Strong X-ray pulsations detected by pulsar 3A 0726-260

Using the AstroSat satellite, Indian astronomers have detected strong X-ray pulses from a binary X-ray pulsar known as 3A 0726-260. The discovery, featured in an article published April 26 in the arXiv preprint repository, sheds more light on the nature of this poorly studied object.

X-ray pulsars (also known as accretion-driven pulsars) are sources that show strict periodic variations in X-ray intensity, consisting of a magnetized neutron star in orbit with a normal stellar companion. In these binary systems, X-ray emission is driven by the release of gravitational potential energy as the material accumulates from a massive companion. X-ray pulsars are among the brightest objects in the x-ray sky.

Located about 20,000 light-years away, 3A 0726-260 (another designation 4U 0728-25) is one of the least-studied binary X-ray pulsars, despite being a persistent X-ray emitter with a brightness level of approximately 100 decillions of ergs / s during its unlit states. The system has an orbital period of approximately 34.55 days and contains an X-ray pulsar with a spin period of approximately 103 seconds.

A team of astronomers led by Jayashree Roy of the University of Mumbai, India investigated 3A 0726-260 in detail. Their study was based on data from the AstroSat Large Area X-Ray Proportional Counter (LAXPC) and the Soft X-Ray Telescope (SXT).

“We present the results of the broadband (0.3-40 keV) study of the source using data from AstroSat observations with the SXT and LAXPC detectors,” the astronomers wrote in the document.

Observations detected a strong X-ray pulse with a period of 103,144 seconds. This suggests that there has only been a marginal change in the pulsar’s rotation period since the last measurements made in 1997. In addition, weak pulsations of the source in the 20-40 keV range were detected for the first time in the LAXPC data.

In general, the pulse profile of 3A 0726-260 was found to be energy dependent. The pulse shape appears to change from a broad single pulse, up to 5.0 keV, to a double pulse at higher energy (more than 5.0 keV). The researchers offer two hypotheses that could explain such behavior.

“The change in pulse profile from a single peak to a weak double-peak structure may be explained by an intrinsic change that occurs in the emission pattern from a pencil beam to a fan beam that causes the beam to move out. from our line of sight (…). The change in pulse profile can also be attributed to a transition in the accretion pattern from a smooth accretion current at low energies to several narrow accretion currents at high energy that are locked in phase with the neutron star, “the document reads.

The study also detected the presence of a broad iron K-alpha line from 1.06 keV to approximately 6.3 keV. Astronomers added that 3A 0726-260 is accumulating at an almost constant accretion rate with no indication of any instability that could trigger bursts.

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