X-ray burst induced spectral variability in 4U 1728-34

TL;DR

This study analyzes how X-ray bursts from the neutron star binary 4U 1728-34 affect its broad-spectrum X-ray emission, revealing spectral state variations and burst-induced cooling effects in the accretion flow.

Contribution

It provides the first detection of a weak hard X-ray tail in the soft state and demonstrates how X-ray bursts alter the spectral properties in the hard state of 4U 1728-34.

Findings

Detection of a weak hard X-ray tail in the soft state (40-80 keV)

X-ray bursts reduce emission above 40 keV by about three times in the hard state

Spectral changes during bursts suggest cooling of electrons in the inner accretion flow

Abstract

Aims. INTEGRAL has been monitoring the Galactic center region for more than a decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow burster". Our aim is to study the connection between the persistent X-ray spectra and the X-ray burst spectra in a broad spectral range. Methods. We performed spectral modeling of the persistent emission and the X-ray burst emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI instruments. Results. We constructed a hardness intensity diagram to track spectral state variations. In the soft state the energy spectra are characterized by two thermal components - likely from the accretion disc and the boundary/spreading layer - together with a weak hard X-ray tail that we detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard state the source is detected up to 200 keV and the spectrum can be described by a thermal Comptonization model plus an additional component: either a powerlaw tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect emission up to 80 keV during the X-ray bursts. We find that during the bursts the emission above 40 keV decreases by a factor of about three with respect to the persistent emission level. Conclusions. Our results suggest that the enhanced X-ray burst emission changes the spectral properties of the accretion disc in the hard state. The likely cause is an X-ray burst induced cooling of the electrons in the inner hot flow near the neutron star.