A NuSTAR Observation of the Low-mass X-Ray Binary GX 349+2 throughout the Z-track

Abstract

Although the most luminous class of neutron star (NS) low-mass X-ray binaries, known as Z sources, have been well studied, their behavior is not fully understood. In particular, what causes these sources to trace out the characteristic Z-shaped pattern on color–color or hardness–intensity diagrams (HIDs) is not well known. By studying the physical properties of the different spectral states of these sources, we may better understand such variability. With that goal in mind, we present a recent NuSTAR observation of the Z source GX 349+2, which spans approximately 2 days and covers all its spectral states. By creating an HID we were able to extract five spectra and trace the change in spectral parameters throughout the Z-track. GX 349+2 shows a strong, broad Fe K α line in all states, regardless of the continuum model used. Through modeling of the reflection spectrum and Fe K α line we find that in most states the inner disk radius is consistent with remaining unchanged at an average radius of 17.5 R g or 36.4 km for a canonical 1.4 M ⊙ NS. During the brightest flaring branch, however, the inner disk radius from reflection is not well constrained.