arXiv subjects

A. Esteban-Gutiérrez

Publications and source records attributed to A. Esteban-Gutiérrez.

6 recordsLinked to original sources

arXiv2026

Joining forces: 30 years of optical monitoring of the Einstein Cross

We present an extended optical monitoring of the quadruply-imaged gravitationally lensed quasar QSO 2237+0305, the Einstein Cross, including observations from different observatories in both hemispheres and using a new photometric technique. This technique uses a region far enough from the lens system to determine accurately the sky background level, and minimises contamination from the lensing galaxy by combining analytical and numerical modeling of its structure. The resulting light curves of the four quasar images describe variations across practically the entire optical spectrum and span about 9000 days in the $VRI$ bands. The multi-band microlensing variability is captured with an unprecedented level of detail, and a preliminary microlensing analysis reveals an almost linear scaling of source radius with wavelength, providing direct evidence for the wavelength-dependent structure of the region contributing to optical passband fluxes. Specifically, assuming a mean microlens mass $\langle M \rangle$ = 0.3 $\rm{M_{\odot}}$ and concentric Gaussian sources that move according to the velocity distribution peaks (speed and direction) reported in a previous microlensing analysis, we find that the half-light radius of the $g$-band source is 9.6 $\pm$ 2.7 lt-day and the size of the sources grows with wavelength with a power-law index of $α$ = 0.94 $\pm$ 0.05. We conclude that these long-term light curves set stringent empirical constraints on models of quasar emission and microlensing physics.

astro-ph.GA↗

arXiv2025

On a model of variable curvature that mimics the observed Universe acceleration

We present a new model based on General Relativity in where a subtle change of curvature at late times is able to produce the observed Universe acceleration and an oscillating behavior in the effective equation of state. This model aims to test the cosmological principle, by introducing a slight modification in the traditional FLRW metric, through a non-constant curvature parameter. This model is defined by a smooth step-like function with a slight transition between two curvature values, fulfilling the premise that the derivative of this curvature parameter is preserved as approximately zero, $\dotκ\approx0$. To test our model, we implemented a MCMC likelihood analysis using Cosmic Chronometers and Type Ia supernovae data in order to constrain the free parameters of the model and reconstruct $H(z)$, $q(z)$, $w_{eff}(z)$, also comparing the results with the $Λ$CDM model. The main result is that this model provides an alternative to the acceleration of the Universe without the need of a dark energy component. In particular, it gives an equivalent phase transition at $z \sim 0.5$, while obtaining the same fraction of matter density, similar to what is expected for the standard $Λ$CDM model. Remarkably, it also predicts a slight decelerated state at $z=0$ in agreement with diverse Dark Energy parameterizations. We conclude that the behavior of our proposed model points towards a new and intriguing way to investigate slight violations to the cosmological principle, in particular the case of inhomogenities during low phase transitions.

astro-ph.CO↗

arXiv2023

Constraints on the Abundance of PBHs from X-ray Quasar Microlensing Observations: Substellar to Planetary Mass Range

We use X-ray observations of quasar microlensing (sensitive to smaller compact objects than in the optical) to study the possible presence of a population of low mass black holes (from $\sim$ $10^{-3}M_{\odot}$ to $10^{-1}M_{\odot}$) in lens galaxies. We compare these observations with microlensing magnification simulations of a mixed population of stars and black holes (BHs) plus a smooth matter component. We estimate the individual mass fractions of both, stars and BHs, for three different BH masses in the range of substellar to planetary masses. Our Bayesian analysis indicates that the contribution of BHs is negligible in the substellar mass range but that a population of BHs of planetary mass (M $\lesssim$ $10^{-3}M_{\odot}$) could pass unnoticed to X-ray microlensing. We provide new upper limits to the contribution of BHs to the fraction of dark matter based on both, the quasar microlensing data in the X-ray band, and our previous estimates in the optical of intermediate-mass BHs with an additional upper limit at $M=3M_{\odot}$.

astro-ph.CO↗

arXiv2022

Abundance of LIGO/Virgo Black Holes from Microlensing Observations of Quasars with Reverberation Mapping Size Estimates

Assuming a population of Black Holes (BHs) with masses in the range inferred by LIGO/Virgo from BH mergers, we use quasar microlensing observations to estimate their abundances. We consider a mixed population of stars and BHs and the presence of a smooth dark matter component. We adopt reverberation mapping estimates of the quasar size. According to a Bayesian analysis of the measured microlensing magnifications, a population of BHs with masses $\sim$ 30$M_{\odot}$ constitutes less than 0.4 % of the total matter at 68 % confidence level (less than 0.9 % at 90 % confidence). We have explored the whole mass range of LIGO/Virgo BHs finding that this upper limit ranges from 0.5 % to 0.4 % at 68 % C.L. (from 1.1 % to 0.9 % at 90 % C.L.) when the BHs mass change from 10 to 60$M_{\odot}$. We estimate a 16 % contribution from the stars, in agreement with previous studies based on a single mass population that do not consider explicitly the presence of BHs. These results are consistent with the estimates of BH abundances from the statistics of LIGO/Virgo mergers and rule out that PBHs (or any type of compact objects), in this mass range constitute a significant fraction of the dark matter.

astro-ph.CO↗

arXiv2022

Limiting the Abundance of LIGO/Virgo Black Holes with Microlensing Observations of Quasars of Finite Size

We present a simple but general argument that strongly limits the abundance of Primordial Black Holes (PBHs) (or other unknown population of compact objects) with masses similar to those determined by LIGO/Virgo from BH binary mergers. We show that quasar microlensing can be very sensitive to the mass of the lenses, and that it is able to distinguish between stars and BHs of high mass, when the finite size of the source is taken into account. A significant presence of massive BHs would produce frequent high flux magnifications (except for unrealistically large sources) which have been very rarely observed. On the contrary, a typical stellar population would induce flux magnifications consistent with the observations. This result excludes PBHs (or any type of compact object) in the mass range determined by LIGO/Virgo as the main dark matter constituents in the lens galaxies.

astro-ph.CO↗

arXiv2020

The Impact of the Mass Spectrum of Lenses in Quasar Microlensing Studies. Constraints on a Mixed Population of Primordial Black Holes and Stars

We show that quasar microlensing magnification statistics induced by a population of point microlenses distributed according to a mass-spectrum can be very well approximated by that of a single-mass, "monochromatic", population. When the spatial resolution (physically defined by the source size) is small as compared with the Einstein radius, the mass of the monochromatic population matches the geometric mean of the mass-spectrum. Otherwise, the best-fit mass can be larger. Taking into account the degeneracy with the geometric mean, the interpretation of quasar microlensing observations under the hypothesis of a mixed population of primordial black holes and stars, makes the existence of a significant population of intermediate mass black holes ($\sim$ 100$M_\odot$) unlikely but allows, within a two-$σ$ confidence interval, the presence of a large population ($\gtrsim 40\%$ of the total mass) of substellar black holes ($\sim$ 0.01$M_\odot$).

astro-ph.CO↗