Observations explore the persistent nature of X-ray binary 4U 0728-25

Using ESA’s XMM-Newton satellite, Italian astronomers have observed a Galactic X-ray binary system known as 4U 0728-25. Results of the new observations, presented in a research paper published March 28 on the arXiv preprint server, shed more light on the persistent nature of this source.

X-ray binaries are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

Be/X-ray binaries (Be/XRBs) are the largest subgroup of HMXBs. Although most BeXRBs are transient sources composed of a neutron star (NS) in a wide and eccentric orbit around a Be star, a growing class of Be/XRBs is that of the persistent Be/NS binaries, which have a rather constant and low luminosity and a long spin period (over 100 seconds).

Discovered in 1978, 4U 0728-25 is a Galactic Be/XRB at a distance of about 24,800 light years. It comprises a Be star and a neutron star (pulsar) with a pulse period of approximately 103.14 seconds. The orbital period of the system was measured to be 34.5 days.

Previous observations of 4U 0728-25 have found that it exhibits almost constant X-ray flux over long timescales. This, together with its relatively long orbital and spin periods, suggests that it belongs to the class of the X-Persei-like persistent Be/XRBs. However, 4U 0728-25 has a high luminosity of a few hundred decillion erg/s, and is therefore greater when compared with other sources of this type.

A team of astronomers led by Nicola La Palombara of the Institute of Space Astrophysics and Cosmic Physics of Milan in Italy decided to further investigate the unusual properties of 4U 0728-25. For this purpose they used XMM-Newton’s European Photon Imaging Camera (EPIC) and Reflection Grating Spectrometer (RGS).

New XMM-Newton observations detected 4U 0728-25 at a luminosity level of 90 decillion erg/s (in the energy range 2–10 keV), which is the lowest level ever observed for this source since its discovery and is also about 50% lower than the minimum luminosity previously observed. However, the researchers underlined that the obtained value is less than one order magnitude lower than the maximum luminosity of 4U 0728-25 in almost 50 years, thus confirming the limited flux variability of the source over long timescales.

The pulse period of the neutron star in 4U 0728-25 was measured to be approximately 103.3 seconds, about 0.15% longer than previously estimated, which indicates a moderate spin down. Such a spin-down rate with a low rate of matter transfer occurs via wind accretion, from the Be companion star to the neutron star. The astronomers added that the pulse shape shows a simple single-peak profile, with a moderate pulsed fraction at all energies.

Moreover, the spectral analysis of 4U 0728-25 revealed the presence of a flux excess above the main power-law component, which can be described with a black body model with high temperature (about 1.5 keV) and small emission radius (some 240 meters).

According to the authors of the paper, all the results conclusively confirm the persistent nature of 4U 0728-25.

“In summary, we verified that the timing and spectral properties of 4U 0728-25 are fully consistent with those of most of the persistent BeXRBs. For this reason, we can consider it as a confirmed member of this class of sources,” the scientists concluded.

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