Looking at A 0535+26 at low luminosities with NuSTAR

Abstract

We report on two NuSTAR observations of the high-mass X-ray binary A 0535+26 taken toward the end of its normal 2015 outburst at very low 3-50 keV luminosities of ∼1.4 × 10 36 erg s −1 and ∼5 × 10 35 erg s −1 , which are complemented by nine Swift observations. The data clearly confirm indications seen in earlier data that the source's spectral shape softens as it becomes fainter. The smooth, exponential rollover at high energies present in the first observation evolves to a much more abrupt steepening of the spectrum at 20-30 keV. The continuum evolution can be well described with emission from a magnetized accretion column, modeled using the compmag model modified by an additional Gaussian emission component for the fainter observation. Between the two observations, the optical depth changes from 0.75 ± 0.04 to 0.56−0.04 , the electron temperature remains constant, and there is an indication that the column decreases in radius. Since the energy resolved pulse profiles remain virtually unchanged in shape between the two observations, the emission properties of the accretion column, however, reflect the same accretion regime. This conclusion is also confirmed by our result that the energy of the cyclotron resonant scattering feature (CRSF) at ∼45 keV is independent of the luminosity, implying that the magnetic field in the region in which the observed radiation is produced is the same in both observations. Finally, we also constrain the evolution of the continuum parameters with rotational phase of the neutron star. The width of the CRSF could only be constrained for the brighter observation. Based on Monte-Carlo simulations of CRSF formation in single accretion columns, its pulse phase dependence supports a simplified fan beam emission pattern. The evolution of the CRSF width is very similar to that of the CRSF depth, which is, however, in disagreement with expectations.

Spectral and Timing Analysis of the Accretion-powered Pulsar 4U 1626–67 Observed with Suzaku and NuSTAR

“…The results indicate that the extra Gaussian is needed to allow the NPEX model-based fit to provide a good description of the physical continuum. Similar broad Gaussian features have been detected from several other accretion-powered pulsars, e.g., 4U 0115+63, Cen X-3, Her X-1 and A 0535+26 (Ferrigno et al 2009;Suchy et al 2008;Vasco et al 2013;Ballhausen et al 2017). Since the broad Gaussian feature appears around the spectral peak, it is inferred that the main difference between the simple NPEX continuum model and the observed spectrum is the spectral shape of the quasi-exponential cutoff.…”

Cyclotron lines in highly magnetized neutron stars

“…The source has been observed over a large range of luminosities. High quality data could be obtained down to comparatively low outburst luminosities (Terada et al 2006;Ballhausen et al 2017) and even in quiescence ). The fundamental cyclotron line energy generally does not change significantly with luminosity (over a wide range) (see Fig.…”

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

“…The fifth line indicates the predicted 4-25 keV flux assuming the compmag model in XSPEC. The compmag model is used by Ballhausen et al (2017) to fit the 4-25 keV NuS-TAR spectrum of a low luminosity pulsar observed with NuSTAR, A 0535+26. The model is cited as a more Vulic et al (2018).…”

“…The first four models in the legend assume a simple power law with the given photon index and Galactic column density. The fifth model represents a more physically motivated model for a low luminosity pulsar observed with NuSTAR by Ballhausen et al (2017). For more details on the comparison between NuSTAR and XMM-Newton flux measurements, see §4.1.…”