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  • 1.
    Chashkina, Anna
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Lipunova, Galina
    Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Abolmasov, Pavel
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;KTH Royal Inst Technol, NORDITA, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Super-Eddington accretion discs with advection and outflows around magnetized neutron stars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A18Article in journal (Refereed)
    Abstract [en]

    We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all the basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once it approaches the local Eddington limit the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase in the magnetospheric radius. Our model can be applied to a wide variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC5907 X-1, the brightest ultra-luminous pulsar currently known, and NGC300 ULX1, which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.

  • 2.
    Krivonos, Roman
    et al.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Sazonov, Sergey
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Natl Res Univ Higher Sch Econ, Myasnitskaya Ul 20, Moscow 101000, Russia..
    Tsygankov, Sergey S.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    NuSTAR observations of the ultraluminous X-ray source M33 X-8: a black hole in a very high state?2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 2357-2364Article in journal (Refereed)
    Abstract [en]

    The closest known ultraluminous X-ray source (ULX), M33 X-8, has been recently observed with NuSTAR during its Extragalactic Legacy program, which includes a hard X-ray survey of the M33 galaxy. We present results of two long observations of M33 taken in 2017 March and July, with M33 X-8 in the field of view. The source demonstrates a nearly constant flux during the observations, and its 3-20 keV spectrum can be well described by two distinct components: a standard accretion disc with a temperature of similar to 1 keV at the inner radius and a power law with a photon index Gamma approximate to 3, which is significantly detected up to 20 keV. There is also an indication of a high-energy cut-off in the spectrum, corresponding to a temperature of the Comptonizing medium of greater than or similar to 10 keV. The broad-band spectral properties of M33 X-8 resemble black hole X-ray binaries in their very high states, suggesting that M33 X-8 is a black hole accreting at a nearly Eddington rate, in contrast to super-Eddington accretion believed to take place in more luminous ULXs.

  • 3.
    Kuuttila, J.
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland..
    Kajava, J. J. E.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Univ Turku, Finnish Ctr Astron ESO FINCA, Vaisalantie 20, Piikkio 21500, Finland.;ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Nättila, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Motta, S. E.
    Univ Oxford, Dept Phys, Astrophys, Denys Wilkinson Bldg,Keble Rd, Oxford OX1 3RH, England..
    Sanchez-Fernandez, C.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Kuulkers, E.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain.;ESA, Estec, Keplerlaan 1, NL-2201 AZ Noordwijk, Netherlands..
    Cumming, A.
    McGill Univ, Dept Phys, 3550 Univ St, Montreal, PQ H3A 2T8, Canada.;McGill Univ, McGill Space Inst, 3550 Univ St, Montreal, PQ H3A 2T8, Canada..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Univ Calif Santa Barbara, Kavli Inst Theoret Phys, Santa Barbara, CA 93106 USA..
    Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, article id A77Article in journal (Refereed)
    Abstract [en]

    The cooling of type-I X-ray bursts can be used to probe the nuclear burning conditions in neutron star envelopes. The flux decay of the bursts has been traditionally modelled with an exponential, even if theoretical considerations predict power-law-like decays. We have analysed a total of 540 type-I X-ray bursts from five low-mass X-ray binaries observed with the Rossi X-ray Timing Explorer. We grouped the bursts according to the source spectral state during which they were observed (hard or soft), flagging those bursts that showed signs of photospheric radius expansion (PRE). The decay phase of all the bursts were then fitted with a dynamic power-law index method. This method provides a new way of probing the chemical composition of the accreted material. Our results show that in the hydrogen-rich sources the power-law decay index is variable during the burst tails and that simple cooling models qualitatively describe the cooling of presumably helium-rich sources 4U 1728-34 and 3A 1820-303. The cooling in the hydrogen-rich sources 4U 1608-52, 4U 1636-536, and GS 1826-24, instead, is clearly different and depends on the spectral states and whether PRE occurred or not. Especially the hard state bursts behave differently than the models predict, exhibiting a peculiar rise in the cooling index at low burst fluxes, which suggests that the cooling in the tail is much faster than expected. Our results indicate that the drivers of the bursting behaviour are not only the accretion rate and chemical composition of the accreted material, but also the cooling that is somehow linked to the spectral states. The latter suggests that the properties of the burning layers deep in the neutron star envelope might be impacted differently depending on the spectral state.

  • 4.
    Li, Z.
    et al.
    Xiangtan Univ, Dept Phys, Xiangtan 411105, Peoples R China.;ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Univ Bern, Albert Einstein Ctr Fundamental Phys, Inst Theoret Phys, Lab High Energy Phys, Bern, Switzerland..
    De Falco, V.
    ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Univ Basel, Dept Phys, Klingelbergstr 82, CH-4056 Basel, Switzerland..
    Falanga, M.
    ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Int Space Sci Inst Beijing, 1 Nanertiao, Beijing 100190, Peoples R China..
    Bozzo, E.
    Univ Geneva, Dept Astron, Chemin Ecogia 16, CH-1290 Versoix, Switzerland..
    Kuiper, L.
    SRON, Netherlands Inst Space Res, Sorbonnelaan 2, NL-3584 CA Utrecht, Netherlands..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.
    Cumming, A.
    McGill Univ, Dept Phys, 3600 Rue Univ, Montreal, PQ H3A 2T8, Canada.;McGill Univ, McGill Space Inst, 3600 Rue Univ, Montreal, PQ H3A 2T8, Canada..
    Galloway, D. K.
    Monash Univ, Sch Phys & Astron, Clayton, Vic 3800, Australia.;Monash Univ, Monash Ctr Astrophys, Clayton, Vic 3800, Australia..
    Zhang, S.
    Inst High Energy Phys, Lab Particle Astrophys, Beijing 100049, Peoples R China..
    Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A114Article in journal (Refereed)
    Abstract [en]

    SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It is also known as an X-ray burster source discovered by Beppo-SAX. We analyzed the persistent emission and type-I X-ray burst properties during its 2015 outburst. The source changed from hard to soft state within half day. We modeled the broadband spectra of the persistent emission in the (1-250) keV energy band for both spectral states using the quasi-simultaneous INTEGRAL and Swift data. The broadband spectra are well fitted by an absorbed thermal Componization model, COMPPS, in a slab geometry. The best-fits for the two states indicate significantly different plasma temperature of 18 and 5 keV and the Thomson optical depths of three and four, respectively. In total, 56 type-I X-ray bursts were observed during the 2015 outburst, of which 26 detected by INTEGRAL in the hard state, 25 by XMM-Newton in the soft state, and five by Swift in both states. As the object transited from the hard to the soft state, the recurrence time for X-ray bursts decreased from approximate to 2 to approximate to 1 h. The relation between the recurrence time, Delta t(rec )and the local mass accretion rate per unit area onto the compact object, in, is fitted by a power-law model, and yielded as best fit at Delta t(rec )similar to <(m) over dot >(-1)(.0)(2 +/-)(0.)(03) using all X-ray bursts. In both cases, the observed recurrence times are consistent with the mixed hydrogen and helium bursts. We also discuss the effects of type-I X-ray bursts prior to the hard to soft transition.

  • 5.
    Li, Zhaosheng
    et al.
    Xiangtan Univ, Dept Phys, Xiangtan 411105, Peoples R China.;Int Space Sci Inst, Hallerstr 6, CH-3012 Bern, Switzerland..
    Suleimanov, Valery F.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Kremlevskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Salmi, Tuomo
    Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland..
    Falanga, Maurizio
    Int Space Sci Inst, Hallerstr 6, CH-3012 Bern, Switzerland.;Int Space Sci Inst Beijing, 1 Nanertiao, Beijing 100190, Peoples R China..
    Nättilä, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Xu, Renxin
    Peking Univ, Sch Phys, Beijing 100871, Peoples R China.;Peking Univ, State Key Lab Nucl Phys & Technol, Beijing 100871, Peoples R China..
    Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 62018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 1, article id 53Article in journal (Refereed)
    Abstract [en]

    Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrich the photosphere with metals, which may imprint photoionization edges on the burst spectra. We report here the discovery of absorption edges in the spectra of the type I X-ray burst from the NS low-mass X-ray binary GRS 1747-312 in Terzan 6 during observations by the Rossi X-ray Timing Explorer. We find that the edge energy evolves from 9.45 +/- 0.51 to similar to 6 keV and then back to 9.44 +/- 0.40 keV during the photospheric radius expansion phase and remains at 8.06 +/- 0.66 keV in the cooling tail. The photoionization absorption edges of hydrogen-like Ni, Fe, or an Fe/Ni mixture and the bound-bound transitions of metals may be responsible for the observed spectral features. The ratio of the measured absorption edge energy in the cooling tail to the laboratory value of the hydrogen-like Ni(Fe) edge energy allows us to estimate the gravitational redshift factor 1 + z = 1.34 +/- 0.11(1 + z = 1.15 +/- 0.09). The evolution of the spectral parameters during the cooling tail are well described by metal-rich atmosphere models. The combined constraints on the NS mass and radius from the direct cooling method and the tidal deformability strongly suggest very high atmospheric abundance of the iron group elements and limit the distance to the source to 11 +/- 1 kpc.

  • 6.
    Mereminskiy, Ilya A.
    et al.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Semena, Andrey N.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Bykov, Sergey D.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia.;Bauman Moscow State Tech Univ, Baumanskaya 2, Moscow 105005, Russia..
    Filippova, Ekaterina V.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Lutovinov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia.;Higher Sch Econ, Myasnitskaya 20, Moscow 101000, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Studying temporal variability of GRS 1739-278 during the 2014 outburst2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 1, p. 1392-1405Article in journal (Refereed)
    Abstract [en]

    We report a discovery of low-frequency quasi-periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS 1739-278 in the hard-intermediate state during its 2014 outburst based on the NuSTAR and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be R-in < 7.3R(g). In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with M-BH a parts per thousand 100 M-aS (TM) is required.

  • 7.
    Monkkonen, Juhani
    et al.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Mushtukov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands.;Univ Amsterdam, Anton Pannekoek Inst, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands..
    Doroshenko, Victor
    Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Suleimanov, Valery F.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Kremlevskaya Str 18, Kazan 420008, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A106Article in journal (Refereed)
    Abstract [en]

    The X-ray pulsar GRO J1744-28 is a unique source that shows both pulsations and type-II X-ray bursts, allowing studies of the interaction of the accretion disk with the magnetosphere at huge mass-accretion rates exceeding 10(19) g s(-1) during its super-Eddington outbursts. The magnetic field strength in the source, B approximate to 5 x 10(11) G, is known from the cyclotron absorption feature discovered in the energy spectrum around 4.5 keV. Here, we have explored the flux variability of the source in context of interaction of its magnetosphere with the radiation-pressure dominated accretion disk. Specifically, we present the results of the analysis of noise power density spectra (PDS) using the observations of the source in 1996-1997 by the Rossi X-ray Timing Explorer (RXTE). Accreting compact objects commonly exhibit a broken power-law PDS shape with a break corresponding to the Keplerian orbital frequency of matter at the innermost disk radius. The observed frequency of the break can thus be used to estimate the size of the magnetosphere. We find, however, that the observed PDS of GRO J1744-28 differs dramatically from the canonical shape. The observed break frequency appears to be significantly higher than expected based on the magnetic field estimated from the cyclotron line energy. We argue that these observational facts can be attributed to the existence of the radiation-pressure dominated region in the accretion disk at luminosities above similar to 2 x10(37) erg s(-1). We discuss a qualitative model for the PDS formation in such disks, and show that its predictions are consistent with our observational findings. The presence of the radiation-pressure dominated region can also explain the observed weak luminosity dependence of the inner radius, and we argue that the small inner radius can be explained by a quadrupole component dominating the magnetic field of the neutron star.

  • 8.
    Nabizadeh, Armin
    et al.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Monkkonen, Juhani
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Doroshenko, Victor
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Molkov, Sergey V.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;KTH Royal Inst Technol, NORDITA, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    NuSTAR observations of wind-fed X-ray pulsar GX301-2 during unusual spin-up event2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 629, article id A101Article in journal (Refereed)
    Abstract [en]

    We report on NuSTAR observations of the well-known wind-accreting X-ray pulsar GX301-2 during a strong spin-up episode that took place in January-March 2019. A measurement of high luminosity of the source in the most recent observation allowed us to detect a positive correlation of the cyclotron line energy with luminosity. Beyond that, only minor differences in spectral and temporal properties of the source during the spin-up, presumably associated with the formation of a transient accretion disk, and the normal wind-fed state could be detected. Finally, we discuss conditions for the formation of the disk and possible reasons for lack of any appreciable variations in most of the observed source properties induced by the change of the accretion mechanism, and conclude that the bulk of the observed X-ray emission is still likely powered by direct accretion from the wind.

  • 9.
    Nabizadeh, Armin
    et al.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Karasev, Dmitrij I.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Monkkonen, Juhani
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Lutovinov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Nagirner, Dmitrij I.
    St Petersburg State Univ, Sobolev Astron Inst, St Petersburg 198504, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Properties of the transient X-ray pulsar Swift J1816.7-1613 and its optical companion2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id A198Article in journal (Refereed)
    Abstract [en]

    We present results of investigation of the poorly studied X-ray pulsar Swift J1816.7-1613 during its transition from the type I outburst to the quiescent state. Our studies are based on the data obtained from X-ray observatories Swift, NuSTAR, and Chandra alongside with the latest IR data from UKIDSS/GPS and Spitzer/GLIMPSE surveys. The aim of the work is to determine the parameters of the system, namely the strength of the neutron star magnetic field and the distance to the source, which are required for the interpretation of the source behaviour in the framework of physically motivated models. No cyclotron absorption line was detected in the broad-band energy spectrum. However, the timing analysis hints at the typical for the X-ray pulsars magnetic field from a few x10(11) to a few x10(12) G. We also estimated the type of the IR-companion as a B0-2e star located at a distance of 7-13 kpc.

  • 10. Salmi, T.
    et al.
    Nättilä, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku, Finland e-mail: thjsal@utu.fi; Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia .
    Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, article id 33348Article in journal (Refereed)
    Abstract [en]

    We present a Bayesian method to constrain the masses and radii of neutron stars (NSs) using the information encoded in the X-ray pulse profiles of accreting millisecond pulsars. We model the shape of the pulses using "oblate Schwarzschild" approximation, which takes into account the deformed shape of the star together with the special and general relativistic corrections to the photon trajectories and angles. The spectrum of the radiation is obtained from an empirical model of Comptonization in a hot slab in which a fraction of seed black-body photons is scattered into a power-law component. By using an affine-invariant Markov chain Monte Carlo ensemble sampling method, we obtain posterior probability distributions for the different model parameters, especially for the mass and the radius. To test the robustness of our method, we first analysed self-generated synthetic data with known model parameters. Similar analysis was then applied for the observations of SAX J1808.4-3658 by the Rossi X-ray Timing Explorer (RXTE). The results show that our method can reproduce the model parameters of the synthetic data, and that accurate constraints for the radius can be obtained using the RXTE pulse profile observations if the mass is a priori known. For a mass in the range 1.5-1.8 M, the radius of the NS in SAX J1808.4-3658 is constrained between 9 and 13 km. If the mass is accurately known, the radius can be determined with an accuracy of 5% (68% credibility). For example, for the mass of 1.7 M the equatorial radius is Req = 11.9+0.5 -0.4 km. Finally, we show that further improvements can be obtained when the X-ray polarization data from the Imaging X-ray Polarimeter Explorer will become available.

  • 11.
    Salmi, Tuomo
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland..
    Suleimanov, Valery F.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Astron Dept, Kremlyovskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, article id A39Article in journal (Refereed)
    Abstract [en]

    The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated, synthetic, phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous, rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in both the size and the temperature of the emitting hotspot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER.

  • 12.
    Suleimanov, V. F.
    et al.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Astron Dept, Kremlyovskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Werner, K.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Accretion heated atmospheres of X-ray bursting neutron stars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A114Article in journal (Refereed)
    Abstract [en]

    Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20-100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30.

  • 13.
    Veledina, Alexandra
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Berdyugin, Andrei, V
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Kosenkov, Ilia A.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;St Petersburg State Univ, Dept Astrophys, Univ Skiy Pr 28, St Petersburg 198504, Russia..
    Kajava, Jari J. E.
    Univ Turku, Finnish Ctr Astron ESO FINCA, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Piirola, Vilppu
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Berdyugin, Svetlana, V
    Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.;Univ Hawaii, Inst Astron, 2680 Woodlawn Dr, Honolulu, HI 96822 USA..
    Sakanoi, Takeshi
    Tohoku Univ, Grad Sch Sci, Aoba Ku, Sendai, Miyagi 9808578, Japan..
    Kagitani, Masato
    Tohoku Univ, Grad Sch Sci, Aoba Ku, Sendai, Miyagi 9808578, Japan..
    Kravtsov, Vadim
    St Petersburg State Univ, Dept Astrophys, Univ Skiy Pr 28, St Petersburg 198504, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Evolving optical polarisation of the black hole X-ray binary MAXI J1820+0702019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A75Article in journal (Refereed)
    Abstract [en]

    Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, similar to 0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between similar to 0.3% and 0.7%, and the polarisation position angle is between similar to 10 degrees-30 degrees. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.

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