Sofia V. Forsblom

University of Turku

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Trends

Author Order

Document Type

Co-Authors

Juri Poutanen

University of Turku

Alessandro Di Marco

Institute for Space Astrophysics and Planetology

Sergey S. Tsygankov

University of Turku

Fabio La Monaca

Institute for Space Astrophysics and Planetology

Fabio Muleri

Institute for Space Astrophysics and Planetology

Michal Dovčiak

Czech Academy of Sciences, Astronomical Institute

S. Bianchi

Roma Tre University

E. Costa

Citigroup

R. Bonino

Istituto Nazionale di Fisica Nucleare, Sezione di Torino

R. Bellazzini

Istituto Nazionale di Fisica Nucleare, Sezione di Pisa

Cooperative Institutions

Marshall Space Flight Center

Institute for Space Astrophysics and Planetology

Istituto Nazionale di Fisica Nucleare

University of Pisa

National Institute for Astrophysics

Istituto Nazionale di Fisica Nucleare, Sezione di Pisa

University of Turku

Centre National de la Recherche Scientifique

Kavli Institute for Particle Astrophysics and Cosmology

Goddard Space Flight Center

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

Studying geometry of the ultraluminous X-ray pulsar Swift J0243.6+6124 using X-ray and optical polarimetry

Astronomy and Astrophysics (2024)

Juri Poutanen Sergey S. Tsygankov Victor Doroshenko Sofia V. Forsblom Peter Jenke

Discovery of pulsations from a number of ultra-luminous X-ray (ULX) sources proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of i p = 15°–40°, the magnetic obliquity of θ p = 60°–80°, and the pulsar spin position angle of χ p ≈ −50°, which significantly differs from the constant component PA of about 10°. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30° misalignment between the pulsar angular momentum and the binary orbital axis.

Swift

10.1051/0004-6361/202450696

Cite

Citations (0)

Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431

Astronomy and Astrophysics (2023)

Victor Doroshenko Juri Poutanen Jeremy Heyl Sergey S. Tsygankov Ilaria Caiazzo

We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January–February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar’s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind.

X-ray pulsar

Position angle

10.1051/0004-6361/202347088

Cite

Citations (14)

Extraction current transients for mobility determination—A comparative study

AIP Advances (2020)

Staffan Dahlström Christian Ahläng Kim Björkström Sofia V. Forsblom Benjamin Granroth

In this work, we measure the hole mobility in the model polymer system poly(3-hexylthiophene-2,5-diyl) by using different measurement techniques. Our main purpose is to determine how the recently developed charge extraction by a linearly increasing voltage technique for metal–insulator–metal devices (MIM-CELIV) compares to other commonly used methods, such as space charge limited currents and time-of-flight. Our findings suggest that the MIM-CELIV technique gives a slightly lower mobility than the other techniques, which is understandable given that the method measures the mobility of relaxed charge carriers in the dark unlike, for example, time-of-flight where charge carriers are photogenerated. In addition, we scrutinize the accuracy and reliability of the techniques used, showing that differences in mobility smaller than a factor of two are not detectable unless statistics from many devices are available.

Electron Mobility

Charge carrier

Mobility model

10.1063/5.0008802

Cite

Citations (9)

A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375

arXiv (Cornell University) (2023)

Christian Malacaria Jeremy Heyl Victor Doroshenko Sergey S. Tsygankov Juri Poutanen

Accreting X-ray pulsars (XRPs) are presumably ideal targets for polarization measurements, as their high magnetic field strength is expected to polarize the emission up to a polarization degree of ~80%. However, such expectations are being challenged by recent observations of XRPs with the Imaging X-ray Polarimeter Explorer (IXPE). Here we report on the results of yet another XRP, EXO 2030+375, observed with IXPE and contemporarily monitored with Insight-HXMT and SRG/ART-XC. In line with recent results obtained with IXPE for similar sources, analysis of the EXO 2030+375 data returns a low polarization degree of 0%-3% in the phase-averaged study and variation in the range 2%-7% in the phase-resolved study. Using the rotating vector model we constrain the geometry of the system and obtain a value for the magnetic obliquity of ~$60^{\circ}$. Considering also the estimated pulsar inclination of ~$130^{\circ}$, this indicates that the magnetic axis swings close to the observer line of sight. Our joint polarimetric, spectral and timing analysis hint to a complex accreting geometry where magnetic multipoles with asymmetric topology and gravitational light bending significantly affect the observed source behavior.

Polarimeter

Line-of-sight

Degree of polarization

10.48550/arxiv.2304.00925

Cite

Citations (2)

Complex rotational dynamics of the neutron star in Hercules X-1 revealed by X-ray polarization

Nature Astronomy (2024)

Jeremy Heyl Victor Doroshenko Denis González–Caniulef Ilaria Caiazzo Juri Poutanen

X-ray pulsar

X-ray burster

X-ray binary

Precession

10.1038/s41550-024-02295-8

Cite

Citations (2)

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

Nature Astronomy (2024)

Alexandra Veledina Fabio Muleri Juri Poutanen Jakub Podgorný Michal Dovčiak

Eddington luminosity

Outflow

10.1038/s41550-024-02294-9

Cite

Citations (12)

X-ray Polarization Reveals the Precessions of the Neutron Star in Hercules X-1

arXiv (Cornell University) (2023)

Jeremy Heyl Victor Doroshenko Denis González–Caniulef Ilaria Caiazzo Juri Poutanen

In an accreting X-ray pulsar, a neutron star accretes matter from a stellar companion through an accretion disk. The high magnetic field of the rotating neutron star disrupts the inner edge of the disc, funneling the gas to flow onto the magnetic poles on its surface. Hercules X-1 is in many ways the prototypical X-ray pulsar; it shows persistent X-ray emission and it resides with its companion HZ Her, a two-solar-mass star, at about 7~kpc from Earth. Its emission varies on three distinct timescales: the neutron star rotates every 1.2~seconds, it is eclipsed by its companion each 1.7~days, and the system exhibits a superorbital period of 35~days which has remained remarkably stable since its discovery. Several lines of evidence point to the source of this variation as the precession of the accretion disc, the precession of the neutron star or both. Despite the many hints over the past fifty years, the precession of the neutron star itself has yet not been confirmed or refuted. We here present X-ray polarization measurements with the Imaging X-ray Polarimetry Explorer (IXPE) which probe the spin geometry of the neutron star. These observations provide direct evidence that the 35-day-period is set by the free precession of the neutron star crust, which has the important implication that its crust is somewhat asymmetric fractionally by a few parts per ten million. Furthermore, we find indications that the basic spin geometry of the neutron star is altered by torques on timescale of a few hundred days.

X-ray pulsar

Precession

X-ray burster

X-ray binary

10.48550/arxiv.2311.03667

Cite

Citations (1)

X-ray polarimetry of the accreting pulsar GX 301-2

arXiv (Cornell University) (2023)

В. Ф. Сулейманов Sofia V. Forsblom Sergey S. Tsygankov Juri Poutanen Victor Doroshenko

The phase- and energy-resolved polarization measurements of accreting X-ray pulsars (XRPs) allow us to test different theoretical models of their emission, and they also provide an avenue to determine the emission region geometry. We present the results of the observations of the XRP GX 301-2 performed with the Imaging X-ray Polarimetry Explorer (IXPE). A persistent XRP, GX 301-2 has one of the longest spin periods known: $\sim$680s. A massive hyper-giant companion star Wray 977 supplies mass to the neutron star via powerful stellar winds. We did not detect significant polarization in the phase-averaged data when using spectro-polarimetric analysis, with the upper limit on the polarization degree (PD) of 2.3% (99% confidence level). Using the phase-resolved spectro-polarimetric analysis, we obtained a significant detection of polarization (above 99% confidence level) in two out of nine phase bins and a marginal detection in three bins, with a PD ranging between $\sim$3% and $\sim$10% and a polarization angle varying in a very wide range from $\sim$0 degree to $\sim$160 degree. Using the rotating vector model, we obtained constraints on the pulsar geometry using both phase-binned and unbinned analyses, finding excellent agreement. Finally, we discuss possible reasons for a low observed polarization in GX 301-2.

10.48550/arxiv.2305.15309

Cite

Citations (1)

X-ray pulsar GRO J1008−57 as an orthogonal rotator

Astronomy and Astrophysics (2023)

Sergey S. Tsygankov Victor Doroshenko Alexander A. Mushtukov Juri Poutanen Alessandro Di Marco

X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008−57 is the first transient X-ray pulsar observed at two different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotating vector model allowed us to estimate the pulsar inclination (130°, which is in good agreement with the orbital inclination), the position angle (75°) of the pulsar spin axis, and the magnetic obliquity (∼74°). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.

X-ray pulsar

Position angle

Orbital inclination

10.1051/0004-6361/202346134

Cite

Citations (16)

Accretion Onto Weakly Magnetized Neutron Stars: Polarization Theory and Its Application to X‐Ray Burster GX 13+1

Astronomische Nachrichten (2024)

Anna Bobrikova Sofia V. Forsblom

ABSTRACT Observations show that the x‐ray emission of the accreting weakly magnetized neutron stars (WMNSs) is polarized. In this article, we summarize the analytical models for the polarized emission of various components of the WMNSs. We introduce a missing theoretical model, where we assume the emission comes from the spreading layer, the extension of the boundary layer between the accretion disk and the neutron star. We show how these models and the results of the simulations provide new insights into the x‐ray polarization from weakly magnetized neutron stars observed with the imaging x‐ray polarimetry explorer (IXPE). We specifically focus on the most peculiar case of the X‐ray burster GX 13+1.

X-ray burster

Accretion disc

X-ray binary

10.1002/asna.20240128

Cite

Citations (0)