arXiv subjects

Beatriz Agís-González

Publications and source records attributed to Beatriz Agís-González.

16 recordsLinked to original sources

arXiv2026

X-ray eclipse tomography: Resolving the extent of X-ray-emitting region(s) in the Seyfert galaxy ESO362-G18

X-ray spectral variability in active galactic nuclei (AGNs) can be either intrinsic or caused by external processes, such as variable absorption. Several transient occultation events from individual clouds or X-ray eclipses have been identified in a number of AGNs in the past two decades. We report results from the analysis of two Swift monitoring campaigns of the Seyfert galaxy ESO 362-G18, a changing-look AGN that showed transitions between optical spectral types 1.5 and 1.9 in the past, most likely induced by variable absorption. We identified two X-ray eclipses during the first Swift campaign, one of which we were able to follow almost entirely from ingress to egress. We used time-resolved spectroscopy to follow the evolution of the X-ray spectrum with time in order to derive the properties of the absorbing cloud and the size of the main X-ray-emitting region. The cloud is located towards the innermost dust sublimation zone, intermediate between the dust-free broad-line region and the dusty torus. The X-ray-emitting region is confined within 35 gravitational radii (Rg) from the centre. A deeper analysis indicated that the eclipsing cloud likely comprises a denser core and a more tenuous atmosphere. We were also able to estimate the size of the soft excess and hot corona X-ray-emitting regions separately. These are two of the most relevant continuum components in the X-ray spectra of unobscured AGNs. Our analysis suggests that the soft-excess-emitting region is 50% more extended than the hot corona one. However, it appears to co-exist with and is not replaced by the hot corona one in the innermost 30 Rg. Our results highlight the potential of X-ray eclipse tomography for providing a clearer view of the innermost accretion flow geometry and of the AGN surroundings out to the torus scale.

astro-ph.HE↗

arXiv2026

Tracking down the broadband polarimetric properties of PG 1553+113

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multi-wavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of $\sim5$ and complex evolution of the optical polarization properties, including one of the largest (exceeding $150^{\circ}$) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree $Π_{\rm X}\,=(\,18.4\,\pm\,5.8)\%$ and $Ψ_{\rm X}\,=\,74^{\circ} \pm 9^{\circ}$ in the 2--8~keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly co-spatial regions within a dynamically evolving, magnetically structured jet.

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arXiv2025

Determining the origin of the X-ray emission in blazars through multiwavelength polarization

The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes that are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent $γ$-ray states. We find that regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies.

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arXiv2025

IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare

The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$σ$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3σ$). We model the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65.

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arXiv2024

A Two-Week $IXPE$ Monitoring Campaign on Mrk 421

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including a $\sim90^\circ$ angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.

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arXiv2024

X-ray and multiwavelength polarization of Mrk 501 from 2022 to 2023

We present multiwavelength polarization measurements of the luminous blazar Mrk~501 over a 14-month period. The 2--8 keV X-ray polarization was measured with the Imaging X-ray Polarimetry Explorer (IXPE) with six 100-ks observations spanning from 2022 March to 2023 April. Each IXPE observation was accompanied by simultaneous X-ray data from NuSTAR, Swift/XRT, and/or XMM-Newton. Complementary optical-infrared polarization measurements were also available in the B, V, R, I, and J bands, as were radio polarization measurements from 4.85 GHz to 225.5 GHz. Among the first five IXPE observations, we did not find significant variability in the X-ray polarization degree and angle with IXPE. However, the most recent sixth observation found an elevated polarization degree at $>3σ$ above the average of the other five observations. The optical and radio measurements show no apparent correlations with the X-ray polarization properties. Throughout the six IXPE observations, the X-ray polarization degree remained higher than, or similar to, the R-band optical polarization degree, which remained higher than the radio value. This is consistent with the energy-stratified shock scenario proposed to explain the first two IXPE observations, in which the polarized X-ray, optical, and radio emission arises from different regions.

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arXiv2024

IXPE observation of PKS 2155-304 reveals the most highly polarized blazar

We report the X-ray polarization properties of the high-synchrotron-peaked (HSP) blazar PKS 2155$-$304 based on observations with the Imaging X-ray Polarimetry Explorer (IXPE). We observed the source between Oct 27 and Nov 7, 2023. We also conducted an extensive contemporaneous multiwavelength (MW) campaign. We find that during the first half ($T_1$) of the IXPE pointing, the source exhibited the highest X-ray polarization degree detected for an HSP blazar thus far, (30.7$\pm$2.0)%, which dropped to (15.3$\pm$2.1)% during the second half ($T_2$). The X-ray polarization angle remained stable during the IXPE pointing at 129.4$^\circ$$\pm$1.8$^\circ$ and 125.4$^\circ$$\pm$3.9$^\circ$ during $T_1$ and $T_2$, respectively. Meanwhile, the optical polarization degree remained stable during the IXPE pointing, with average host-galaxy-corrected values of (4.3$\pm$0.7)% and (3.8$\pm$0.9)% during the $T_1$ and $T_2$, respectively. During the IXPE pointing, the optical polarization angle changed achromatically from $\sim$140$^\circ$ to $\sim$90$^\circ$ and back to $\sim$130$^\circ$. Despite several attempts, we only detected (99.7% conf.) the radio polarization once (during $T_2$, at 225.5 GHz): with degree (1.7$\pm$0.4)% and angle 112.5$^\circ$$\pm$5.5$^\circ$. The direction of the broad pc-scale jet is rather ambiguous and has been found to point to the east and south at different epochs; however, on larger scales (> 1.5 pc) the jet points toward the southeast ($\sim$135$^\circ$), similar to all of the MW polarization angles. Moreover, the X-ray to optical polarization degree ratios of $\sim$7 and $\sim$4 during $T_1$ and $T_2$, respectively, are similar to previous IXPE results for several HSP blazars. These findings, combined with the lack of correlation of temporal variability between the MW polarization properties, agree with an energy-stratified shock-acceleration scenario in HSP blazars.

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arXiv2024

Detection of X-ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-ray Polarimetry Explorer

Observations of linear polarization in the 2-8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating non-thermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray selected BL Lac-type blazar 1ES 1959+650 in 2022 May 3-4 showed a significant linear polarization degree of $Π_\mathrm{x} = 8.0\% \pm 2.3\%$ at an electric-vector position angle $ψ_\mathrm{x} = 123^\circ \pm 8^\circ$. However, in 2022 June 9-12, only an upper limit of $Π_\mathrm{x} \leq 5.1\%$ could be derived (at the 99% confidence level). The degree of optical polarization at that time $Π_\mathrm{O} \sim 5\%$ is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.

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arXiv2023

Polarized Blazar X-rays imply particle acceleration in shocks

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to $\sim 1$ TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization - the only range available until now - probe extended regions of the jet containing particles that left the acceleration site days to years earlier (Jorstad et al., 2005; Marin et al., 2018; Blinov et al., 2021), and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian~501 (Mrk~501). We measure an X-ray linear polarization degree $Π_X \sim10\%$, a factor of $\sim2$ higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration, and also implies that the plasma becomes increasingly turbulent with distance from the shock.

astro-ph.HE↗

arXiv2023

IXPE and multi-wavelength observations of blazar PG 1553+113 reveal an orphan optical polarization swing

The lower energy peak of the spectral energy distribution of blazars has commonly been ascribed to synchrotron radiation from relativistic particles in the jets. Despite the consensus regarding jet emission processes, the particle acceleration mechanism is still debated. Here, we present the first X-ray polarization observations of PG 1553+113, a high-synchrotron-peak blazar observed by the Imaging X-ray Polarimetry Explorer (IXPE). We detect an X-ray polarization degree of $(10\pm2)\%$ along an electric-vector position angle of $ψ_X=86^{\circ}\pm8^{\circ}$. At the same time, the radio and optical polarization degrees are lower by a factor of $\sim$3. During our IXPE pointing, we observed the first orphan optical polarization swing of the IXPE era, as the optical angle of PG 1553+113 underwent a smooth monotonic rotation by about 125$^\circ$, with a rate of $\sim$17 degrees per day. We do not find evidence of a similar rotation in either radio or X-rays, which suggests that the X-ray and optically emitting regions are separate or, at most, partially co-spatial. Our spectro-polarimetric results provide further evidence that the steady-state X-ray emission in blazars originates in a shock-accelerated and energy-stratified electron population.

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arXiv2022

X-ray Polarization Observations of BL Lacertae

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus the Synchrotron emission to be responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae performed with the Imaging X-ray Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization degree $Π_X<$12.6\% was found in the 2-8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.

astro-ph.HE↗

arXiv2022

A search for ionised gas outflows in an Halpha imaging atlas of nearby LINERs

Outflows play a major role in the evolution of galaxies. However, we do not have yet a complete picture of their properties (extension, geometry, orientation and clumpiness). For low-luminosity Active Galactic Nuclei (AGNs), in particular, low-ionisation nuclear emission line regions (LINERs), the rate of outflows and their properties are largely unknown. The main goal of this work is to create the largest, up-to-date atlas of ionised gas outflow candidates in a sample of 70 nearby LINERs. We use narrow-band, imaging data to analyse the morphological properties of the ionised gas nuclear emission of these galaxies and to identify signatures of extended emission with distinctive outflow-like morphologies. We obtained new imaging data from ALFOSC/NOT for 32 LINERs. We complemented it with HST archival data for 6 objects and with results from the literature for other 32 targets. We additionally obtained soft X-ray data from Chandra archive to compare with the ionised gas. The distribution of the ionised gas in these LINER shows that $\sim$32% have bubble-like emission, $\sim$28% show a 'Core-halo', unresolved emission, and $\sim$21% have a disky-like distribution. Dust lanes prevent a detailed classification for $\sim$11% of the sample ('Dusty'). If we account for the kinematical information, available for 60 galaxies, we end up with 48% of the LINERs with detected outflows/inflows (50% considering only kinematical information based on Integral Field Spectroscopy). Our results suggest that the incidence of outflows in LINERs may vary from 41% up to 56%, based on both the Halpha morphology and the kinematical information from the literature. The ionised gas is co-spatial with the soft X-ray emission for the majority of cases ($\sim$60%), so that they may have a common origin. We discuss the use of Halpha imaging for the pre-selection of candidates likely hosting ionised gas outflows.

astro-ph.GA↗

arXiv2015

The ionised X-ray outflowing torus in ESO 323-G77: low-ionisation clumps confined by homogeneous warm absorbers

We report on the long- and short-term X-ray spectral analysis of the polar-scattered Seyfert 1.2 galaxy ESO 323-G77, observed in three epochs between 2006 and 2013 with Chandra and XMM-Newton. Four high-resolution Chandra observations give us a unique opportunity to study the properties of the absorbers in detail, as well as their short time-scale (days) variability. From the rich set of absorption features seen in the Chandra data, we identify two warm absorbers with column densities and ionisations that are consistent with being constant on both short and long time-scales, suggesting that those are the signature of a rather homogeneous and extended outflow. A third absorber, ionised to a lesser degree, is also present and it replaces the strictly neutral absorber that is ubiquitously inferred from the X-ray analysis of obscured Compton-thin sources. This colder absorber appears to vary in column density on long time-scales, suggesting a non-homogeneous absorber. Moreover, its ionisation responds to the nuclear luminosity variations on time-scales as short as a few days, indicating that the absorber is in photoionisation equilibrium with the nuclear source on these time-scales. All components are consistent with being co-spatial and located between the inner and outer edges of the so-called dusty, clumpy torus. Assuming co-spatiality, the three phases also share the same pressure, suggesting that the warm / hot phases confine the colder, most likely clumpy, medium. We discuss further the properties of the outflow in comparison with the lower resolution XMM-Newton data.

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arXiv2015

Eclipsing the innermost X-ray emitting regions in AGN

Variable X-ray absorption has been observed in active galactic nuclei (AGN) on several time scales. Observations allow us to identify the absorber with clouds associated either with the clumpy torus (parsec scales, long timescales) or with the broad line region (BLR) (short timescales). In the latter, the cloud size has been estimated to be of the order of few gravitational radii from the observed absorption variability. Such small cloud sizes are comparable to the X-ray emitting regions so that a detailed modeling of occultation events in AGN has the potential of enabling us to infer accurately the geometry of the system. We have developed a relativistic X-ray spectral model for occultation events and we present here theoretical predictions on the different observables that can be inferred by studying X-ray eclipses in simulated XMM-Newton data. These include the size of the X-ray emitting regions as well as more fundamental parameters such as the black hole spin and the system inclination. We find that absorption varies as a function of the energy range and that its maximum takes place when the approaching part of the accretion disc is covered. Therefore we study the hard-to-soft (H/S) ratio light curves produced during an eclipse and use them to characterise the properties of the inner accretion disc in a new model-independent way.

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arXiv2014

Black hole spin and size of the X-ray emitting region(s) in the Seyfert 1.5 galaxy ESO 362-G18

We report results from multi-epoch X-ray observations of the Seyfert 1.5 galaxy ESO 362-G18 performed between November 2005 and June 2010. ESO 362-G18 generally exhibits the typical X-ray spectrum of type 1 Active Galactic Nuclei (AGN). A disc-reflection component accounts for broad residuals in the iron K band and above 10~keV, as well as for a significant soft excess. From our best-fitting reflection model, we measure a black hole spin greater or equal to 0.92 at the 99.99 per cent confidence level. ESO 362-G18 is also (typically) mildly absorbed by a column of neutral gas. The absorber is variable and one observation, performed ~2 months after a typical mildly absorbed one, is heavily absorbed by a cold column density two orders of magnitude higher than that during any other observation. UV variability between the heavily absorbed observation and the others suggests that the absorber can be identified with a dusty, clumpy torus. The absorption variability timescale enables us to locate the X-ray emitting region within the innermost ~50 gravitational radii. Such result holds not only for the X-ray continuum, but also for the soft excess.

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arXiv2013

The size of the X-ray emitting region in SWIFT J2127.4+5654 via a broad line region cloud X-ray eclipse

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ~130 ks XMM-Newton observation. We reveal large spectral variations, especially during the first ~90 ks of the XMM-Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density N_H = 2.0^{+0.2}_{-0.3}e22 cm^-2 which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ~43 per cent. Our analysis enables us to constrain the size, number density, and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of $D_c < 1.5e13 cm, corresponding to a density of n_c > 1.5e9 cm^-3 at a distance of R_c > 4.3e16 cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be D_s < 2.3e13 cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5e7 M_sun), translates into D_s < 10.5 gravitational radii (r_g) with larger sizes (in r_g) being associated with smaller black hole masses, and viceversa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation.

astro-ph.HE↗