arXiv subjects

Niccolò Di Lalla

Publications and source records attributed to Niccolò Di Lalla.

At least 19 recordsLinked to original sources

arXiv2026

IXPE view of the Crab pulsar following the 17 July and 6 August 2025 glitches

The Crab pulsar experienced two relatively small glitches separated by only 20 days in September and October 2025. IXPE observed the source twice, with delay times since the glitch epoch ranging between 35 and 75 days, depending on the observation. We carried out a multi-method analysis to investigate whether there is evidence for significant changes in the polarization properties of the pulsar, underlying possible variations in the pulsar magnetosphere itself following the glitches. Specifically, we performed: (1) phase-averaged polarimetry of the Crab pulsar before and after the glitches, following an approach similar to that adopted in 2019 by PolarLight, a non-imaging CubeSat-class photoelectric polarimeter which observed a change in the X-ray polarization within 100 days after a stronger glitch in July 2019; (2) a comparison, before and after the glitch, of phase-resolved X-ray polarimetry with IXPE, not possible with PolarLight. Furthermore, we investigated, by means of phase-resolved optical (OPTIMA) polarimetry, whether a significant change in the X-to-optical lag was present in the data before and after the glitch. We find no evidence of a change in the polarization for the pulsar emission before and after the glitch, We use the upper limits obtained to estimate the maximum change in magnetic obliquity allowed by the data, using the standard rotating vector model and assuming that the glitch is due to a neutron-star quake. We constrain this maximum change to be no greater than $\pm4^{o}$ at the 95\% confidence level.

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arXiv2026

Testing Magnetic Field Configurations in Spider Pulsar PSR J1723-2837 with IXPE

We present the first X-ray polarimetry observations of a redback millisecond pulsar binary, PSR J1723-2837, with the Imaging X-ray Polarimetry Explorer (IXPE). We conduct a spectro-polarimetric analysis combining IXPE data with archival Chandra, XMM-Newton, NuSTAR, and Swift observations. We explore two limiting magnetic field configurations, parallel and perpendicular to the bulk flow, and simulate their expected polarization signatures using the 3DPol radiative transport code. To account for the rapid rotation of the polarization angle predicted by these models, we implement a phase-dependent Stokes alignment procedure that preserves the polarization degree while correcting for a phase-rotating PA. We also devise a new maximum-likelihood fitting strategy to determine the phase-dependence of the polarization angle by minimizing the polarization degree uncertainty. This technique hints that the binary may be rotating clockwise relative to the celestial north pole. We find no significant detection of polarization in the IXPE data, with PD<~50% at 99% confidence level. Our results excludes the high-polarization degree scenario predicted by the perpendicular field model during the brightest orbital phase bin. Simulations show that doubling the current exposure would make the parallel configuration detectable. The new PA rotation technique is also applicable to IXPE data of many sources whose intrinsic PA variation is a priori not known but is strictly periodic.

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arXiv2026

Polarization of reflected X-ray emission from the Sgr A molecular complex: multiple flares, multiple sources?

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ($δt<$1.5 yr) and bright ($L_{\rm X}>10^{39} {\rm erg s^{-1}}$) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We reveal an inconsistent polarization pattern for the brightest reflection region in its center. The X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination and shows smaller degree of polarization compared to the large region. This could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

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arXiv2025

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

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arXiv2025

From Rare Events to a Population: Discovering Overlooked Extragalactic Magnetar Giant Flare Candidates in Archival Fermi Gamma-ray Burst Monitor Data

Magnetar giant flares (MGFs) are rare, extremely bright bursts of gamma-rays from highly magnetized neutron stars. These events are challenging to identify because, at extragalactic distances, they can appear similar to other astrophysical phenomena. Only a handful have been confidently identified to date, limiting our understanding of their origin and physical properties. This study focuses on expanding the sample of known events and enabling a more detailed characterization of their observational features and intrinsic properties, while introducing significant improvements in the methods used to identify and analyze them. When applied to archival data from the Gamma-ray Burst Monitor (GBM) on the \Fermi Gamma-ray Space Telescope, this approach added four previously unidentified events the known sample, expanding the total to 13 MGFs. This demonstrates both the effectiveness of the method and the likelihood that additional MGFs remain hidden in existing gamma-ray burst catalogs. We utilize this expanded sample to gain a deeper understanding of the broader population of MGFs. We develop a statistical modeling framework that combines previously considered data with modern observations from Fermi/GBM. The model accounts for instrumental sensitivity and the expected diversity in event characteristics. We infer a volumetric rate of events above $1.2\times10^{44}\,\rm{erg}$ of $R_{MGF}=5.5^{+4.5}_{-2.7}\times10^5\rm{Gpc^{-3}yr^{-1}}$. The results show that individual magnetars must produce multiple flares throughout their lifetimes, reinforcing the idea that these are recurring phenomena rather than singular explosive events. Expanding the sample of known MGFs improves our understanding of magnetars and their role in other astrophysical phenomena, including possible links to fast radio bursts, gravitational waves, and the creation of heavy elements in extreme astrophysical environments.

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arXiv2025

X-ray Polarization Detection of the Pulsar Wind Nebula in G21.5-0.9 with IXPE

We present the X-ray polarization observation of G21.5-0.9, a young Galactic supernova remnant (SNR), conducted with the Imaging X-ray Polarimetry Explorer (IXPE) in October 2023, with a total livetime of approximately 837 ks. Using different analysis methods, such as a space-integrated study of the entire region of the PWN and a space-resolved polarization map, we detect significant polarization from the pulsar wind nebula (PWN) at the center of the SNR, with an average polarization degree of ~10% oriented at ~33° (north through east). No significant energy-dependent variation in polarization is observed across the IXPE band (2-8 keV). The polarization map, corrected for the effect of polarization leakage, reveals a consistent pattern in both degree and angle, with little change across the nebula. Our findings indicate the presence of a highly polarized central torus, suggesting low levels of turbulence at particle acceleration sites. Unlike Vela, but similar to the Crab Nebula, we observe substantial differences between radio and X-ray polarization maps. This suggests a clear separation in energy of the emitting particle populations and hints at an important, yet poorly understood, role of instabilities in the turbulence dynamics of PWNe.

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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

Evidence for a shock-compressed magnetic field in the northwestern rim of Vela Jr. from X-ray polarimetry

Synchrotron X-ray emission has been detected from nearly a dozen young supernova remnants (SNRs). X-rays of synchrotron origin exhibit linear polarization in a regular, non-randomly oriented magnetic field. The significant polarized X-ray emission from four such SNRs has already been reported on the basis of observations with the Imaging X-ray Polarimetry Explorer (IXPE). The magnetic-field structure as derived from IXPE observations is radial for Cassiopeia A, Tycho's SNR, and SN 1006, and tangential for RX J1713.7-3946. The latter together with the recent detection of a tangential magnetic field in SNR 1E 0102.2-7219 by the Australia Telescope Compact Array in the radio band shows that tangential magnetic fields can also be present in young SNRs. Thus, the dichotomy in polarization between young and middle-aged SNRs (radial magnetic fields in young SNRs, but tangential magnetic fields in middle-aged SNRs), previously noticed in the radio band, deserves additional attention. The present analysis of IXPE observations determines, for the first time, a magnetic-field structure in the northwestern rim of Vela Jr, also known as RX J0852.0-4622, and provides a new example of a young SNR with a tangential magnetic field.

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arXiv2024

Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE

The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds the critical value, known as the Eddington limit. Discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity significantly exceeds this limit. High apparent luminosity might be produced thanks to geometric beaming of the radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we made the measurement of X-ray polarization in the Galactic X-ray binary Cyg X-3. We find high, over 20%, nearly energy-independent linear polarization, orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow in the X-ray binary Cyg~X-3 and constrain its half-opening angle, <15 degrees. Thus, the source can be used as a laboratory for studying the super-critical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources.

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arXiv2024

Discovery of a shock-compressed magnetic field in the north-western rim of the young supernova remnant RX J1713.7-3946 with X-ray polarimetry

Supernova remnants (SNRs) provide insights into cosmic-ray acceleration and magnetic field dynamics at shock fronts. Recent X-ray polarimetric measurements by the Imaging X-ray Polarimetry Explorer (IXPE) have revealed radial magnetic fields near particle acceleration sites in young SNRs, including Cassiopeia A, Tycho, and SN 1006. We present here the spatially-resolved IXPE X-ray polarimetric observation of the northwestern rim of SNR RX J1713.7-3946. For the first time, our analysis shows that the magnetic field in particle acceleration sites of this SNR is oriented tangentially with respect to the shock front. Because of the lack of precise Faraday-rotation measurements in the radio band, this was not possible before. The average measured polarization degree (PD) of the synchtrotron emission is 12.5 {\pm} 3.3%, lower than the one measured by IXPE in SN 1006, comparable to the Tycho one, but notably higher than the one in Cassiopeia A. On sub-parsec scales, localized patches within RX J1713.7-3946 display PD up to 41.5 {\pm} 9.5%. These results are compatible with a shock-compressed magnetic field. However, in order to explain the observed PD, either the presence of a radial net magnetic field upstream of the shock, or partial reisotropization of the turbulence downstream by radial magneto-hydrodynamical instabilities, can be invoked. From comparison of PD and magnetic field distribution with γ-rays and 12 CO data, our results provide new inputs in favor of a leptonic origin of the γ-ray emission.

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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

X-ray Polarization of the Black Hole X-ray Binary 4U 1630-47 Challenges Standard Thin Accretion Disk Scenario

Large energy-dependent X-ray polarization degree is detected by the Imaging X-ray Polarimetry Explorer ({IXPE}) in the high-soft emission state of the black hole X-ray binary 4U 1630--47. The highly significant detection (at $\approx50σ$ confidence level) of an unexpectedly high polarization, rising from $\sim6\%$ at $2$ keV to $\sim10\%$ at $8$ keV, cannot be easily reconciled with standard models of thin accretion discs. In this work we compare the predictions of different theoretical models with the {IXPE} data and conclude that the observed polarization properties are compatible with a scenario in which matter accretes onto the black hole through a thin disc, covered by a partially-ionized atmosphere flowing away at mildly relativistic velocities.

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arXiv2024

Pulsar-wind-nebula-powered Galactic center X-ray filament G0.13-0.11: Proof of the synchrotron nature by IXPE

We report the discovery of X-ray polarization from the X-ray-bright filament. G0.13-0.11 in the Galactic center (GC) region. This filament features a bright, hard X-ray source that is most plausibly a pulsar wind nebula (PWN) and an extended and structured diffuse component. Combining the polarization signal from IXPE with the imaging/spectroscopic data from Chandra, we find that X-ray emission of G0.13-0.11 is highly polarized PD=$57(\pm18)$% in the 3-6 keV band, while the polarization angle is PA=$21^\circ(\pm9^\circ)$. This high degree of polarization proves the synchrotron origin of the X-ray emission from G0.13-0.11. In turn, the measured polarization angle implies that the X-ray emission is polarized approximately perpendicular to a sequence of nonthermal radio filaments that may be part of the GC Radio Arc. The magnetic field on the order of $100\,{\rmμG}$ appears to be preferentially ordered along the filaments. The above field strength is the fiducial value that makes our model self-consistent, while the other conclusions are largely model independent.

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arXiv2024

First detection of polarization in X-rays for PSR B0540-69 and its nebula

We report on X-ray polarization measurements of the extra-galactic Crab-like PSR B0540-69 and its Pulsar Wind Nebula (PWN) in the Large Magellanic Cloud (LMC), using a ~850 ks Imaging X-ray Polarimetry Explorer (IXPE) exposure. The PWN is unresolved by IXPE. No statistically significant polarization is detected for the image-averaged data, giving a 99% confidence polarization upper limit (MDP99) of 5.3% in 2-8 keV energy range. However, a phase-resolved analysis detects polarization for both the nebula and pulsar in the 4-6 keV energy range. For the PWN defined as the off-pulse phases, the polarization degree (PD) of (24.5 ${\pm}$ 5.3)% and polarization angle (PA) of (78.1 ${\pm}$ 6.2)° is detected at 4.6$σ$ significance level, consistent with the PA observed in the optical band. In a single on-pulse window, a hint of polarization is measured at 3.8$σ$ with polarization degree of (50.0 ${\pm}$ 13.1)% and polarization angle of (6.2 ${\pm}$ 7.4)°. A 'simultaneous' PSR/PWN analysis finds two bins at the edges of the pulse exceeding 3$σ$ PD significance, with PD of (68 ${\pm}$ 20)% and (62 ${\pm}$ 20)%; intervening bins at 2-3$σ$ significance have lower PD, hinting at additional polarization structure.

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arXiv2024

Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

The Imaging X-ray Polarimetry Explorer (IXPE) measured with high significance the X-ray polarization of the brightest Z-source Scorpius X-1, resulting in the nominal 2-8 keV energy band in a polarization degree of 1.0(0.2)% and a polarization angle of 8(6)° at 90% of confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broad-band spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasi-periodic oscillations. From a spectro-polarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% of confidence level, compatible with expectations for an electron-scattering dominated optically thick atmosphere at the Sco X-1 inclination of 44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3(0.4)%, in agreement with expectations for a slab of Thomson optical depth of ~7 and an electron temperature of ~3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or to a change in the corona geometry with the accretion flow.

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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

Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5-1

We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) \gx in X-rays (IXPE, NICER, Nustar and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by \IXPE twice in March-April 2023 (Obs. 1 and 2). In the radio band, the source was detected, but only upper-limits to the linear polarization were obtained at a $3σ$ level of $6.1\%$ at 5.5 GHz and $5.9\%$ at 9 GHz in Obs.~1 and $12.5\%$ at 5.5~GHz and $20\%$ at 9~GHz in Obs.~2. The mid-IR, near-IR and optical observations suggest the presence of a compact jet which peaks in the mid- or far-IR. The X-ray polarization degree was found to be $3.7\% \pm 0.4 \%$ (at $90\%$ confidence level) during Obs.~1 when the source was in the horizontal branch of the Z-track and $1.8\% \pm 0.4 \%$ during Obs.~2 when the source was in the normal-flaring branch. These results confirm the variation of polarization degree as a function of the position of the source in the color-color diagram as for previously observed Z-track sources (Cyg~X-2 and XTE~1701$-$462). Evidence for a variation of the polarization angle $\sim 20^\circ$ with energy is found in both observations, likely related to the different, non-orthogonal polarization angles of the disk and Comptonization components which peak at different energies.

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arXiv2023

The detection of polarized x-ray emission from the magnetar 1E 2259+586

We report on IXPE, NICER and XMM-Newton observations of the magnetar 1E 2259+586. We find that the source is significantly polarized at about or above 20% for all phases except for the secondary peak where it is more weakly polarized. The polarization degree is strongest during the primary minimum which is also the phase where an absorption feature has been identified previously (Pizzocaro et al. 2019). The polarization angle of the photons are consistent with a rotating vector model with a mode switch between the primary minimum and the rest of the rotation of the neutron star. We propose a scenario in which the emission at the source is weakly polarized (as in a condensed surface) and, as the radiation passes through a plasma arch, resonant cyclotron scattering off of protons produces the observed polarized radiation. This confirms the magnetar nature of the source with a surface field greater than about 10 15 G

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