arXiv subjects

Rodrigo Nemmen

Publications and source records attributed to Rodrigo Nemmen.

At least 19 recordsLinked to original sources

arXiv2026

$\texttt{GPUmonty}$: A GPU-accelerated relativistic Monte Carlo radiative transfer code

We introduce $\texttt{GPUmonty}$, a CUDA/C-based Monte Carlo radiative transfer code accelerated using graphics processing units (GPUs). $\texttt{GPUmonty}$ derives from the CPU-based code $\texttt{grmonty}$ and offloads the most computationally expensive stages of the calculation -- superphoton generation, sampling, tracking, and scattering -- to the GPU. Whereas $\texttt{grmonty}$ handles photons sequentially, $\texttt{GPUmonty}$ processes large numbers of superphotons concurrently, leveraging the single-instruction, multiple-thread (SIMT) execution model of modern GPUs. Benchmarks demonstrate a speedup of about $12\times$ relative to the original CPU implementation on a single GPU, with runtime limited primarily by register pressure rather than compute or memory bandwidth saturation. We validate the implementation through analytic tests for a optically thin synchrotron sphere, as well as comparisons with $\texttt{igrmonty}$ for scattering synchrotron sphere and GRMHD simulation data. Relative errors remain below a percent level and convergence is consistent with the expected $N_{\rm s}^{-1/2}$ Monte Carlo scaling. By significantly reducing computational costs, GPUmonty enables the extensive parameter space surveys and faster spectra modeling required to interpret horizon-scale observations of supermassive black holes. $\texttt{GPUmonty}$ is publicly available under the GNU General Public License.

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arXiv2025

Spectral Learning of Magnetized Plasma Dynamics: A Neural Operator Application

Fourier neural operators (FNOs) provide a mesh-independent way to learn solution operators for partial differential equations, yet their efficacy for magnetized turbulence is largely unexplored. Here we train an FNO surrogate for the 2-D Orszag-Tang vortex, a canonical non-ideal magnetohydrodynamic (MHD) benchmark, across an ensemble of viscosities and magnetic diffusivities. On unseen parameter settings the model achieves a mean-squared error of $\approx 6 \times 10^{-3}$ in velocity and $\approx 10^{-3}$ in magnetic field, reproduces energy spectra and dissipation rates within $96\%$ accuracy, and retains temporal coherence over long timescales. Spectral analysis shows accurate recovery of large- and intermediate-scale structures, with degradation at the smallest resolved scales due to Fourier-mode truncation. Relative to a UNet baseline the FNO cuts error by $97\%$, and compared with a high-order finite-volume solver it delivers a $25\times$ inference speed-up, offering a practical path to rapid parameter sweeps in MHD simulations.

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arXiv2025

Black hole spectral states revealed in GRMHD simulations with texture memory accelerated cooling

X-ray binaries (XRBs) display spectral state transitions that are accompanied by substantial changes in the hardness, luminosity, and structure of the accretion flow. We developed a GPU-accelerated cooling toolkit for general relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes that uses texture memory for fast retrieval of pre-computed values. The toolkit incorporates bremsstrahlung, synchrotron, inverse Compton radiation and Coulomb collision processes. We implemented our toolkit into a GRMHD code and used it to simulate a magnetically arrested disk in the context of the XRB low/hard state around a Kerr black hole. We explored the mass accretion rate in the $\sim (10^{-6}-0.3) \dot{M}_{\rm Edd}$ range, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate. Our simulations reveal that for low accretion rates ($\dot{M} \lesssim 0.01 \dot{M}_{\rm Edd}$), the flow settles into a geometrically thick, low-density, two-temperature hot accretion flow. At higher accretion rates, the flow turns into a cold single-temperature thin disk at $r_{\rm in} \gtrsim 50 r_g$. Inside, the disk breaks up into single-temperature thin filaments embedded into a two-temperature hot thick flow. Our GPU texture memory accelerated cooling prescription is $3-5$ times faster than the standard radiation M1 closure methods, and $\sim5$ times faster than storing the lookup table in global memory.

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arXiv2025

First X-ray polarimetric view of a Low-Luminosity Active Galactic Nucleus: the case of NGC 2110

Low-Luminosity Active Galactic Nuclei (LLAGN) provides a unique view of Comptonization and non-thermal emission from accreting black holes in the low-accretion rate regime. However, to decipher the exact nature of the Comptonizing corona in LLAGN, its geometry and emission mechanism must be understood beyond the limits of spectro-timing techniques. Spectro-polarimetry offers the potential to break the degeneracies between different coronal emission models. Compton-thin LLAGN provide an opportunity for such spectro-polarimetric exploration in the 2-8 keV energy range using IXPE. In this work, we carry out a spectro-polarimetric analysis of the first IXPE observation, in synergy with a contemporaneous NuSTAR observation, of an LLAGN: NGC 2110. Using 554.4 ks of IXPE data from October 2024, we constrain the 99% upper limit on the Polarization Degree (PD) to be less than 8.3% assuming the corresponding Polarization Angle (PA) to be aligned with the radio jet, and less than 3.6% if in the perpendicular direction. In the absence of a significant PD detection, the PA remains formally unconstrained, yet the polarization significance contours appear to be aligned with the radio jet, tentatively supporting models in which the corona is radially extended in the plane of the disk. We also carry out detailed Monte Carlo simulations using MONK and STOKES codes to test different coronal models against our results and compare the polarization properties between NGC 2110 and brighter Seyferts.

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arXiv2024

NuSTAR Observation of the TeV-Detected Radio Galaxy 3C 264: Core Emission and the Hot Accretion Flow Contribution

3C 264 is one of the few FRI radio galaxies with detected TeV emission. It is a low-luminosity AGN (LLAGN) and is generally associated with a radiatively inefficient accretion flow (RIAF). Earlier multiwavelength studies suggest that the X-ray emission originates from a jet. However, the possibility that the RIAF can significantly contribute to the X-rays cannot be ruled out. In particular, hard X-ray emission $\gtrsim$10 keV has never been detected, making it challenging to distinguish between X-ray models. Here we report a NuSTAR detection up to 25 keV from 3C 264. We also present subpixel deconvolved Chandra images to resolve jet emission down to ~0.2 arcsec from the center of the unresolved X-ray core. Together with a simultaneous Swift observation, we have constrained the dominant hard X-ray emission to be from its unresolved X-ray core, presumably in its quiescent state. We found evidence of a cutoff in the energy around 20 keV, indicating that at least some of the X-rays from the core can be attributed to the RIAF. The Comptonization model suggests an electron temperature of about 15 keV and an optical depth ranging between 4 and 7, following the universality of coronal properties of black hole accretion. The cutoff energy or electron temperature of 3C 264 is the lowest among those of other LLAGNs. The detected hard X-ray emission is at least an order of magnitude higher than that predicted by synchrotron self-Compton models introduced to explain $\gamma$-ray and TeV emission, suggesting that the synchrotron electrons might be accelerated to higher energies than previously thought.

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arXiv2024

Optical and UV Flares from Binary Black Hole Mergers in Active Galactic Nuclei

Stellar mass, binary black hole (BBH) mergers dominates the sources of gravitational wave (GW) events so far detected by the LIGO/Virgo/KAGRA (LVK) experiment. The origin of these BBHs is unknown, and no electromagnetic (EM) counterpart has been undoubtedly associated to any of such GW events. The thin discs of active galactic nuclei (AGNs) might be viable environments where BBHs can form and at the same time produce observable radiation feedback. This paper presents new physically motivated light-curve (LC) solutions for thermal flares driven by the remnant of a BBH merger within the disc of an AGN. Following previous analyses, we consider that the BBH likely creates an under-density cavity in the disc prior to its coalescence. Depending on the merger conditions, the black hole (BH) remnant can leave the cavity, interact with the unperturbed disc, and drive a transient BH wind. The wind expels disc material that expands above and below the disc plane and we consider the emission of these plasma ejections as the EM counterpart to the merger GW. We model the LC of such eruptions as mini supernovae explosions finding that stellar mass merger remnants can drive distinguishable flares in optical and UV bands, with time lags of $\sim 10-100$ days after the GW event, when occurring in AGN with central engines of $\sim 10^{6-8}$ M$_\odot$. The Vera C. Rubin Observatory can potentially detect the optical component of these EM counterparts up to redshifts of $z \sim 0.7 - 1.2$, accordingly. The present model provide constraints on multi-messenger observables, such as the binary effective spin, mass ratio, remnant mass, and the time delay of the EM signature. These constraints are useful to localise flaring AGNs as sources of multi-messenger emission following GW alerts.

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arXiv2023

Universality of coronal properties in accreting black holes across mass and accretion rate

Through their radio loudness, lack of thermal UV emission from the accretion disk and power-law dominated spectra, Low Luminosity AGN (LLAGN) display similarity with the hard state of stellar-mass black hole X-Ray Binaries (BHBs). In this work we perform a systematic hard X-ray spectral study of a carefully selected sample of unobscured LLAGN using archival $NuSTAR$ data, to understand the central engine properties in the lower accretion regime. We analyze the $NuSTAR$ spectra of a sample of 16 LLAGN. We model the continuum emission with detailed Comptonization models. We find a strong anti-correlation between the optical depth and the electron temperature of the corona, previously also observed in the brighter Seyferts. This anti-correlation is present irrespective of the shape of the corona, and the slope of this anti-correlation in the log space for LLAGN (0.68-1.06) closely matches that of the higher accretion rate Seyferts (0.55-1.11) and hard state of BHBs ($\sim$0.87). This anti-correlation may indicate a departure from a fixed disk-corona configuration in radiative balance. Our result, therefore, demonstrates a possible universality in Comptonization processes of black hole X-ray sources across multiple orders of magnitude in mass and accretion rate.

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arXiv2023

Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey

With its capacity to observe $\sim 10^{5-6}$ faint active galactic nuclei (AGN) out to redshift $z\approx 6$, Roman is poised to reveal a population of $10^{4-6}\, {\rm M_\odot}$ black holes during an epoch of vigorous galaxy assembly. By measuring the light curves of a subset of these AGN and looking for periodicity, Roman can identify several hundred massive black hole binaries (MBHBs) with 5-12 day orbital periods, which emit copious gravitational radiation and will inevitably merge on timescales of $10^{3-5}$ years. During the last few months of their merger, such binaries are observable with the Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave mission set to launch in the mid-2030s. Roman can thus find LISA precursors, provide uniquely robust constraints on the LISA source population, help identify the host galaxies of LISA mergers, and unlock the potential of multi-messenger astrophysics with massive black hole binaries.

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arXiv2023

Emergence of hot corona and truncated disk in simulations of accreting stellar mass black holes

Stellar mass black holes in X-ray binaries (XRBs) are known to display different states characterized by different spectral and timing properties, understood in the framework of a hot corona coexisting with a thin accretion disk whose inner edge is truncated. There are several open questions related to the nature and properties of the corona, the thin disk, and dynamics behind the hard state. This motivated us to perform two-dimensional hydrodynamical simulations of accretion flows onto a 10 solar masses black hole. We consider a two-temperature plasma, incorporate radiative cooling with bremmstrahlung, synchrotron and comptonization losses and approximate the Schwarzschild spacetime via a pseudo-Newtonian potential. We varied the mass accretion rate in the range 0.02 <= Mdot/Mdot_Edd <= 0.35. Our simulations show the natural emergence of a colder truncated thin disk embedded in a hot corona, as required to explain the hard state of XRBs. We found that as Mdot increases, the corona contracts and the inner edge of the thin disk gets closer to the event horizon. At a critical accretion rate 0.02 <= Mdot_crit\Mdot_Edd <= 0.06, the thin disk disappears entirely. We discuss how our simulations compare with XRB observations in the hard state.

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arXiv2023

Quenching star formation with low-luminosity AGN winds

We present a simple model for low-luminosity active galactic nucleus (LLAGN) feedback through winds produced by a hot accretion flow. The wind carries considerable energy and deposits it on the host galaxy at kiloparsec scales and beyond, heating the galactic gas thereby quenching star formation. Our model predicts that the typical LLAGN can quench more than $10\%$ of star formation in its host galaxy. We find that long-lived LLAGN winds from supermassive black holes (SMBH) with masses $\geq 10^8 M_{\odot}$ and mass accretion rates $\dot{M} > 10^{-3} \medd \ (0.002 \msun / yr)$ can prevent gas collapse and significantly quench galactic star formation compared to a scenario without AGN, if the wind persists over 1 Myr. For sustained wind production over timescales of 10 Myr or longer, SMBHs with $10^8 M_{\odot}$ or larger masses have important feedback effects with $\dot{M} > 10^{-4} \medd \ (0.0002 \msun / yr)$.

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arXiv2021

The Eddington ratio-dependent changing look events in NGC 2992

We present an analysis of historical multi-wavelength emission of the Changing Look (CL) Active Galactic Nucleus (AGN) in NGC 2992, covering epochs ranging from 1978 to 2021, as well as new X-ray and optical spectra. The galaxy presents multiple Seyfert type transitions from type 2 to intermediate-type, losing and regaining its H$\alpha$ BEL recurrently. In X-rays, the source shows intrinsic variability with the absorption corrected luminosity varying by a factor of $\sim$ 40. We rule out tidal disruption events or variable obscuration as causes of the type transitions and show that the presence and the flux of the broad H$\alpha$ emission line are directly correlated with the 2-10 keV X-ray luminosity (L$_{2-10}$): the component disappears at L$_{2-10} \leq 2.6\times10^{42}$\ergcms, this value translates into an Eddington ratio ($\lambda_{\rm Edd}$) of $\sim$ 1\%. The $\lambda_{\rm Edd}$ in which the BEL transitions occur is the same as the critical value at which there should be a state transition between a radiatively inefficient accretion flow (RIAF) and a thin accretion disk, such similarity suggests that the AGN is operating at the threshold mass accretion rate between the two accretion modes. We find a correlation between the narrow Fe K$\alpha$ flux and $\lambda_{\rm Edd}$, and an anti-correlation between full-width at half maximum of H$\alpha$ BEL and $\lambda_{\rm Edd}$, in agreement with theoretical predictions. Two possible scenarios for type transitions are compatible with our results: either the dimming of the AGN continuum, which reduces the supply of ionising photons available to excite the gas in the Broad Line Region (BLR), or the fading of the BLR structure itself occurs as the low accretion rate is not able to sustain the required cloud flow rate in a disk-wind BLR model.

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arXiv2021

Fermi LAT observations of Sagittarius A*: Imaging Analysis

Sagittarius A* (Sgr A*) -- the supermassive black hole (SMBH) in the center of our galaxy -- has been observed in most of the electromagnetic spectrum, from radio to X-rays. Diffuse gamma-ray emission has been observed around Sgr A* and a gamma-ray point source has been detected coinciding with the SMBH's position, although there is no definitive association between the two. In this work, we have used ~11 years of Fermi Large Area Telescope (LAT) observations of the point source 4FGL J1745.6-2859 and performed a detailed imaging analysis across four energy bands. Our goal is to elucidate the nature of the gamma-ray emission at the Galactic Center (GC) and whether it is associated with the SMBH. We find that the centroid of the emission approaches Sgr A*'s location as the energy increases. Assuming that the gamma-ray point source is located at the GC, we estimate a luminosity of 2.61E36 erg/s in the 100 MeV to 500 GeV energy range. This is consistent with Sgr A*'s bolometric luminosity. Based on the point source properties, we ruled out several potential candidates for its nature and favor a cosmic ray origin either from protons, electrons or both, accelerated by -- or in the vicinity of -- the SMBH. Our results indicate that the point source at the GC is indeed the gamma-ray counterpart of Sgr A* in the GeV range.

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arXiv2021

Deep learning model for multiwavelength emission from low-luminosity active galactic nuclei

Most active supermassive black holes (SMBH) in present-day galaxies are underfed and consist of low-luminosity active galactic nuclei (LLAGN). They have multiwavelength broadband spectral energy distributions (SED) dominated by non-thermal processes which are quite different from those of the brighter, more distant quasars. Modelling the observed SEDs of LLAGNs is currently challenging, given the large computational expenses required. In this work, we used machine learning (ML) methods to generate model SEDs and fit sparse observations of LLAGNs. Our ML model consisted of a neural network and reproduced with excellent precision the radio-to-X-rays emission from a radiatively inefficient accretion flow around a SMBH and a relativistic jet, at a small fraction of the computational cost. The ML method performs the fit $4 \times 10^5$ times faster than previous semianalytic models. As a proof-of-concept, we used the ML model to reproduce the SEDs of the LLAGNs M87, NGC 315 and NGC 4261.

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arXiv2021

Black Hole Weather Forecasting with Deep Learning: A Pilot Study

In this pilot study, we investigate the use of a deep learning (DL) model to temporally evolve the dynamics of gas accreting onto a black hole in the form of a radiatively inefficient accretion flow (RIAF). We have trained a machine to forecast such a spatiotemporally chaotic system -- i.e. black hole weather forecasting -- using a convolutional neural network (CNN) and a training dataset which consists of numerical solutions of the hydrodynamical equations, for a range of initial conditions. We find that deep neural networks seem to learn well black hole accretion physics and evolve the accretion flow orders of magnitude faster than traditional numerical solvers, while maintaining a reasonable accuracy for a long time. For instance, CNNs predict well the temporal evolution of a RIAF over a long duration of $8\times 10^4 GM/c^3$, which corresponds to 80 dynamical times at $r=100 GM/c^2$. The DL model is able to evolve flows from initial conditions not present in the training dataset with good accuracy. Our approach thus seems to generalize well. Once trained, the DL model evolves a turbulent RIAF on a single GPU four orders of magnitude faster than usual fluid dynamics integrators running in parallel on 200 CPU cores. We speculate that a data-driven machine learning approach should be very promising for accelerating not only fluid dynamics simulations, but also general relativistic magnetohydrodynamic ones.

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arXiv2020

The first AI simulation of a black hole

We report the results from our ongoing pilot investigation of the use of deep learning techniques for forecasting the state of turbulent flows onto black holes. Deep neural networks seem to learn well black hole accretion physics and evolve the accretion flow orders of magnitude faster than traditional numerical solvers, while maintaining a reasonable accuracy for a long time.

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arXiv2020

Jet efficiencies and black hole spins in jetted quasars

The mechanisms responsible for the production of relativistic jets from supermassive black holes (SMBHs) accreting at near-Eddington rates are not well-understood. Simple theoretical expectations indicate that SMBHs in quasars accrete via thin discs which should produce at most very weak jets. This is contradicted by observations of powerful jets in flat-spectrum radio quasars (FSRQs). We use gamma-ray luminosities observed with the \emph{fermi} Large Area Telescope as a proxy of the jet power for a population of 154 FSRQs. Assuming typical quasar accretion rates and using black hole mass measurements from a variety of methods, we find a mean jet production efficiency of about 10 per cent for FSRQs, with values as high as 222 per cent. We find that this is consistent with FSRQs hosting moderately thin, magnetically arrested accretion discs around rapidly spinning black holes (BHs). Modeling our observations using general relativistic magnetohydrodynamic (GRMHD) simulations of jets from thin discs, we find an average lower limit of $a_* = 0.59$ for the SMBH spins of FSRQs, with tendency for the spins to decrease as the black hole mass increases. Our results are consistent with the merger-driven evolution of SMBHs. 3 per cent of the sample cannot be explained by current GRMHD models of jet production from Kerr BHs due to the high efficiencies. Along the way, we find a correlation between BH masses and $L_\gamma$ which may be an useful mass estimator in blazar gamma-ray studies.

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arXiv2020

On the physical association of Fermi-LAT blazars with their low-energy counterparts

Associating gamma-ray sources to their low-energy counterparts is one of the major challenges of modern gamma-ray astronomy. In the context of the Fourth Fermi Large Area Telescope Source Catalog (4FGL), the associations rely mainly on parameters as apparent magnitude, integrated flux, and angular separation between the gamma-ray source and its low-energy candidate counterpart. In this work we propose a new use of likelihood ratio and a complementary supervised learning technique to associate gamma-ray blazars in 4FGL, based only on spectral parameters as gamma-ray photon index, mid-infrared colors and radio-loudness. In the likelihood ratio approach, we crossmatch the WISE Blazar-Like Radio-Loud Sources catalog with 4FGL and compare the resulting candidate counterparts with the sources listed in the gamma-ray blazar locus to compute an association probability for 1138 counterparts. In the supervised learning approach, we train a random forest algorithm with 869 high confidence blazar associations and 711 fake associations, and then compute an association probability for 1311 candidate counterparts. A list with all 4FGL blazar candidates of uncertain type associated by our method is provided to guide future optical spectroscopic follow up observations.

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arXiv2020

Gamma-ray observations of low-luminosity active galactic nuclei

The majority of the activity around nearby (z ~ 0) supermassive black holes is found in low-luminosity active galactic nuclei (LLAGN), the most of them being classified as low ionization nuclear emission regions. Although these sources are well studied from radio up to X-rays, they are poorly understood in gamma-rays. In this work we take advantage of the all sky-surveying capabilities of the Large Area Telescope on board Fermi Gamma ray Space Telescope to study the whole Palomar sample of LLAGN in gamma-rays. Precisely, the four radio-brightest LLAGN in the sample are identified as significant gamma-ray emitters, all of which are recognized as powerful Fanaroff-Riley I galaxies. These results suggest that the presence of powerful radio jets is of substantial importance for observing a significant gamma-ray counterpart even if these jets are misaligned with respect to the line of sight. We also find that most of the X-ray-brightest LLAGN do not have a significant gamma-ray and strong radio emission, suggesting that the X-rays come mainly from the accretion flow in these cases. A detailed analysis of the spectral energy distributions (SEDs) of NGC 315 and NGC 4261, both detected in gamma-rays, is provided where we make a detailed comparison between the predicted hadronic gamma-ray emission from a radiatively inefficient accretion flow (RIAF) and the gamma-ray emission from a leptonic jet-dominated synchrotron self-Compton (SSC) model. Both SEDs are better described by the SSC model while the RIAF fails to explain the gamma-ray observations.

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