Alessia Tortosa
Astrophysicist researcher

I am currently working on the reduction, analysis and interpretation of X-ray observations from the XMM-Newton Multi-Year Heritage programme HYPERION. The HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) project aims at investigating the titans among the first quasars to get insight on the formation of the first cosmic structures during the first billion years of the Universe by measuring the physical properties (from the nucleus to the host galaxy) of a sample of quasars a the Epoch of Reionization (z>6).

In 2019 I awarded three year FONDECYT Post-Doc research grant with the project "Accretion and emission processes in supermassive black holes, from the lowest to the highest Eddington ratios", which I carried out at the Instituto de Estudios Astrofisicos at the Universidad Diego Portales, Santiago, Chile. The project was focused on the spectral and timing analysis of the X- ray broad-band emission of Active Galactic Nuclei (AGN), with the aim of reaching a global view on the accretion and emission process in AGN, from the lower to the higher Eddington ratios. The project aim was answering to some open questions in the field of AGN physics, such as: how do AGN accrete? How is the AGN energy detected as radiation produced? How do AGN interact with their host galaxy? The spectral parameters of the primary X-ray emission are related with the physical characteristics of the X-ray emitting plasma, but today very little is known about its potential relation with the accretion properties of supermassive black holes. A deep comprehension of the typical characteristics of the coronal plasma which emits X-ray for different intervals of the accretion rate is crucial to assess the impact of radiative heating in the feedback process linking AGN to their host galaxies. During my period in Chile I carried out the following sub-projects:

X-ray Variability of XMM-Newton observations from BASS sample of type 1 AGN.

As part of my FONDECYT Post-Doc Project, I performed a variability study of the public XMM-Newton observations of the unobscured type 1 AGN belonging to the BAT AGN Spectroscopic Survey (BASS) which includes ∼ 385 observations of ∼ 150 sources with well measured black hole masses. This project is part of the BASS Collaboration. Check the dedicated paper for more details: "BASS-XL: X-ray variability properties of unobscured active galactic nuclei".

Super-Eddington Accreting Active Galactic Nuclei.

I carried out a detailed spectral and timing analysis of a sample of AGN accreting matter in the Super and Hyper Eddington regime with the aim of better understand the behaviour of the X-ray direct emission and reprocessed radiation in the regime of extreme accretion, and to spread light on the physical properties of the accretion flow of the X-ray sources at very high accretion rates.
"The extreme properties of the nearby hyper-Eddington accreting active galactic nucleus in IRAS 04416+1215" is the first paper we published related to this project and is about the X-ray broad-band analysis of the most extreme hyper-Eddington AGN known so far in the Universe.
"Systematic broad-band X-ray study of super-Eddington accretion on to supermassive black holes – I. X-ray continuum" is the second paper we published related to this project and it is the first systematic broad-band X-ray study of super-Eddington accretion onto supermassive black holes with simultaneous NuSTAR and XMM–Newton or Swift/XRT observations of a sample of eight super-Eddington accreting AGN.

X-ray monitoring campaign of IC 4329A

I also worked on the analysis of the joint XMM-Newton and NuSTAR campaign on the AGN namely IC 4329A, consisting of 9x20 ks XMM-Newton observations, and 5x20 ks NuSTAR observations within nine days, performed in August 2021 of the source. All the results are presented in the dedicated paper: "XMM-Newton - NuSTAR monitoring campaign of the Seyfert 1 galaxy IC 4329A" .

"Calibrations and simulations of the instrument onboard the NASA IXPE mission, Instrument system tests and support for the phase B/C/D."

After my PhD defense, I worked for 1 year at the Institute for Space Astrophysics and Planetology in Rome (Italy) doing research and laboratory activity in the field of high energy astrophysics and X-ray polarisation. I have collaborated with the best experts in X-ray polarimetry in Italy in order to test and calibrate the Gas Pixel Detectors (GPS), the X-ray polarisation detectors which will be placed at the focus of eachImaging X-ray Polarimetry Explorer (IXPE) X-ray telescopes. I contributed to the first measure of the polarisation of a laboratory source with a continuum energy spectrum, which simulates the effect of a real astrophysical source, carried out with a prototype of the GPD, which is reported in the SPIE proceeding 10699. 

IXPE is a NASA's Small Explorer program (SMEX) mission. It was announced on 3 January 2017 and launched on December 2021. IXPE exploit the X-ray polarisation from astrophysical sources to provide insight into our understanding of X-ray production in objects such as neutron stars and pulsar wind nebulae, as well as stellar and supermassive black holes. IXPE provides simultaneous spectral, spatial, and temporal measurements of all the observed sources. Some of it's technical and science objectives include also determining the geometry and the emission mechanism of Active Galactic Nuclei and Microquasars. IXPE will have three identical X-ray telescopes, with polarisation detectors (Gas Pixel Detectors, GPD) at each focus. GPD are the polarisation detectors based on proportional counters which will detect the polarised light in IXPE. It is an X-ray polarimeter that exploits the photoelectric effect to measure the polarization and to obtain the image of astrophysical sources. The polarised X-ray photon interacts with the gaseous medium and creates photoelectons that are preferentially emitted in the polarisation direction. The photoelectron tracks ionize the gas and an electric field allows electron/ion pairs to drift respectively to the Gas Electron Multiplier (GEM) and top plane. An analysis of the distribution of the initial directions of the tracks gives the degree of polarization and the position angle from the incident X ray. 

My thesis project was based mainly on the study of the X-ray emission spectrum of Active Galactic Nuclei (AGN), to constrain the coronal parameters of AGN through a detailed spectral analysis of high-quality X-rays data. In this project I looked for correlations between these parameters and other physical parameters, such as the geometry and the position of the emitting corona, with the aim of better understanding the complex environment that can be found in AGN. The approach of this work was based both on new observations of NuSTAR, XMM-Newton and Swift X-ray satellites and on archival data. The detailed analysis of single sources allows to built and constraints physical models while the analysis of a large sample give us insights into the average properties of AGN. I present the data analysis of three sources, namely GRS 1734-292, MCG +8-11-11 and NGC 6814. These three sources are nearby X-ray-bright Seyfert 1 galaxies. GRS 1734-292 is a Seyfert 1 galaxy, located near the Galactic plane. It shows one of the lowest high energy cutoff measurements made up so far by NuSTAR. The results of the analysis are presented in Tortosa et al., 2017. MCG +8-11-11 and NGC 6814 are two bright Seyfert 1 galaxies that show very similar coronal properties even if they had different properties overall (black hole masses, luminosity and Eddington ratios). The analysis of the data of these sources is presented in Tortosa et al., 2018a. After the analysis of single sources I discussed a project based on the analysis of a small catalogue of AGN we build up choosing the unobscured nearby, non-jetted, Seyfert galaxies that have been observed by NuSTAR (often in coordination with XMM-Newton, Suzaku or Swift) present on literature. The aim of the project, presented in Tortosa et al., 2018b,  is to look for correlations between coronal spectral parameters, i.e. the photon index and cut-off energy, and physical parameters, i.e. the optical depth and coronal temperature, (when a Comptonization model is used), and other parameters of the systems, such as the black hole mass or the Eddington ratio. To this end we analysed the coronal parameters of the selected sample, founding on one hand, an anti-correlation with a significance level >98% between the coronal optical depth and the coronal temperature of the sources from our sample. On the other hand, no correlation between the above parameters and the black hole mass, the accretion rate, and the intrinsic spectral slope of the sources is found.
The full pdf of my PhD Thesis can be reached here!