Astronomers observe a burst of X-ray binary Swift J1858.6-0814

(ORDO NEWS) — Using the MeerKAT radio telescope and the Arcminute Microkelvin Imager-Large Array (AMI-LA), astronomers radio monitored the outburst of a newly discovered X-ray binary known as Swift J1858.6-0814. The results of these observations, presented April 4 on the arXiv website, shed more light on the nature of this source.

X-ray binaries (XRDs) consist of a normal star or white dwarf that transfers mass to a compact neutron star or black hole. Most ERDs with black holes and some ERDs with neutron stars exhibit transient events that are characterized by X-ray flares.

Swift J1858.6-0814 (or J1858 for short) is an RHD neutron star that was first discovered by NASA’s Swift spacecraft during an outburst in October 2018. The system is about 41,700 light-years away and has an estimated orbital period of about 0.83 days.

To better understand the nature and behavior of J1858, a group of astronomers led by Lauren Rhodes from the University of Oxford (UK) began radio observations of this source.

MeerKAT observations were made at 1.28 GHz with a bandwidth of 856 MHz divided into 4096 channels. For AMI-LA, observations were made at 15.5 GHz with a 5 GHz bandwidth divided into 8 channels.

“We present the results of our long-term radio monitoring campaign at 1.3 GHz (MeerKAT) and 15.5 GHz (Arcminute Microkelvin Imager-Large Array, AMI-LA) for the outburst of the recently discovered X-ray binary neutron star Swift J1858.6-0814,” – researchers write in their article.

While monitoring, the team observed self-absorbed radio emission from Swift J1858.6-0814 and registered two radio flares.

This corresponds to a quasi-stationary compact jet, as expected in the hard X-ray spectral state. The radio emission curves of the source show little long-term variability.

After analyzing one of the flares, the researchers were able to determine the magnetic field and the minimum energy of the J1858 jet.

It was found that the magnetic field is at the level of 2.0 G, and the energy is estimated at about 50×10 36 erg. The astronomers added that at the peak of the outbreak, the size of the ejecta was about 100 million kilometers.

Based on the results obtained, the authors of the article came to the conclusion that J1858, apparently, is a very “radio-luminous” binary source of an atoll-type neutron star or a weak Z-source.

The so-called atoll sources have some features in common with ERB black holes, since they have similar X-ray spectra and temporal characteristics.

However, they differ in their radio characteristics, where atoll-type sources are 27 times less radio bright. As for Z-sources, they evolve much faster than atoll-type sources. In general, they are more X-ray and radio luminous than atoll sources.

The astronomers stressed that despite a large number of observations and data, they could not conclusively determine whether J1858 is the source of an atoll or Z. Further research is needed to properly classify this system.

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