arXiv subjects

R. Ferrazzoli

Publications and source records attributed to R. Ferrazzoli.

13 recordsLinked to original sources

arXiv2025

IXPE Observations of the Blazar Mrk 501 in 2022: A Multiwavelength View

The blazar Markarian 501 (Mrk 501) was observed on three occasions over a 4-month period between 2022 March and 2022 July with the Imaging X-ray Polarimetry Explorer (IXPE). In this paper, we report for the first time on the third IXPE observation, performed between 2022 July 9 and 12, during which IXPE detected a linear polarization degree of $Π_X=6\pm2$ per cent at a polarization angle, measured east of north, of $Ψ_X=143^\circ\pm11^\circ$ within the 2-8 keV X-ray band. The X-ray polarization angle and degree during this observation are consistent with those obtained during the first two observations. The chromaticity of the polarization across radio, optical, and X-ray bands is likewise consistent with the result from the simultaneous campaigns during the first two observations. Furthermore, we present two types of models to explain the observed spectral energy distributions (SEDs) and energy-resolved polarization: a synchrotron self-Compton model with an anisotropic magnetic field probability distribution in the emitting volume, as well as an energy-stratified shock model. Our results support both the shock scenario as well as support that small levels of magnetic field anisotropy can explain the observed polarization.

astro-ph.HE↗

arXiv2024

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei : NGC 1068

We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2-8 keV polarization of NGC 1068. We pointed IXPE for a net exposure time of 1.15 Ms on the target, in addition to two ~ 10 ks each Chandra snapshots in order to account for the potential impact of several ultraluminous X-ray source (ULXs) within IXPE's field-of-view. We measured a 2 - 8 keV polarization degree of 12.4% +/- 3.6% and an electric vector polarization angle of 101° +/- 8° at 68% confidence level. If we exclude the spectral region containing the bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises up to 21.3% +/- 6.7% in the 3.5 - 6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec scale radio jet. Using a combined Chandra and IXPE analysis plus multi-wavelength constraints, we estimated that the circumnuclear "torus" may sustain a half-opening angle of 50° - 55° (from the vertical axis of the system). Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of polarization angle orientation with respect to the radio-jet and torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN : the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic object is now feasible thanks to X-ray polarimetry.

astro-ph.HE↗

arXiv2024

IXPE observation confirms a high spin in the accreting black hole 4U 1957+115

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The $\sim$571 ks observation reveals a linear polarisation degree of $1.9\% \pm 0.6\%$ and a polarisation angle of $-41^\circ.8 \pm 7^\circ.9$ in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than $50^\circ$ are disfavoured.

astro-ph.HE↗

arXiv2023

The X-ray polarisation of the Seyfert 1 galaxy IC 4329A

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy IC 4329A. The Imaging X-ray Polarimetry Explorer (IXPE) observed the source for ~500 ks, supported by XMM-Newton (~60 ks) and NuSTAR (~80 ks) exposures. We detect polarisation in the 2-8 keV band with 2.97 sigma confidence. We report a polarisation degree of $3.3\pm1.1$ per cent and a polarisation angle of $78\pm10$ degrees (errors are 1 sigma confidence). The X-ray polarisation is consistent with being aligned with the radio jet, albeit partially due to large uncertainties on the radio position angle. We jointly fit the spectra from the three observatories to constrain the presence of a relativistic reflection component. From this, we obtain constraints on the inclination angle to the inner disc (< 39 degrees at 99 per cent confidence) and the disc inner radius (< 11 gravitational radii at 99 per cent confidence), although we note that modelling systematics in practice add to the quoted statistical error. Our spectro-polarimetric modelling indicates that the 2-8 keV polarisation is consistent with being dominated by emission directly observed from the X-ray corona, but the polarisation of the reflection component is completely unconstrained. Our constraints on viewer inclination and polarisation degree tentatively favour more asymmetric, possibly out-flowing, coronal geometries that produce more highly polarised emission, but the coronal geometry is unconstrained at the 3 sigma level.

astro-ph.HE↗

arXiv2023

Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC4151 with IXPE

We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy NGC4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree $Π = 4.9 {\pm} 1.1 \%$ and angle $Ψ= 86° {\pm} 7°$ east of north ($68\%$ confidence level) are measured in the 2-8 keV energy range. The spectro-polarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires however a reflection with a very large ($> 38 \%$) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ${\sim}4$ to ${\sim}8\%$ is found. The observed polarization degree excludes a spherical lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC4151 is the first AGN with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry.

astro-ph.GA↗

arXiv2023

X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: an in-depth study with IXPE and NuSTAR

We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4--8 keV band, with a degree of $2.2\% \pm 0.5\%$ (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disk emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disk of 4% (at 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which gives a significant contribution in any realistic scenario.

astro-ph.HE↗

arXiv2023

The geometry of the hot corona in MCG-05-23-16 constrained by X-ray polarimetry

We report on the second observation of the radio-quiet active galactic nucleus (AGN) MCG-05-23-16 performed with the Imaging X-ray Polarimetry Explorer (IXPE). The observation started on 2022 November 6 for a net observing time of 640 ks, and was partly simultaneous with NuSTAR (86 ks). After combining these data with those obtained in the first IXPE pointing on May 2022 (simultaneous with XMM-Newton and NuSTAR) we find a 2-8 keV polarization degree $Π$ = 1.6 $\pm$ 0.7 (at 68 per cent confidence level), which corresponds to an upper limit $Π$ = 3.2 per cent (at 99 per cent confidence level). We then compare the polarization results with Monte Carlo simulations obtained with the MONK code, with which different coronal geometries have been explored (spherical lamppost, conical, slab and wedge). Furthermore, the allowed range of inclination angles is found for each geometry. If the best fit inclination value from a spectroscopic analysis is considered, a cone-shaped corona along the disc axis is disfavoured.

astro-ph.HE↗

arXiv2023

X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

X Persei is a persistent low-luminosity X-ray pulsar of period of $\sim$835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above $10^{13}$ G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to $\sim$20 per cent over the pulse with a positive correlation with the pulsed X-ray flux. The polarization angle shows significant variation and makes two complete revolutions during the pulse period resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008$-$57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered with its value increasing from 0% at $\sim$2 keV to 30% at 8 keV.

astro-ph.HE↗

arXiv2022

Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements

We report spectro-polarimetric results of an observational campaign of the bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by IXPE, NICER and INTEGRAL. Consistently with previous results, the broad-band spectrum is characterized by a lower-energy component, attributed to the accretion disc with $kT_{\rm in} \approx$ 1 keV, plus unsaturated Comptonization in thermal plasma with temperature $kT_{\rm e} = 3$ keV and optical depth $τ\approx 4$, assuming a slab geometry. We measure the polarization degree in the 2-8 keV band $P=1.8 \pm 0.3$ per cent and polarization angle $ϕ= 140^{\circ} \pm 4^{\circ}$, consistent with the previous X-ray polarimetric measurements by OSO-8 as well as with the direction of the radio jet which was earlier observed from the source. While polarization of the disc spectral component is poorly constrained with the IXPE data, the Comptonized emission has a polarization degree $P =4.0 \pm 0.7$ per cent and a polarization angle aligned with the radio jet. Our results strongly favour a spreading layer at the neutron star surface as the main source of the polarization signal. However, we cannot exclude a significant contribution from reflection off the accretion disc, as indicated by the presence of the iron fluorescence line.

astro-ph.HE↗

arXiv2022

Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer

We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of $18° \pm 5°$, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45°-55°. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.

astro-ph.HE↗

arXiv2022

Polarization constraints on the X-ray corona in Seyfert Galaxies: MCG-05-23-16

We report on the first observation of a radio-quiet Active Galactic Nucleus (AGN) using polarized X-rays: the Seyfert 1.9 galaxy MCG-05-23-16. This source was pointed with the Imaging X-ray Polarimetry Explorer (IXPE) starting on May 14, 2022 for a net observing time of 486 ks, simultaneously with XMM-Newton (58 ks) and NuSTAR (83 ks). A polarization degree smaller than $Π<4.7\%$ (at the 99% c.l.) is derived in the 2-8 keV energy range, where emission is dominated by the primary component ascribed to the hot corona. The broad-band spectrum, inferred from a simultaneous fit to the IXPE, NuSTAR, and XMM-Newton data, is well reproduced by a power law with photon index $Γ=1.85\pm0.01$ and a high-energy cutoff $E_{\rm C}=120\pm15$ keV. A comparison with Monte Carlo simulations shows that a lamp-post and a conical geometry of the corona are consistent with the observed upper limit, a slab geometry is allowed only if the inclination angle of the system is less than 50$^{\circ}$.

astro-ph.HE↗

arXiv2021

Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission

Due to be launched in late 2021, the Imaging X-Ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission designed to perform polarization measurements in the 2-8 keV band, complemented with imaging, spectroscopy and timing capabilities. At the heart of the focal plane is a set of three polarization-sensitive Gas Pixel Detectors (GPD), each based on a custom ASIC acting as a charge-collecting anode. In this paper we shall review the design, manufacturing, and test of the IXPE focal-plane detectors, with particular emphasis on the connection between the science drivers, the performance metrics and the operational aspects. We shall present a thorough characterization of the GPDs in terms of effective noise, trigger efficiency, dead time, uniformity of response, and spectral and polarimetric performance. In addition, we shall discuss in detail a number of instrumental effects that are relevant for high-level science analysis -- particularly as far as the response to unpolarized radiation and the stability in time are concerned.

astro-ph.IM↗

arXiv2020

Prospects for IXPE and eXTP polarimetric archaeology of the reflection nebulae in the Galactic center

The X-ray polarization properties of the reflection nebulae in the Galactic center inform us about the direction of the illuminating source (through the polarization angle) and the cloud position along the line of sight (through the polarization degree). However, the detected polarization degree is expected to be lowered because the polarized emission of the clouds is mixed with the unpolarized diffuse emission that permeates the Galactic center region. In a real observation, also the morphological smearing of the source due to the point spread function and the unpolarized instrumental background contribute in diluting the polarization degree. So far, these effects have never been included in the estimation of the dilution. We evaluate the detectability of the X-ray polarization predicted for the MC2, Bridge-B2, G0.11-0.11, Sgr B2, Sgr C1, Sgr C2, and Sgr C3 molecular clouds with modern X-ray imaging polarimeters such as the Imaging X-ray Polarimetry Explorer (IXPE) and the Enhanced X-ray Timing and Polarimetry mission (eXTP). We perform realistic simulations of X-ray polarimetric observations considering (with the aid of Chandra maps and spectra) the spatial, spectral, and polarization properties of all the diffuse emission and background components in each region of interest. We find that in the 4.0-8.0 keV band, where the emission of the molecular clouds outshines the other components, the dilution of the polarization degree, including the contribution due to the morphological smearing of the source, ranges between $\sim$19\% and $\sim$55\%. We conclude that for some distance values reported in the literature, the diluted polarization degree of G0.11-0.11, Sgr B2, Bridge-B2, Bridge-E, Sgr C1, and Sgr C3 may be detectable in a 2 Ms long IXPE observations. The enhanced capabilities of eXTP may allow detecting the 4.0-8.0 keV of all the targets considered here.

astro-ph.HE↗