arXiv subjects

Valentina La Parola

Publications and source records attributed to Valentina La Parola.

13 recordsLinked to original sources

arXiv2025

How the spin-phase variability of cyclotron lines shapes the pulsed fraction spectra: insights from 4U 1538-52

We study the energy-dependent pulse profile of 4U 1538-52 and its phase-dependent spectral variability, with emphasis on the behavior around the cyclotron resonant scattering feature at around 21 keV. We analyze all available NuSTAR observations of 4U 1538-52. We decompose energy-resolved pulse profiles into Fourier harmonics to study their energy dependence. Specifically, we compute pulsed fraction spectra, cross-correlation and lag spectra, identifying discontinuities and linking them to features in the phase-averaged spectra. We perform phase-averaged and phase-resolved spectral analyses to probe spectral variability and its relation to pulse profile changes. Finally, we interpret our findings via physical modeling of energy- and angle-dependent pulse profile emission, performing radiative transfer in a homogeneous slab-like atmosphere under conditions relevant to 4U 1538-52. The emission is projected onto the observer's sky plane to derive expected observables. In contrast to the dips in pulsed fraction spectra observed in other sources (e.g., Her X-1), we find a broad bump near the cyclotron resonance energy in 4U 1538-52. This increase is driven primarily by phase-dependent spectral variability, especially by strong variations in cyclotron line depth across different phase intervals. We interpret the observed contrast between dips and bumps in various sources as arising from phase-dependent variations of cyclotron line depth relative to the phase-modulated flux. We model the X-ray emission from an accreting neutron star and find that our simulations indicate high values of both the observer's inclination and the magnetic obliquity, along with a 10-15 degrees asymmetry between the locations of the magnetic poles. Assuming this geometry, we satisfactorily reproduce the observed pulse profiles and introduce general trends in the observables resulting from the system's geometry.

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arXiv2025

Energy-resolved pulse profile changes in V 0332+53: Indications of wings in the cyclotron absorption line profile

We aim to investigate the energy-resolved pulse profile changes of the accreting X-ray pulsar V 0332+53, focusing in the cyclotron line energy range, using the full set of available NuSTAR observations. We applied a tailored pipeline to study the energy dependence of the pulse profiles and to build the pulsed fraction spectra (PFS) for the different observations. We studied the profile changes also using cross-correlation and lag spectra. We re-analysed the energy spectra to search for links between the local features observed in the PFS and spectral emission components associated with the shape of the fundamental cyclotron line. In the PFS data, with sufficiently high statistics, we observe a consistent behaviour around the cyclotron line energy. Specifically, two Gaussian-shaped features appear symmetrically on either side of the putative cyclotron line. These features exhibit minimal variation with source luminosity, and their peak positions consistently remain on the left and right of the cyclotron line energy. Associated with the cyclotron line-forming region, we interpret them as evidence for the resonant cyclotron absorption line wings, as predicted by theoretical models of how the cyclotron line profile should appear along the observer's line of sight. A phase-resolved analysis of the pulse in the energy bands surrounding these features enables us to determine both the spectral shape and the intensity of the photons responsible for these peaks in the PFS. Assuming these features correspond to a spectral component, we used their shapes as priors for the corresponding emission components, finding a statistically satisfactory description of the spectra. To explain these results, we propose that our line of sight is close to the direction of the spin axis, while the magnetic axis is likely orthogonal to it.

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arXiv2022

Compton-thick AGN in the NuSTAR Era. IX. A Joint NuSTAR and XMM-Newton Analysis of Four Local AGN

We present the results of the broadband X-ray spectral analysis of simultaneous NuSTAR and XMM-Newton observations of four nearby Compton-thick active galactic nuclei (AGN) candidates selected from the Swift-Burst Alert Telescope (BAT) 150-month catalog. This work is part of a larger effort to identify and characterize all Compton-thick (NH >= 10^24 cm^-2) AGN in the local Universe (z < 0.05). We used three physically motivated models -- MYTorus, borus02, and UXClumpy -- to fit and characterize these sources. Of the four candidates analyzed, 2MASX J02051994-0233055 was found to be an unobscured (NH < 10^22 cm^-2) AGN, 2MASX J04075215-6116126 and IC 2227 to be Compton-thin (10^22 cm^-2 < NH < 10^24 cm^-2) AGN, and one, ESO 362-8, was confirmed to be a Compton-thick AGN. Additionally, every source was found to have a statistically significant difference between their line-of-sight and average torus hydrogen column density, further supporting the idea that the obscuring material in AGN is inhomogeneous. Furthermore, half of the sources in our sample (2MASX J02051994-0233055 and 2MASX J04075215-6116126) exhibited significant luminosity variation in the last decade, suggesting that this might be a common feature of AGN.

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arXiv2020

Unveiling sub-parsec supermassive black hole binary candidates in active galactic nuclei

Elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to their origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool to unveil the presence of SMBHBs, by means of either double Fe K$α$ emission lines or periodicity in their light curve. Here we report on a new method to select SMBHBs by means of the presence of a periodic signal in their Swift-BAT 105-months light curves. Our technique is based on the Fisher's exact g-test and takes into account the possible presence of colored noise. Among the 553 AGN selected for our investigation, only the Seyfert 1.5 Mrk 915 emerged as possible candidate for a SMBHB; from the subsequent analysis of its light curve we find a period $P_0=35\pm2$ months, and the null hypothesis is rejected at the $3.7σ$ confidence level. We also present a detailed analysis of the BAT light curve of the only previously X-ray-selected binary candidate source in the literature, the Seyfert 2 galaxy MCG+11-11-032. We find $P_0=26.3\pm0.6$ months, consistent with the one inferred from previously reported double Fe K$α$ emission lines.

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arXiv2019

GROWTH on S190425z: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR

The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg$^2$ of the initial skymap over the next two nights, corresponding to 46\% integrated probability, ZTF system achieved a depth of $\approx$\,21 $m_\textrm{AB}$ in $g$- and $r$-bands. Palomar Gattini-IR covered 2200 square degrees in $J$-band to a depth of 15.5\,mag, including 32\% integrated probability based on the initial sky map. The revised skymap issued the following day reduced these numbers to 21\% for the Zwicky Transient Facility and 19\% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical lightcurves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled-out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

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arXiv2019

Compton-thick AGNs in the NuSTAR era. V: Joint NuSTAR and XMM-Newton spectral analysis of three "soft-Gamma" candidate CT-AGNs in the Swift-BAT 100-month catalog

We present the joint NuSTAR and XMM-Newton spectral analysis in the 0.6-70 keV band of three candidate Compton thick (CT-) AGN selected in the 100-month Swift-BAT catalog. These objects were previously classified as CT-AGNs based on low quality Swift-XRT and Swift-BAT data, and had soft photon indices (Gamma>2.2) that suggested a potential overestimation of the line of sight column density. Thanks to the high-quality NuSTAR and XMM-Newton data we were able to determine that in all three objects the photon index was significantly overestimated, and two out of three sources are reclassified from CT to Compton thin, confirming a previously observed trend, i.e., that a significant fraction of BAT-selected, candidate CT-AGNs with poor soft X-ray data are reclassified as Compton thin when the NuSTAR data are added to the fit. Finally, thanks to both the good XMM-Newton spatial resolution and the high NuSTAR and XMM-Newton spectral quality, we found that the third object in our sample was associated to the wrong counterpart: the correct one, 2MASX J10331570+5252182, has redshift z=0.14036, which makes it one of the very few candidate CT-AGNs in the 100-month BAT catalog detected at z>0.1, and a rare CT quasar.

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arXiv2018

Compton-thick AGN in the NuSTAR era III: A systematic study of the torus covering factor

We present the analysis of a sample of 35 candidate Compton thick (CT-) active galactic nuclei (AGNs) selected in the nearby Universe (average redshift ~0.03) with the Swift-BAT 100-month survey. All sources have available NuSTAR data, thus allowing us to constrain with unprecedented quality important spectral parameters such as the obscuring torus line-of-sight column density (N_{H, z}), the average torus column density (N_{H, tor}) and the torus covering factor (f_c). We compare the best-fit results obtained with the widely used MyTorus (Murphy et al. 2009) model with those of the recently published borus02 model (Balokovic et al. 2018) used in the same geometrical configuration of MyTorus (i.e., with f_c=0.5). We find a remarkable agreement between the two, although with increasing dispersion in N_{H, z} moving towards higher column densities. We then use borus02 to measure f_c. High-f_c sources have, on average, smaller offset between N_{H, z} and N_{H, tor} than low-f_c ones. Therefore, low f_c values can be linked to a "patchy torus" scenario, where the AGN is seen through an over-dense region in the torus, while high-f_c objects are more likely to be obscured by a more uniform gas distribution. Finally, we find potential evidence of an inverse trend between f_c and the AGN 2-10 keV luminosity, i.e., sources with higher f_c values have on average lower luminosities.

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arXiv2017

Chandra and NuSTAR follow-up observations of Swift-BAT selected AGN

Based on current models of the Cosmic X-ray Background (CXB), heavily obscured Active Galactic Nuclei (AGN) are expected to make up ~10% of the peak emission of the CXB and ~20% of the total population of AGN, yet few of these sources have been recorded and characterized in current surveys. Here we present the Chandra follow-up observation of 14 AGN detected by Swift-BAT. For five sources in the sample, NuSTAR observations in the 3-80 keV band are also available. The X-ray spectral fitting over the 0.3-150 keV energy range allows us to determine the main X-ray spectral parameters, such as the photon index and the intrinsic absorption, of these objects, and to make hypotheses on the physical structures responsible for the observed spectra. We find that 13 of the 14 objects are absorbed AGN, and one is a candidate Compton thick AGN, having intrinsic absorption NH>1E24 cm^{-2}. Finally, we verified that the use of NuSTAR observations is strategic to strongly constrain the properties of obscured AGN, since the best-fit values we obtained for parameters such as the power-law photon index and the intrinsic absorption NH changed sometimes significantly fitting the spectra with and without the use of NuSTAR data.

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arXiv2017

A broadband self-consistent modelling of the X-ray spectrum of 4U 1626--67

The accretion-powered X-ray pulsar 4U 1626--67 is one of the few highly magnetized pulsars that accretes through Roche-lobe overflow from a low-mass companion. The characteristics of its broadband spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary systems, with a broad resonant cyclotron scattering feature (CRSF) at $\sim$37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broadband spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broadband model for the disk reflected spectrum. In the softer X-ray band, we need to add a soft black-body component with $kT_{\rm bb}$ $\sim$0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at $\sim$61 keV.

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arXiv2017

X-ray spectral properties of seven heavily obscured Seyfert 2 galaxies

We present the combined Chandra and Swift-BAT spectral analysis of seven Seyfert 2 galaxies selected from the Swift-BAT 100-month catalog. We selected nearby (z<=0.03) sources lacking of a ROSAT counterpart and never previously observed with Chandra in the 0.3-10 keV energy range, and targeted these objects with 10 ks Chandra ACIS-S observations. The X-ray spectral fitting over the 0.3-150 keV energy range allows us to determine that all the objects are significantly obscured, having NH>=1E23 cm^(-2) at a >99% confidence level. Moreover, one to three sources are candidate Compton thick Active Galactic Nuclei (CT-AGN), i.e., have NH>=1E24 cm^(-2). We also test the recent "spectral curvature" method developed by Koss et al. (2016) to find candidate CT-AGN, finding a good agreement between our results and their predictions. Since the selection criteria we adopted have been effective in detecting highly obscured AGN, further observations of these and other Seyfert 2 galaxies selected from the Swift-BAT 100-month catalog will allow us to create a statistically significant sample of highly obscured AGN, therefore better understanding the physics of the obscuration processes.

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arXiv2010

The broad-band X-ray spectrum of the Seyfert 1 galaxy, MCG+8-11-11

We present a long (100 ks) Suzaku observation of one of the X-ray brightest AGN, MCG+8-11-11. These data were complemented with the 54-month Swift BAT spectrum, allowing us to perform a broad-band fit in the 0.6-150 keV range. The fits performed in the 0.6-10 keV band give consistent results with respect to a previous XMM-Newton observation, i.e. the lack of a soft excess, warm absorption along the line of sight, a large Compton reflection component (R~1) and the absence of a relativistic component of the neutral iron K$α$ emission line. However, when the PIN and Swift BAT data are included, the reflection amount drops significantly (R~0.2-0.3), and a relativistic iron line is required, the latter confirmed by a phenomenological analysis in a restricted energy band (3-10 keV). When a self-consistent model is applied to the whole broadband data, the observed reflection component appears to be all associated to the relativistic component of the iron K$α$ line. The resulting scenario, though strongly model-dependent, requires that all the reprocessing spectral components from Compton-thick material must be associated to the accretion disc, and no evidence for the classical pc-scale torus is found. The narrow core of the neutral iron K$α$ line is therefore produced in a Compton-thin material, like the BLR, similarly to what found in another Seyfert galaxy, NGC7213, but with the notable difference that MCG+8-11-11 presents spectral signatures from an accretion disc. The very low accretion rate of NGC7213 could explain the lack of relativistic signatures in its spectrum, but the absence of the torus in both sources is more difficult to explain, since their luminosities are comparable, and their accretion rates are completely different.

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arXiv2009

Variability and spectral energy distributions of low-luminosity active galactic nuclei: a simultaneous X-ray/UV look with Swift

We have observed four low-luminosity active galactic nuclei classified as Type 1 LINERs with the X-ray Telescope (XRT) and the UltraViolet-Optical Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering mechanism of this class of nearby sources. Among our targets, we detect X-ray variability in NGC 3998 for the first time. The light curves of this object reveal variations of up to 30% amplitude in half a day, with no significant spectral variability on this time scale. We also observe a decrease of ~30% over 9 days, with significant spectral softening. Moreover, the X-ray flux is ~40% lower than observed in previous years. Variability is detected in M 81 as well, at levels comparable to those reported previously: a flux increase in the hard X-rays (1-10 keV) of 30% in ~3 hours and variations by up to a factor of 2 within a few years. This X-ray behaviour is similar to that of higher-luminosity, Seyfert-type, objects. Using previous high-angular-resolution imaging data from the Hubble Space Telescope (HST), we evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear flux in our UVOT observations. All sources are detected in the UV band, at levels similar to those of the previous observations with HST. The XRT (0.2-10 keV) spectra are well described by single power-laws and the UV-to-X-ray flux ratios are again consistent with those of Seyferts and radio-loud AGNs of higher luminosity. The similarity in X-ray variability and broad-band energy distributions suggests the presence of similar accretion and radiation processes in low- and high-luminosity AGNs.

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arXiv2006

Swift XRT Observations of the Afterglow of XRF 050416A

Swift discovered XRF 050416A with the BAT and began observing it with its narrow field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast, subsequently flattens and eventually steepens again, similar to many X-ray afterglows. The first and second phases end about 172 and 1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index Gamma ~ 3.0 during the initial steep decay, to a harder emission with Gamma ~ 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy. The consistency of the initial photon index with the high energy BAT photon index suggests that the initial phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.

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