Dramatic spectral transition of X-ray pulsar GX 304−1 in low luminous state

Tsygankov, Sergey S.; Escorial, A. Rouco; Suleimanov, V.; Mushtukov, Alexander A.; Doroshenko, V.; Lutovinov, A.; Wijnands, Rudy; Poutanen, Juri

doi:10.1093/mnrasl/sly236

Cited by 49 publications

(38 citation statements)

References 34 publications

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“…The hard component is produced by inverse Compton scattering of the photons of the soft compo-nent by the electrons in the atmosphere just outside the magnetosphere during the flares and the low luminosity states (if magnetic reconnections takes place). According to the recent findings of Tsygankov et al (2019), bulk Comptonization of the leaking matter should be negligible because of the small optical depth expected at the accretion rates occurring during the low luminosity level of A0538−66. When the accretion on the surface decreases dramatically, the soft component decreases suddenly.…”

Section: Discussionmentioning

confidence: 95%

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗

Ducci¹,

Mereghetti

Santangelo³

2019

ApJL

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A0538−66 is a Be/X-ray binary (Be/XRB) hosting a 69 ms pulsar. It emitted bright X-ray outbursts with peak luminosity up to ∼ 10 39 erg s −1 during the first years after its discovery in 1977. Since then, it was always seen in quiescence or during outbursts with L x 4 × 10 37 erg s −1 . In 2018 we carried out XMM-Newton observations of A0538−66 during three consecutive orbits when the pulsar was close to periastron. In the first two observations we discovered a remarkable variability, with flares of typical durations between ∼2−50 s and peak luminosities up to ∼ 4 × 10 38 erg s −1 (0.2−10 keV). Between the flares the luminosity was ∼ 2 × 10 35 erg s −1 . The flares were absent in the third observation, during which A0538−66 had a steady luminosity of 2 × 10 34 erg s −1 . In all observations, the X-ray spectra consist of a softer component, well described by an absorbed power law with photon index Γ 1 ≈ 2 − 4 and N H ≈ 10 21 cm −2 , plus a harder power-law component (Γ 2 ≈ 0 − 0.5) dominating above ∼2 keV. The softer component shows larger flux variations than the harder one, and a moderate hardening correlated with the luminosity. The fast flaring activity seen in these observations was never observed before in A0538−66, nor, to our best knowledge, in other Be/XRBs. We explore the possibility that during our observations the source was accreting in a regime of nearly spherically symmetric inflow. In this case, an atmosphere can form around the neutron star magnetosphere and the observed variability can be explained by transitions between the accretion and supersonic propeller regimes.

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Section: Discussionmentioning

confidence: 95%

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗

Ducci¹,

Mereghetti

Santangelo³

2019

ApJL

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“…Motivated by recent NuSTAR observations of the transient X-ray pulsars GX 304−1 (Tsygankov et al 2019a) and A 0535+262 (Tsygankov et al 2019b), which allow the direct study of the broadband X-ray spectrum emitted by highly magnetized neutron stars at lowṀ, in this paper we consider the spectral formation in the atmosphere of a neutron star. Specifically, the NuSTAR observations were performed at luminosities of ∼10 34 erg s −1 for GX 304−1 and ∼7 × 10 34 erg s −1 for A 0535+262.…”

Section: Introductionmentioning

confidence: 99%

X-ray emission from magnetized neutron star atmospheres at low mass-accretion rates. I. Phase-averaged spectrum

Sokolova-Lapa

Gornostaev²,

Wilms³

et al. 2021

A&A

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Recent observations of X-ray pulsars at low luminosities allow, for the first time, the comparison of theoretical models of the emission from highly magnetized neutron star atmospheres at low mass-accretion rates (Ṁ ≲ 1015 g s−1) with the broadband X-ray data. The purpose of this paper is to investigate spectral formation in the neutron star atmosphere at low Ṁ and to conduct a parameter study of the physical properties of the emitting region. We obtain the structure of the static atmosphere, assuming that Coulomb collisions are the dominant deceleration process. The upper part of the atmosphere is strongly heated by the braking plasma, reaching temperatures of 30–40 keV, while its denser isothermal interior is much cooler (∼2 keV). We numerically solve the polarized radiative transfer in the atmosphere with magnetic Compton scattering, free–free processes, and nonthermal cyclotron emission due to possible collisional excitations of electrons. The strongly polarized emitted spectrum has a double-hump shape that is observed in low-luminosity X-ray pulsars. A low-energy “thermal” component is dominated by extraordinary photons that can leave the atmosphere from deeper layers because of their long mean free path at soft energies. We find that a high-energy component is formed because of resonant Comptonization in the heated nonisothermal part of the atmosphere even in the absence of collisional excitations. However, these latter, if present, affect the ratio of the two components. A strong cyclotron line originates from the optically thin, uppermost zone. A fit of the model to NuSTAR and Swift/XRT observations of GX 304−1 provides an accurate description of the data with reasonable parameters. The model can thus reproduce the characteristic double-hump spectrum observed in low-luminosity X-ray pulsars and provides insights into spectral formation.

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“…The spectra of X Persei (green points;Doroshenko et al 2012), GX 304−1 (grey points;Tsygankov et al 2019) and A 0535+262 (this work) observed at low accretion rates, along with the best-fitting models consisting of two Comptonization components (black solid lines). Separate model components (comptt in xspec) are shown with dashed lines.…”

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confidence: 99%

Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

Tsygankov

Doroshenko

Mushtukov

et al. 2019

Monthly Notices of the Royal Astronomical Society: Letters

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Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low luminosity of ∼ 7 × 10 34 erg s −1 , revealed the presence of two spectral components. We argue that the high-energy component is associated with cyclotron emission from recombination of electrons collisionally excited to the upper Landau levels. The cyclotron line energy of E cyc = 47.7 ± 0.8 keV was measured at the luminosity of almost an order of magnitude lower than what was achieved before. The data firmly exclude a positive correlation of the cyclotron energy with the mass accretion rate in this source.

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Dramatic spectral transition of X-ray pulsar GX 304−1 in low luminous state

Cited by 49 publications

References 34 publications

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗

X-ray emission from magnetized neutron star atmospheres at low mass-accretion rates. I. Phase-averaged spectrum

Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

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Dramatic spectral transition of X-ray pulsar GX 304−1 in low luminous state

Cited by 49 publications

References 34 publications

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66∗

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66∗

X-ray emission from magnetized neutron star atmospheres at low mass-accretion rates. I. Phase-averaged spectrum

Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

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Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗

Awakening of the Fast-spinning Accreting Be/X-Ray Pulsar A0538–66^∗