The Transient Accreting X-Ray Pulsar Xte J1946+274: Stability of X-Ray Properties at Low Flux and Updated Orbital Solution

Santangelo

et al. 2017

A&A

We report on the BeppoSAX monitoring of a giant outburst of the transient X-ray pulsar XTE J1946+274 in 1998. The source was detected with a flux of ∼ 4 · 10 −9 erg cm −2 s −1 (in 0.1 − 120 keV range). The broadband spectrum, typical for accreting pulsars, is well described by a cutoff power law with a cyclotron resonance scattering feature (CRSF) at ∼ 38 keV. This value is consistent with earlier reports based on the observations with Suzaku at factor of ten lower luminosity, which implies that the feature is formed close to the neutron star surface rather than in the accretion column. Pulsations with P ∼ 15.82 s were observed up to ∼ 70 keV. The pulse profile strongly depends on energy and is characterised by a "soft" and a "hard" peaks shifted by half period, which suggests a strong phase dependence of the spectrum, and that two components with roughly orthogonal beam patterns are responsible for the observed pulse shape. This conclusion is supported by the fact that the CRSF, despite its relatively high energy, is only detected in the spectrum of the soft peak of the pulse profile. Along with the absence of correlation of the line energy with luminosity, this could be explained in the framework of the recently proposed "reflection" model for CRSF formation. However more detailed modelling of both line and continuum formation are required to confirm this interpretation.

Section: Introductionmentioning

confidence: 84%

BeppoSAX observations of XTE J1946+274

Santangelo

et al. 2017

A&A

“…Thus, a difference of the peak separation up to a factor of ∼3 in orbital phase is not astonishing. Note that the outbursts of XTE J1946+274 occurred twice per orbital period (P orb ∼ 172 d; see, e.g., Marcu-Cheatham et al 2015, and references therein), i.e., the peak separation in this system is about 0.5 in orbital phase. As the system shows ≥ 5 outbursts in a row, this morphology is, however, different compared to the "double-peaked" outbursts.…”

Section: "Triple-peaked" Outburst Morphologymentioning

confidence: 98%

Evidence for different accretion regimes in GRO J1008−57

Kühnel

Fürst

Pottschmidt

et al. 2017

A&A

Self Cite

We present a comprehensive spectral analysis of the BeXRB GRO J1008−57 over a luminosity range of three orders of magnitude using NuSTAR , Suzaku and RXTE data. We find significant evolution of the spectral parameters with luminosity. In particular the photon index hardens with increasing luminosity at intermediate luminosities between 10 36 -10 37 erg s −1 . This evolution is stable and repeatedly observed over different outbursts. However, at the extreme ends of the observed luminosity range, we find that the correlation breaks down, with a significance level of at least 3.7σ. We conclude that these changes indicate transitions to different accretion regimes, which are characterized by different deceleration processes, such as Coulomb or radiation breaking. We compare our observed luminosity levels of these transitions to theoretical predications and discuss the variation of those theoretical luminosity values with fundamental neutron star parameters. Finally, we present detailed spectroscopy of the unique "triple peaked" outburst in 2014/15 which does not fit in the general parameter evolution with luminosity. The pulse profile on the other hand is consistent with what is expected at this luminosity level, arguing against a change in accretion geometry. In summary, GRO J1008−57 is an ideal target to study different accretion regimes due to the well constrained evolution of its broad-band spectral continuum over several orders of magnitude in luminosity.

“…The exact proportionality factor depends on different factors, like the magnetic field strength and the way the accretion disk couples to the magnetosphere. For simplicity, we describe it with a simple spin-up parameter b, for which we can fit directly (Marcu-Cheatham et al 2015;Bissinger 2016). The theory is based on a sub-critical, geometrically thin accretion disk, and we therefore caution that the coupling might be significantly different in the super-Eddington case.…”

Section: Determination Of the Orbitmentioning

confidence: 99%

A tale of two periods: determination of the orbital ephemeris of the super-Eddington pulsar NGC 7793 P13

Fuerst

Walton

Heida

et al. 2018

A&A

We present a timing analysis of multiple XMM-Newton and NuSTAR observations of the ultra-luminous pulsar NGC 7793 P13 spread over its 65 d variability period. We use the measured pulse periods to determine the orbital ephemeris, confirm a long orbital period with P orb = 63.9 +0.5 −0.6 d, and find an eccentricity of e ≤ 0.15. The orbital signature is imprinted on top of a secular spin-up, which seems to get faster as the source becomes brighter. We also analyse data from dense monitoring of the source with Swift and find an optical photometric period of 63.9 ± 0.5 d and an X-ray flux period of 66.8 ± 0.4 d. The optical period is consistent with the orbital period, while the X-ray flux period is significantly longer. We discuss possible reasons for this discrepancy, which could be due to a super-orbital period caused by a precessing accretion disk or an orbital resonance. We put the orbital period of P13 into context with the orbital periods implied for two other ultra-luminous pulsars, M82 X-2 and NGC 5907 ULX and discuss possible implications for the system parameters.