Evidence for different accretion regimes in GRO J1008−57

Kühnel, Matthias; Fürst, Felix; Pottschmidt, K.; Kreykenbohm, I.; Ballhausen, Ralf; Falkner, Sebastian; Rothschild, R. E.; Klochkov, D.; Wilms, J.

doi:10.1051/0004-6361/201629131

Cited by 20 publications

(17 citation statements)

References 61 publications

(105 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Assuming that the spectral formation for X Persei is similar to that for GX 304−1 and A 0535+262, as suggested by Tsygankov et al (2019a,b), we expect the cyclotron line to be located above ∼100 keV which implies a magnetic field B 9 × 10 12 G (Mushtukov et al 2021). Similarly, the magnetic field for GRO J1008−57 is estimated to be B 6.5 × 10 12 G, with a cyclotron line observed at 78 keV (Kühnel et al 2017b;Bellm et al 2014;Yamamoto et al 2014), and a recent claim of a significantly higher energy of ∼90 keV (Ge et al 2020). In the case of such high magnetic fields, collisional excitations in the atmosphere are barely possible.…”

Section: Collisional Excitationmentioning

confidence: 92%

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

View full text Add to dashboard Cite

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.

show abstract

Section: Collisional Excitationmentioning

confidence: 92%

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

View full text Add to dashboard Cite

show abstract

“…GRO J1008-57 has an orbital period of 250 days and shows giant outbursts (e.g. Kühnel et al 2017). For this system to undergo KL oscillations, the disc aspect ratio must be close to constant with radius for the mechanism to operate.…”

Section: Discussionmentioning

confidence: 99%

The frequency of Kozai–Lidov disc oscillation driven giant outbursts in Be/X-ray binaries

Martin

Franchini

2019

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

Giant outbursts of Be/X-ray binaries may occur when a Be-star disc undergoes strong eccentricity growth due to the Kozai-Lidov (KL) mechanism. The KL effect acts on a disc that is highly inclined to the binary orbital plane provided that the disc aspect ratio is sufficiently small. The eccentric disc overflows its Roche lobe and material flows from the Be star disc over to the companion neutron star causing X-ray activity. With N-body simulations and steady state decretion disc models we explore system parameters for which a disc in the Be/X-ray binary 4U 0115+634 is KL unstable and the resulting timescale for the oscillations. We find good agreement between predictions of the model and the observed giant outburst timescale provided that the disc is not completely destroyed by the outburst. This allows the outer disc to be replenished between outbursts and a sufficiently short KL oscillation timescale. An initially eccentric disc has a shorter KL oscillation timescale compared to an initially circular orbit disc. We suggest that the chaotic nature of the outbursts is caused by the sensitivity of the mechanism to the distribution of material within the disc. The outbursts continue provided that the Be star supplies material that is sufficiently misaligned to the binary orbital plane. We generalise our results to Be/X-ray binaries with varying orbital period and find that if the Be star disc is flared, it is more likely to be unstable to KL oscillations in a smaller orbital period binary, in agreement with observations.

show abstract

“…It is worth noting that GRO J1008−57, another Be/XRB with a long orbital period (∼ 250 d) showed an anomalous variability in 2014/2015, namely three outbursts in a single orbit, with the peak of the third reached at apastron (Kühnel et al 2017). Kühnel et al (2017) discussed the peculiar variability of GRO J1008−57 in the framework of misaligned orbital plane and circumstellar disc, with outbursts occurring at the intersection between these two planes. We point out that variabil-ity displayed by GRO J1008−57 in 2014/2015, shows some similarities with that observed in AX J0049.4−7323 and presented in this work.…”

Section: Perturbed Circumstellar Discmentioning

confidence: 94%

In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4−7323

Ducci

Romano

Malacaria

et al. 2018

A&A

View full text Add to dashboard Cite

AX J0049.4−7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a ∼ 750 s pulsar which has been observed over the last ∼17 years by several X-ray telescopes. Despite numerous observations, little is known about its X-ray behaviour. Therefore, we coherently analysed archival Swift, Chandra, XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already published ASCA data, to study its X-ray long-term spectral and flux variability. AX J0049.4−7323 shows a high X-ray variability, spanning more than three orders of magnitudes, from L ≈ 1.6 × 10 37 erg s −1 (0.3−8 keV, d = 62 kpc) down to L ≈ 8 × 10 33 erg s −1 . RXTE, Chandra, Swift, and ASCA observed, in addition to the expected enhancement of X-ray luminosity at periastron, flux variations by a factor of ∼ 270 with peak luminosities of ≈ 2.1 × 10 36 erg s −1 far from periastron. These properties are difficult to reconcile with the typical long-term variability of Be/XRBs, traditionally interpreted in terms of type I and type II outbursts. The study of AX J0049.4−7323 is complemented with a spectral analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend when the emission becomes fainter, and an analysis of optical/UV data collected by the UVOT telescope on board Swift. In addition, we measured a secular spin-up rate ofṖ = (−3.00 ± 0.12) × 10 −3 s day −1 , which suggests that the pulsar has not yet achieved its equilibrium period. Assuming spherical accretion, we estimated an upper limit for the magnetic field strength of the pulsar of ≈ 3 × 10 12 G.

show abstract

Evidence for different accretion regimes in GRO J1008−57

Cited by 20 publications

References 61 publications

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

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

The frequency of Kozai–Lidov disc oscillation driven giant outbursts in Be/X-ray binaries

In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4−7323

Contact Info

Product

Resources

About