK)
Collaboration providing crucial insights. In this study, our objective is to
investigate the empirical chi_eff distribution (and constrain individual spin
components) of binary BH mergers and compare them with extensive simulations,
assuming that they originate purely from isolated binaries or a mixture of
formation channels. We explore scenarios using BH kicks with and without the
effect of spin-axis tossing during BH formation. We employ simple yet robust
Monte Carlo simulations of the final core collapse forming the second-born BH,
using minimal assumptions to ensure transparency and reproducibility. The
synthetic chi_eff distribution is compared to the empirical data from LVK
science runs O1-O3 using functional data analysis, kernel density estimations,
and three different statistical tests, accounting for data uncertainties. We
find strong indications for spin-axis tossing during BH formation if LVK
sources are dominated by the isolated binary channel. Simulations with
spin-axis tossing achieve high p-values (up to 0.882) using Kolmogorov-Smirnov,
Cramer-von Mises, and Anderson-Darling tests, while without tossing, all
p-values drop below 0.001 for isolated binaries. A statistically acceptable
solution without tossing, however, emerges if ~72+/-8% of detected binary BH
mergers result from dynamical interactions causing random BH spin directions.
Finally, for an isolated binary origin, we find a preference for mass reversal
in ~30% of the progenitor binaries. Predictions from this study can be tested
with LVK O4+O5 data as well as the 3G detectors, Einstein Telescope and Cosmic
Explorer, enabling improved constraints on formation channel ratios and the
critical question of BH spin-axis tossing.
comment: New Astronomy, in press (30 pages, incl. 24 figures, 1 table, 2
appendices), small typos fixed and DOI added
♻ ☆ Hybrid Approaches for Black Hole Spin Estimation: From Classical Spectroscopy to Physics-Informed Machine Learning
The measurement of black hole spin is considered one of the key problems in
relativistic astrophysics. Existing methods, such as continuum fitting, X-ray
reflection spectroscopy and quasi-periodic oscillation analysis, have
systematic limitations in accuracy, interpretability and scalability. In this
work, a hybrid approach is proposed in which theoretical models based on the
Teukolsky formalism are integrated with Physics-Informed Neural Networks
(PINNs). A PINN model is developed to solve the linearized spin problem in the
scalar case, with physical constraints directly embedded into the training
process. Annotated data are not required; instead, the model is trained using
the differential operator and boundary conditions as supervision. It is
demonstrated that the PINN converges reliably, with residual loss values below
1e-7 and a root mean squared error (RMSE) of the order of 1e-6 (final approx
5.4 x 1e-8). Benchmarking results indicate that the proposed method outperforms
both classical and data-driven machine learning approaches in terms of AUC and
sensitivity, while also exhibiting superior interpretability, generalizability
and adherence to physical principles, with moderate computational cost.
Potential extensions include integration with general relativistic
magnetohydrodynamics (GRMHD) solvers and application to real observational
data. These findings support the viability of physics-based machine learning as
a robust framework for accurate and interpretable black hole spin estimation.
comment: 8 pages, 4 figures, 4 tables, 4 equations
♻ ☆ So long Kolmogorov: the forward and backward turbulence cascades in a supernovae-driven, multiphase interstellar medium
The interstellar medium (ISM) of disk galaxies is turbulent, and yet the
fundamental nature of ISM turbulence, the energy cascade, is not understood in
detail. In this study, we use high-resolution simulations of a hydrodynamical,
gravitationally stratified, supernova (SNe)-driven, multiphase ISM to probe the
nature of a galactic turbulence cascade. Through the use of velocity flux
transfer functions split into interactions between compressible $\mathbf{u}_c$
and incompressible $\mathbf{u}_s$ modes, we show that there exists a
large-to-small-scale cascade in both $\mathbf{u}_c$ and $\mathbf{u}_s$ when
mediated by an additional $\mathbf{u}_s$ mode. But the $\mathbf{u}_s$ cascade
is highly non-local. Moreover, there is a $\mathbf{u}_c$ mediated component of
the $\mathbf{u}_s$ cascade that proceeds in the opposite direction -- an
inverse cascade from small-to-large scales. The cascade feeds flux into scales
well beyond the scale height, energizing the winds and fueling the direct
cascades. Both the strongly non-local and the inverse $\mathbf{u}_s$ cascades
happen on scales that have a power law $\mathbf{u}_s$ energy spectrum,
highlighting how degenerate the spectrum is to the true underlying physical
processes. We directly show that the inverse cascade comes from $\mathbf{u}_s$
modes interacting with expanding SNe remnants (SNRs) and that $\mathbf{u}_s$
modes are generated to leading order via baroclinic, highly corrugated cooling
layers between warm $(T\lesssim 10^4\,\rm{K})$ and hot $(T\gg10^4\,\rm{K})$ gas
in these SNRs. Finally, we outline a complete phenomenology for SNe-driven
turbulence in a galactic disk, estimate a $10^{-16}\,\rm{G}$ Biermann field
generated from SNR cooling layers, and highlight the strong deviations that
SNe-driven turbulence has from the conventional Kolmogorov model.
comment: Main text 26 pages, 14 figures. Appendix, 7 pages, 8 figures.
Submitted to ApJ. Comments welcome