arXiv subjects

S. Bernal

Publications and source records attributed to S. Bernal.

15 recordsLinked to original sources

arXiv2026

Spatially resolved optical and mid-infrared spectroscopy of SDSS1335+0728: implications for the origin of the Ansky event

The galaxy SDSS1335+0728 brightened abruptly in December 2019 (the Ansky event) and has since been confirmed as the host of extreme X-ray quasi-periodic eruptions (QPEs) of debated origin. We constrain the origin of its transient activity by characterising the galaxy properties and nuclear accretion history with spatially resolved VLT/MUSE and JWST MIRI/MRS spectroscopy. We extract stellar and gas kinematics and emission-line fluxes, construct emission-line ionisation diagnostic maps, reconstruct the nuclear ionisation history via a Balmer-line light-echo analysis, and measure the mid-infrared silicate feature strength. The stellar kinematics reveal two counter-rotating stellar regions and kinematically cold gas ($\sigma_{\rm gas} \lesssim 60$ km s$^{-1}$), consistent with a past minor merger. Stellar populations show an old host with ongoing star formation confined to a ring at intermediate radii. Ionisation diagnostics reveal a three-zone structure: a central region powered by SMBH accretion, where high-ionisation coronal lines ([NeVI]$\lambda7.65\mu$m, [NeV]$\lambda14.32\mu$m, [OIV]$\lambda25.89\mu$m) are confined, a star-forming ring, and a LINER-like outer region. A Balmer-line light-echo analysis yields a minimum ionising luminosity $\log L_{\rm ion,min} \approx 40.5$ erg s$^{-1}$ sustained over at least $\sim 1\,500$ yr. Broad silicate emission at 9.7 and 18$\mu$m indicates optically thin dust, inconsistent with a classical active galactic nucleus (AGN) dusty torus. The data are consistent with two scenarios for the pre-2019 accretion: a persisting or gradually fading low-luminosity AGN, or a long-lived tidal disruption event (TDE) remnant disc. In both, Ansky corresponds to a slow, faint transient in a $\sim\!10^6\,M_{\odot}$ SMBH with already ongoing accretion, challenging the "faded AGN" interpretation proposed for some QPE hosts.

astro-ph.GA↗

arXiv2026

Spectral Handling and Estimation of AGN Parameters (SHEAP), The first AGN fitting GPU-based code

In the coming years, the number of discovered active galactic nuclei (AGN) is expected to increase significantly due to upcoming spectroscopic surveys. This growth will challenge current analysis and modeling techniques, requiring scalable methods for large, heterogeneous datasets with diverse signal-to-noise ratios, spectral resolutions, and host-galaxy contamination. We present SHEAP (Spectral Handling and Estimation of AGN Parameters), a spectral-fitting framework designed to analyze large AGN samples efficiently while preserving physical interpretability, reproducibility, and robust uncertainty estimation. SHEAP uses JAX, a Python GPU-powered framework, to implement a flexible model with modular components, including continuum, host galaxy, FeII pseudo-continuum, and multi-component emission lines, together with parameter tying and physically motivated constraints. By combining gradient-based optimization with automatic differentiation, vectorization, and just-in-time compilation, SHEAP achieves stable convergence in blended regions, such as H$\beta$, while substantially reducing runtime. We compare SHEAP measurements with literature results and public fitting pipelines across four samples covering the CIV, MgII, H$\beta$, and H$\alpha$ regions. We find good agreement for the main AGN spectral parameters, with $\sim85$--$100%$ of objects lying within the $\pm0.3$ dex band and reduced chi-square distributions close to unity. Relative to the runtime reported by \citet{2026Bernal} using \texttt{pPXF}, the fitting stage requires only $\sim1.7%$ of the computational time, corresponding to an improvement of approximately $100$ times. These results show that \texttt{SHEAP} delivers reliable AGN spectral decompositions at substantially lower computational cost, making it suitable for massive spectroscopic datasets.

astro-ph.GA↗

arXiv2026

Robust AGN and host-galaxy decomposition in optical spectral fitting

Unraveling the growth of supermassive black holes and their connection to host galaxies requires disentangling the Active Galactic Nuclei (AGN) emission from that of the stellar populations. When an AGN spectrum is observed at different activity phases, if the spectral decomposition properly recognizes the nuclear and stellar components, key physical properties - such as black-hole mass, stellar mass, and stellar velocity dispersion - should remain consistent. We present a novel optical spectral-fitting approach that combines pPXF and PyQSOFit to robustly decompose spectra into stellar and AGN components. We apply this technique to three SDSS samples with repeated optical spectra of the same objects at z<0.55: 32 changing-look AGN in bright and dim states, and 15 quasars and 15 galaxies with three single-epoch and one stacked spectrum each. To compare with the literature, we use SDSS spectra and photometric data from AGN in the eFEDS field, as well as Gemini and VLT observations of some of our changing-look AGN. We evaluate the reliability of stellar mass, velocity dispersion, and black-hole mass measurements, especially in relation to the AGN-to-total continuum contribution (fAGN). For host-derived properties, especially when fAGN<0.8, our method yields consistent results. For single-epoch black-hole mass estimates from Ha and Hb, 3-sigma confidence in the broad-line flux and FWHM provides effective criteria for selecting reliable measurements. After applying these quality cuts, measurements across different epochs agree within uncertainties, and their reliability is confirmed by the alignment with previously established scaling relations. Many changing-look AGN in our sample exhibit "breathing" broad-line regions, as determined from Ha analysis, while some deviate significantly, suggesting non-virialized systems across the spectral transition.

astro-ph.GA↗

arXiv2024

The Success of Optical Variability in Uncovering AGNs in Low-stellar Mass Galaxies

We used random forest algorithms to classify all objects in a large portion of the sky, using optical light curves obtained, or built from images provided, by the Zwicky Transient Facility (ZTF). We compare different selection sets based on alerts or complete light curves derived from different photometric selection algorithms. The AGN candidates thus selected are cross-matched with objects in the NASA-Sloan Atlas (NSA) of local galaxies, with $M_*<2\times10^{10}M_\odot$. The AGN nature of these candidates is verified and characterized using archival optical spectra from SDSS. We further establish the fraction of candidates with counterparts in the eROSITA data release 1 catalog of X-ray sources. From an initial sample of 506 candidates, 415 have good-quality spectra. Among these 415 objects, we found significant broad Balmer lines in the spectra for $86\%$ (357) of the candidates. When considering BPT classifications, an additional 5 candidates were confirmed, resulting in $87\%$ (362) confirmed candidates. Specifically, broad Balmer lines were detected in $94\%$-$98\%$ of the AGN candidates selected from complete light curves and in $80\%$ of those selected from the less frequent ZTF alerts. The black hole masses estimated from the spectra range from $2.2\times10^6M_\odot$ to $4.2\times10^7M_\odot$, reaching lower values for the candidates selected using the more sensitive light curves. The black hole masses obtained cluster around $0.1\%$ of the stellar mass of the host from the NSA catalog. Two-thirds of the AGN candidates are classified as Seyfert or Composite by their narrow emission line ratios (BPT diagnostics) while the rest are star-forming. Almost all the candidates classified as Seyfert and over $50\%$ of those classified as star-forming have significant BELs. We found X-ray counterparts for $67\%$ of the candidates that fall in the footprint of the eROSITA-DE DR1.

astro-ph.GA↗

arXiv2024

AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system

With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT 2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT 2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every about 60-90 days with amplitudes of around 0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT 2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of about 0.83 mpc and would merge in about 70000 years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.

astro-ph.GA↗

arXiv2024

Probing the rest-frame wavelength dependence of quasar variability : Insights from the Zwicky Transient Facility Survey

Context: Quasar variability can potentially unlock crucial insights into the accretion process. Understanding how this variability is influenced by wavelength is crucial for validating and refining quasar variability models. Aims: This paper aims to enhance the understanding of the dependence of variability on the rest-frame wavelength ($\lambda_{RF}$) by isolating the variance in different timescales in well-defined wavelength bins and examining the corona-heated accretion-disk (CHAR) model. Methods: We investigated the relation between variance and rest-frame wavelength ($\lambda_{RF}$) using optical g- and r-band light curves from the Zwicky Transient Facility (ZTF) Data Release 15 for $\sim 5000$ quasars within narrow ranges of black hole mass ($M_{BH}$) and Eddington ratio ($R_{Edd}$). A spectral model taking into account disk continuum emission, Balmer transitions, Fe II pseudo-continuum emission, and other emission lines is necessary to best interpret the variance spectrum. Results: Our analysis indicates a strong anticorrelation between median variance and $\lambda_{RF}$ for quasars with $M_{BH} = 10^{8}$ and $R_{Edd} = 10^{-1}$ at different timescales. This anti-correlation is more pronounced at shorter timescales. The results align well with a bending power-law power spectrum density (PSD) model with both the damping timescale and the high-frequency slope of the PSD depending on the wavelength. The predictions provided by the CHAR model on the variance spectrum across most timescales studied showcase its potential in constraining temperature variations within the accretion disk. Key words. accretion, accretion discs; galaxies:active; quasars: supermassive black holes

astro-ph.GA↗

arXiv2024

SDSS1335+0728: The awakening of a $\sim 10^6 M_{\odot}$ black hole

The galaxy SDSS1335+0728, which had exhibited no prior optical variations during the preceding two decades, began showing significant nuclear variability in the Zwicky Transient Facility (ZTF) alert stream from December 2019 (as ZTF19acnskyy). Its behaviour suggests that SDSS1335+0728 hosts a $\sim 10^6 M_{\odot}$ black hole (BH) that is currently in the process of `turning on'. We present a multi-wavelength photometric analysis and spectroscopic follow-up performed with the aim of better understanding the origin of the nuclear variations detected in SDSS1335+0728. We used archival photometry and spectroscopic data to study the state of SDSS1335+0728 prior to December 2019, and new observations from Swift, SOAR/Goodman, VLT/X-shooter, and Keck/LRIS taken after its turn-on to characterise its current state. We find that: (a) since 2021, the UV flux is four times brighter than the flux reported by GALEX in 2004; (b) since June 2022, the mid-infrared flux has risen more than two times, and the W1-W2 WISE colour has become redder; (c) since February 2024, the source has begun showing X-ray emission; (d) the narrow emission line ratios are now consistent with a more energetic ionising continuum; (e) broad emission lines are not detected; and (f) the [OIII] line increased its flux $\sim 3.6$ years after the first ZTF alert, which implies a relatively compact narrow-line-emitting region. We conclude that the variations observed in SDSS1335+0728 could be either explained by an AGN that is just turning on or by an exotic tidal disruption event (TDE). If the former is true, SDSS1335+0728 is one of the strongest cases of an AGN observed in the process of activating. If the latter, it would correspond to the longest and faintest TDE ever observed (or another class of still unknown nuclear transient). Future observations of SDSS1335+0728 are crucial to further understand its behaviour.

astro-ph.GA↗

arXiv2024

A Newborn AGN in a Starforming Galaxy

We report on the finding of a newborn AGN, i.e. current AGN activity in a galaxy previously classified as non-active, and characterize its evolution. Black hole ignition event candidates were selected from a parent sample of spectrally classified non-active galaxies (2.394.312 objects), that currently show optical flux variability indicative of a type I AGN, according to the ALeRCE light curve classifier. A second epoch spectrum for a sample of candidate newborn AGN were obtained with the SOAR telescope to search for new AGN features. We present spectral results for the most convincing case of new AGN activity, for a galaxy with a previous star-forming optical classification, where the second epoch spectrum shows the appearance of prominent, broad Balmer lines without significant changes in the narrow line flux ratios. Long term optical lightcurves show a steady increase in luminosity starting 1.5 years after the SDSS spectrum was taken and continuing for at least 7 years. MIR colors from the WISE catalog have also evolved from typical non active galaxy colors to AGN-like colors and recent X-ray flux detections confirm its AGN nature.

astro-ph.GA↗

arXiv2023

Improving the selection of changing-look AGNs through multi-wavelength photometric variability

We present second epoch optical spectra for 30 changing-look (CL) candidates found by searching for Type-1 optical variability in a sample of active galactic nuclei (AGNs) spectroscopically classified as Type 2. We use a random-forest-based light curve classifier and spectroscopic follow-up, confirming 50 per cent of candidates as turning-on CLs. In order to improve this selection method and to better understand the nature of the not-confirmed CL candidates, we perform a multi-wavelength variability analysis including optical, mid-infrared (MIR) and X-ray data, and compare the results from the confirmed and not-confirmed CLs identified in this work. We find that most of the not-confirmed CLs are consistent with weak Type 1s dominated by host-galaxy contributions, showing weaker optical and MIR variability. On the contrary, the confirmed CLs present stronger optical fluctuations and experience a long (from five to ten years) increase in their MIR fluxes and the colour W1-W2 over time. In the 0.2-2.3 keV band, at least four out of 11 CLs with available SRG/eROSITA detections have increased their flux in comparison with archival upper limits. These common features allow us to select the most promising CLs from our list of candidates, leading to nine sources with similar multi-wavelength photometric properties to our CL sample. The use of machine learning algorithms with optical and MIR light curves will be very useful to identify CLs in future large-scale surveys.

astro-ph.GA↗

arXiv2023

The universal power spectrum of Quasars in optical wavelengths: Break timescale scales directly with both black hole mass and accretion rate

Aims: Establish the dependence of variability properties, such as characteristic timescales and variability amplitude, on basic quasar parameters such as black hole mass and accretion rate, controlling for the rest-frame wavelength of emission. Methods: Using large catalogs of quasars, we selected the g-band light curves for 4770 objects from the Zwicky Transient Facility archive. All selected objects fall into a narrow redshift bin, $0.6<z<0.7$, but cover a wide range of accretion rates in Eddington units (REdd) and black hole masses ($M$). We grouped these objects into 26 independent bins according to these parameters, calculated low-resolution $g$-band variability power spectra for each of these bins, and approximated the power spectra with a simple analytic model that features a break at a timescale $t_b$. Results: We found a clear dependence of the break timescale $t_b$ on REdd, on top of the known dependence of $t_b$ on the black hole mass $M$. In our fits, $t_b\propto M^{0.65 - 0.55}$ REdd $^{0.35 - 0.3}$, where the ranges in the exponents correspond to the best-fitting parameters of different power spectrum models. Scaling $t_b$ to the orbital timescale of the innermost stable circular orbit (ISCO), $t_{\rm ISCO}$, results approximately in $t_{b}/t_{\rm ISCO} \propto ($REdd$/M)^{0.35}$. The observed values of $t_b$ are $\sim 10$ longer than the orbital timescale at the light-weighted average radius of the disc region emitting in the (observer frame) $g$-band. The different scaling of the break frequency with $M$ and REdd shows that the shape of the variability power spectrum cannot be solely a function of the quasar luminosity, even for a single rest-frame wavelength. Finally, the best-fitting models have slopes above the break in the range -2.5 and -3. A slope of -2, as in the damped random walk models, fits the data significantly worse.

astro-ph.GA↗

arXiv2022

The Type 1 and Type 2 AGN dichotomy according to their ZTF optical variability

The scarce optical variability studies in spectrally classified Type 2 active galactic nuclei (AGNs) have led to the discovery of anomalous objects that are incompatible with the simplest unified models (UM). This paper focuses on the exploration of different variability features that allows to separate between obscured, Type 2 AGNs, and the variable, unobscured Type 1s. We analyse systematically the Zwicky Transient Facility, 2.5 years long light curves of ~ 15000 AGNs from the Sloan Digital Sky Survey Data Release 16, which are generally considered Type 2s due to the absence of strong broad emission lines (BELs). Consistently with the expectations from the UM, the variability features are distributed differently for distinct populations, with spectrally classified weak Type 1s showing 1 order of magnitude larger variances than the Type 2s. We find that the parameters given by the damped random walk model leads to broader H{\alpha} equivalent width for objects with {\tau}_g > 16 d and long term structure function SF{\infty},g> 0.07 mag. By limiting the variability features, we find that ~ 11 per cent of Type 2 sources show evidence for optical variations. A detailed spectral analysis of the most variable sources (~1 per cent of the Type 2 sample) leads to the discovery of misclassified Type 1s with weak BELs and changing-state candidates. This work presents one of the largest systematic investigations of Type 2 AGN optical variability to date, in preparation for future large photometric surveys.

astro-ph.GA↗

arXiv2022

Confirming new changing--look AGNs discovered through optical variability using a random-forest based light curve classifier

Determining the frequency and duration of changing--look (CL) active galactic nuclei (AGNs) phenomena, where the optical broad emission lines appear or disappear, is crucial to understand the evolution of the accretion flow around supermassive black holes. We present a strategy to select new CL candidates starting from a spectroscopic type 2 AGNs sample and searching for current type 1 photometric variability. We use the publicly available Zwicky Transient Facility (ZTF) alert stream and the Automatic Learning for the Rapid Classification of Events (ALeRCE) light curve classifier to produce a list of CL candidates with a highly automated algorithm, resulting in 60 candidates. Visual inspection reduced the sample to 30. We performed new spectroscopic observations of six candidates of our clean sample, without further refinement, finding the appearance of clear broad Balmer lines in four of them and tentative evidence of type changes in the remaining two, which suggests a promising success rate of $\geq66$ per cent for this CL selection method.

astro-ph.GA↗

arXiv2021

Optimization of Flat to Round Transformers with Self-fields using Adjoint Techniques

A continuous system of moment equations is introduced that models the transverse dynamics of a beam of charged particles as it passes through an arbitrary lattice of quadrupoles and solenoids in the presence of self-fields. Then, figures of merit are introduced specifying system characteristics to be optimized. The resulting model is used to optimize the parameters of the lattice elements of a flat to round transformer with self-fields, as could be applied in electron cooling. Results are shown for a case of no self-fields and two cases with self-fields. The optimization is based on a gradient descent algorithm in which the gradient is calculated using adjoint methods that prove to be very computationally efficient. Two figures of merit are studied and compared: one emphasizing radial force balance in the solenoid, the other emphasizing minimization of transverse beam energy in the solenoid.

physics.acc-ph↗

arXiv2020

Small scale star formation as revealed by VVVX galactic cluster candidates

We report a search and analysis of obscured cluster candidates in the "VISTA Variables in the Via Lactea eXtended (VVVX)" ESO Public Survey area encompassing the region between 229.4 < l < 295.2 and -4.3 < b < 4.4 of the southern Galactic disk. We discover and propose 88 new clusters. We improve the completeness of the embedded cluster population in this region, adding small size (linear diameters of 0.2-1.4 pc) and relatively far objects (heliocentric distance between 2 and 4 kpc) to existing catalogues. Nine candidates are proposed to be older open cluster candidates. Three of them (VVVX CL204, 207, 208) have sufficient numbers of well-resolved stellar members to allow us to determine some basic cluster parameters. We confirm their nature as older, low-mass open clusters. Photometric analysis of 15 known clusters shows that they have ages above 20Myr, and masses below 2000Msun: in general, their proper motions follow the motion of the disk. We outline some groups of clusters, most probably formed within the same dust complex. Broadly, our candidates follow the network of filamentary structure in the remaining dust. Thus, in this part of the southern disk of the Galaxy, we have found recent star formation, producing small size and young clusters, in addition to the well known, massive young clusters, including NGC3603, Westerlund2 and the Carina Nebula Complex.

astro-ph.SR↗

arXiv2018

Low Space-Charge Intensity Beams in UMER via Collimation and Solenoid Focusing

The University of Maryland Electron Ring (UMER) has operated traditionally in the regime of strong space-charge dominated beam transport, but small-current beams are desirable to significantly reduce the direct (incoherent) space-charge tune shift as well as the tune depression. This regime is of interest to model space-charge effects in large proton and ion rings similar to those used in nuclear physics and spallation neutron sources, and also for nonlinear dynamics studies of lattices inspired on the Integrable Optics Test Accelerator (IOTA). We review the definition of space-charge intensity, show a comparison of space-charge parameters in UMER and other machines, and discuss a simple method involving double collimation and solenoid focusing for varying the space-charge intensity of the beam injected into UMER.

physics.acc-ph↗