arXiv subjects

Antonella Tarana

Publications and source records attributed to Antonella Tarana.

16 recordsLinked to original sources

arXiv2026

X-ray polarization of Z-type neutron star low-mass X-ray binaries -- II. Spectropolarimetric analysis

IXPE has provided for the first time detailed energy- and time-resolved X-ray polarimetry of Z-type neutron star low-mass X-ray binaries (NS-LMXBs) as they move along their color-color diagrams (CCDs). These sources can reach the highest polarization observed for NS-LXMBs in the 2-8 keV range when they move along the horizontal branch. In a previous paper, we characterized the spectral state of a sample of Z-sources using the CCD and estimated the polarization with model-independent analysis. Here, we present detailed spectropolarimetric analysis for each source on each branch using data from IXPE, NICER, and NuSTAR. The continuum X-ray emission of all the sources is well described with a combination of thermal accretion disk emission plus a harder Comptonized component. In addition, reflection features, in particular the relativistically broadened Fe line, are observed for our sources, except GX 5-1. For most of the sources and branches, the main contribution to the X-ray emission and polarization is due to Comptonization: moving from the horizontal branch (HB) to the normal branch (NB), the polarization degree (PD) in the 2-8 keV band varies from about 6% to 3-4%, while the PD is loosely constrained in the flaring branch (FB), due to the shorter exposures. These PD values are significantly higher than theoretical expectations for typical spreading or boundary layer configurations. The polarization of the disk is generally lower (below 3%) but still higher than predictions for an electron scattering-dominated, plane-parallel atmosphere above the disk observed at the corresponding inclination. Moreover, the polarization angle (PA) of the disk seems to be significantly misaligned and not perpendicular to that of Comptonization. We find no correlation between the polarization signal and the inclination, nor with the contribution of reflected photons throughout the Z-track.

astro-ph.HE↗

arXiv2026

A multiwavelength study of the Galactic center black hole candidate MAXI J1744-294

For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or -- as it was known in its previous life -- Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of $a>0.92$ and viewing inclination $θ=28^{+3}_{-4}$ deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) -- the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr~A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

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arXiv2026

Discovery of High X-Ray Polarization from the Neutron Star Low-Mass X-Ray Binary Cyg X-2 in the Horizontal Branch

We present results from simultaneous X-ray polarimetric and spectroscopic observations of the bright neutron star low-mass X-ray binary Cyg X-2, performed by the Imaging X-ray Polarimetry Explorer (IXPE) and the Nuclear Spectroscopic Telescope Array (NuSTAR). IXPE detected significant polarization (15 sigma) from the source in the 2-8 keV energy band with an average polarization degree (PD) of 4.5% +/- 0.3% and a polarization angle (PA) of 128 +/- 2 degrees as the source moved along the horizontal branch of its Z-track. The PD increases with energy reaching 9.9% +/- 2.8% in the 7-8 keV band, with no evidence for energy-dependent variation in the PA. The PA is roughly consistent with previous measurements obtained during the normal and flaring branches and also with the known radio jet axis. From spectropolarimetric analysis, the main contribution to the polarized radiation is due to Comptonized photons, but the polarization is higher than predicted in typical spreading layer geometries. The observed high polarization may be due to a combination of a highly polarized reflected component and a moderately polarized spreading layer on the neutron star surface or produced by electron scattering in an equatorial wind.

astro-ph.HE↗

arXiv2025

The First X-Ray Polarimetry of an Eclipsing Low-Mass X-Ray Binary: Serendipitous IXPE Observation of AX J1745.6-2901

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6-2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within $ \sim $1.5' of the Galactic center, was observed serendipitously during a MAXI J1744-294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6-2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744-294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7$\% \pm$ 4.0$\%$ and polarization angle PA = 122$^\circ \pm 8^\circ$. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2$\% \pm$ 8.7$\%$, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

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arXiv2025

Crystal Eye: all sky MeV monitor with high precision real-time localization

Crystal Eye is a space-based all-sky monitor optimized for the autonomous detection and localization of transients in the 10 keV to 30 MeV energy range, a region where extensive observations and monitoring of various astrophysical phenomena are required. By focusing on the operating environment and its impact on the observation process, we optimized the detector design and assessed its scientific potential. We explored the use of novel techniques to achieve the science goals of the experiment. We assumed the orbit of a potential future mission at approximately 550 km altitude near the equatorial region with a 20° inclination. In such an orbit, the main background contributions for this kind of detector are from different particles and radiation of cosmic origin and secondaries produced by their interaction in the Earth's atmospheric and geomagnetic environment. We studied the response of the detector in this background environment, using the Geant4 Monte Carlo simulation toolkit. We also calculated other detector performance parameters to estimate its scientific capabilities. The detector effective area and efficiency are calculated for low energy gamma-ray sources and used to estimate its sensitivity to short-duration transient sources. The calculation shows a better effective area and sensitivity by several factors compared to existing instruments of similar type. A method is also developed and discussed to estimate the online transient-localization performance of the detector, suggesting a better localization precision by about an order of magnitude than those typically reported by existing gamma-ray monitors. We present here the simulation study and results of an innovative detector design concept that can make a significant contribution in the multi-messenger era. Moreover, this study can be useful as a technical reference for similar future experiments.

astro-ph.IM↗

arXiv2025

X-ray polarization of Z-type neutron star low-mass X-ray binaries -- I. Model-independent, time-resolved X-ray polarimetry

Z-sources are a particular class of neutron star low-mass X-ray binaries characterized by a wide Z-like track in their hard colorsoft color (or hardness-intensity) diagrams, with three branches: the horizontal (HB), the normal (NB), and the flaring branch (FB). Spectropolarimetric observations with the Imaging X-ray Polarimetry Explorer (IXPE) show that the polarization in these sources varies along the Z-track, reaching unexpectedly high values in the HB. In this work, we collected all the polarimetric results obtained so far from observations of Z-sources with IXPE, using a model-independent analysis with ixpeobssim. We first performed a detailed characterization of the spectral state of each source along the Z-track using IXPE, along with the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) data and then estimated the polarization for each branch. Although we confirm that the average polarization in the 2-8 keV band decreases moving from the HB to the NB for all three Z-sources observed in these branches, we also observe a qualitatively increasing trend from the NB to the FB. Whereas this increase is clearly significant for Cyg X-2 and Sco X-1, the polarization remains consistent at the 90% confidence level for GX 5-1 and GX 349+2, while for XTE J1701-462 and GX 340+0 only upper limits are found in the FB. For most sources, the average polarization angle in the 2-8 keV range remains consistent along the CCD; however, we observe a significant rotation for both Sco X1 and GX 349+2 (at the 90% confidence level) as they move from the NB to the FB. In addition, we observe a significant increase in the polarization degree with energy in most of the observed Z-sources, with some also exhibiting a rotation of the polarization angle with energy (approximately by 20-30 deg).

astro-ph.HE↗

arXiv2025

Exploring MAXI J1744-294: IXPE insights into a Newly Discovered X-ray Transient

We present the first IXPE spectro-polarimetric observation of the black hole candidate MAXI J1744$-$294, a transient X-ray source discovered during a bright 2025 outburst in the Galactic center region. During the $\sim$150 ks observation, the source was found in the soft state, and its spectrum was well described by an absorbed multicolor disk with a minor high-energy tail. No significant polarization was detected, and we derived a 3$σ$ upper limit on the polarization degree of $1.3\%$ in the 2--8 keV energy band. This result is consistent with previous findings for soft-state black hole binaries observed at low to intermediate inclination angles. By comparing the polarization degree upper limit with theoretical predictions for standard accretion disk emission, we constrain the disk inclination to $i \lesssim 38^\circ$--$71^\circ$, depending on the black hole spin and the disk atmosphere albedo.

astro-ph.HE↗

arXiv2025

X-ray spectro-polarimetry analysis of the weakly magnetized neutron star X-ray binary GX 9+1

We present an X-ray spectro-polarimetric study of the weakly magnetized neutron star low-mass X-ray binary GX 9+1, utilizing data from the Imaging X-ray Polarimetry Explorer (IXPE), alongside simultaneous NuSTAR, NICER, and INTEGRAL observations. GX 9+1, located in the Galactic bulge, is a persistently bright Atoll source known for its spectral variability along the color-color diagram. Our spectral analysis during the soft state confirms emission dominated by a soft blackbody and thermal Comptonization components, with no evidence of a hard X-ray tail. These observations suggest a relatively low-inclination system (23 deg < i < 46 deg) with a weak reflection component, consistent with emission from the accretion disk and neutron star boundary layer. Spectro-polarimetric analysis reveals no significant polarization in the 2-8 keV range, with a 3-sigma upper limit for the polarization degree of 1.9%. However, marginal evidence of polarization was detected in the 2-3 keV band at the 95.5% confidence level (2-sigma), suggesting potential contributions from scattering effects in the individual spectral components (disk, reflection, and Comptonization) that could cancel each other out due to the different orientations of their polarization angles. This behavior aligns with other Atoll sources observed by IXPE, which typically exhibit lower and less variable polarization degrees compared to Z-class sources.

astro-ph.HE↗

arXiv2024

First spectropolarimetric observation of the neutron star low-mass X-ray binary GX 3+1

We report the first simultaneous X-ray spectropolarimetric observation of the bright atoll neutron star low-mass X-ray binary GX 3+1, performed by the Imaging X-ray Polarimetry Explorer (IXPE) joint with NICER and NuSTAR. The source does not exhibit significant polarization in the 2-8 keV energy band, with an upper limit of 1.3% at a 99% confidence level on the polarization degree. The observed spectra can be well described by a combination of thermal disk emission, the hard Comptonization component, and reflected photons off the accretion disk. In particular, from the broad Fe K$α$ line profile, we were able to determine the inclination of the system ($i \approx 36^\circ$), which is crucial for comparing the observed polarization with theoretical models. Both the spectral and polarization properties of GX 3+1 are consistent with those of other atoll sources observed by IXPE. Therefore, we may expect a similar geometrical configuration for the accreting system and the hot Comptonizing region. The low polarization is also consistent with the low inclination of the system.

astro-ph.HE↗

arXiv2023

First detection of X-ray polarization from the accreting neutron star 4U 1820-303

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz.

astro-ph.HE↗

arXiv2013

High energy characteristics of Neutron star low mass X-ray binaries as seen by INTEGRAL: the 4U 1722-30 case

We report here results on the long time behaviour of the variable source 4U 1722-30, obtained from INTEGRAL non-continuous observations carried out between 2003 and 2009. During this period 4U 1722-30 shows a general persistent emission along with several bright outbursts resembling those of transient X-ray sources/X-ray novae. However the source never switches into a real quiescent state, with very weak or undetectable flux level, as it normally occurs in transient sources. We compare here flux ratios and spectral state variations measured during the different outbursts and highlight the peculiar character of the 2008 outburst, for which we also propose a specific physical explanation.

astro-ph.HE↗

arXiv2008

INTEGRAL and BeppoSAX observations of the transient atoll source 4U 1608-522: from quiescent to hard spectral state

In this paper we report on the spectral evolution of 4U 1608-522 performed as part of the long Galactic Bulge monitoring with INTEGRAL. The data set include the April 2005 outburst. BeppoSAX archival data (two observations, in 1998 and 2000) have been also analysed and compared with the INTEGRAL ones. Three different spectral states have been identified from the hard Color-Intensity diagram derived from INTEGRAL: the canonical Hard and Soft as well as an Intermediate state. The hard state spectrum is well described by a weak black body component plus a Comptonised plasma component with high electron temperature (kTe~60 keV) extending up to 200 keV without any additional cut-off. The soft spectra are characterized by a cold Comptonised plasma (kTe= 2-3 keV, and 7 keV for the intermediate state) and a strong disk black body component. A reflection component, indicating reflection of the X-ray radiation from the accretion disc, is also present in the soft state revealed by BeppoSAX in 1998. The 2000 BeppoSAX observation revealed the source in quiescent state modelled by a neutron star atmosphere (assuming a neutron star with radius 10 km and mass 1.4 Msun) with an effective temperature, kTeff of 0.1 keV plus a power law component with Gamma~3 detected for the first time for this source. This spectrum can also be modelled with a simple black body compatible with emission originating from a small fraction of the NS surface of radius of 0.4 km.

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arXiv2006

Observing the high energy behaviour of the low mass X-ray binary XB 1832-330 with INTEGRAL

We report INTEGRAL/IBIS results on temporal and spectral behaviour of the Low Mass X-ray Binary XB 1832-330 located in the globular cluster NGC 6652. During the 2003-2005 monitoring of the Galactic Centre, the source shows a weak flux variability and an everage flux in the 20-150 keV of about 2*10^{-10} erg s^{-1} cm^{-2}. The overall energy spectrum extends up to 150 keV and is well described by a Comptonization model with an electrons temperature of $\sim$ 22 keV and a plasma optical depth of 1.8. We thus confirm the persistent nature of this burtser.

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arXiv2006

Following the Colour of the Low Mass X-ray Binary 4U 1820-30 with INTEGRAL

The 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 has been studied with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail above 50 keV in the hard state has been revealed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~ 2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e ~ 6 keV and optical depth of $τ~ 4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During the observations, the source spent most of the time in the soft state, as previously reported and the $\ge$4 keV spectra can be represented by thermal Comptonization with kT_e ~ 3 keV and $τ~ 6-7.

astro-ph↗

arXiv2006

INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission

We study the 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e~6 keV and optical depth of τ~4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the >~4 keV spectra are represented by thermal Comptonization with kT_e~3 keV and τ~6-7.

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arXiv2004

INTEGRAL, XMM-Newton and Rossi-XTE Observations of the State Transition of the X-ray Transient and Black Hole Candidate XTE J1720-318

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton and RXTE. The source, which underwent an outburst in January 2003, was observed in February in a spectral state dominated by a soft component with a weak high-energy tail. The XMM-Newton data provided a high column density Nh of 1.2*e22 cm^{-2} which suggests that the source lies at the Galactic Center distance. The simultaneous RXTE and INTEGRAL Target of Opportunity observations allowed us to measure the weak and steep tail, typical of a black-hole binary in the so-called High/Soft State. We could follow the evolution of the source outburst over several months using the INTEGRAL Galactic Center survey observations. The source regained activity at the end of March: it showed a clear transition towards a much harder state, and then decayed to a quiescent state in summer. In the hard state, the source was detected up to 200 keV with a typical power law index of 1.8 and a peak luminosity of 7.5*e36 ergs s^{-1} in the 2-100 keV band, for an assumed distance of 8 kpc. We conclude that XTE J1720-318 is indeed representative of the class of the black hole X-ray novae which populate our Galactic bulge and we discuss its properties in the frame of the spectral models used for transient black hole binaries.

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