It is still a mystery where the center of the Milky Way lies. Astronomers believe there is a supermassive dark hole there. However, it could also be dark matter. This region is dominated by red giants and densely packed stars. Because of the dense dust between Earth's galactic center and Earth, it is impossible to see any visible light, ultraviolet light or low-energy radiation.
However, we can detect radio waves and some are not explained.
Astronomers discovered a transient source for radio waves at the Milky Ways centre. Their findings were presented in a paper entitled Discovery of ASKAPJ173608.2321635 as Highly Polarized Transient Source with the Australian SKA Pathfinder. The paper was published in The Astrophysical Journal.
They knew they had found something extraordinary. We found ASKAP J173608.2-321635 in the center of the Galaxy. Professor Tara Murphy, co-author, described how it was named after its coordinates. The uniqueness of this object is that it appeared to be invisible at first, then became visible, then faded away, and then returned. This behavior was amazing.
This new signal has the most bizarre property: it has very high polarization. According to the lead author Wang, this means that its light oscillates only in one direction. However, it rotates with time. The object's brightness also changes dramatically by a factor 100. Additionally, the signal switches on/off at random. It's unlike anything we have ever seen.
What is it? There are many types of variable stars and other objects in the sky. They emit varying levels of light across the spectrum.
Could it be a star of low mass or a substellar object Could it be a magnetar or a pulsar? None of these possibilities match the observations, according to the authors.
We initially thought it might be a pulsar, a dense type of spin dead star or a star that emits large solar flares. Mr. Wang stated that the signals coming from this source are not consistent with what we would expect from celestial objects. Although the object is highly polarized just like a Pulsar, no pulsations were detected by the team.
They also considered magnetars, which are neutron star with extremely strong magnetic fields, as the source. However, the data did not match what we know about magnetars. Radio magnetars all show high levels of polarization. However, their flat radio spectrums, which are not like what we see for ASKAPJ173608.2?321635 makes it difficult to interpret a magnetar, they write in their paper.
This image shows the location and characteristics of the variable radio source as well as other objects within the galactic center. The ASKAP detection is shown in yellow, and MeerKAT detection is shown in cyan. ASKAP and MeerKAT's best-fit positions are indicated with yellow + and cyan symbols. The sources of the VVV catalog, which is a list of variables in infrared, are shown as red inverted Y symbols. Gemini Observatory's red Gemini star is an example of a well-known source. Image Credit: Wang et al, 2021
Over the period of nine months, six radio signals were detected by the team from the object. They couldn't find the object when they tried to search for it in visible light. They decided to use another radio telescope, the Parkes Observatory in Australia, to detect the object. They did not find anything.
The team continued to observe the MeerKAT radio telescope in South Africa. This telescope is more sensitive than the MeerKAT. The team kept checking the MeerKAT to confirm if the intermittent signal was still present. The more sensitive MeerKAT radio telescope was then used in South Africa. We observed the signal intermittently for 15 minutes each week in hopes of seeing it again.
This is how the centre of the Milky Way appears to the MeerKAT radio telescope located in South Africa. Image Credit: By MeerKAT Public release photo https://gizmodo.com/new-south-african-telescope-releases-epic-image-of-the-1827572028, Public Domain, https://commons.wikimedia.org/w/index.php?curid=70866309
They were lucky. They received the signal. They were not surprised when the signal returned.
Murphy said that although the signal was returned, we noticed that the behavior of the source was drastically different. The source vanished in one day, despite having been there for several weeks in previous ASKAP observations.
Although detecting the transient signal was a great boost, it did not help the team to identify the source. It could have been a Galactic Center Radio Transient (GCRT) type of object. The object was only four degrees away from the galactic centre. Although it shared some similarities with a GCRT (Global Center for Radio Telemetry), astronomers don't know what a GCRT is.
We have some similarities to another class of mysterious objects, known as Galactic Centre Radio Transients. One of these objects is the cosmic burper. Professor David Kaplan, University of Wisconsin-Milwaukee, was Mr. Wang's co-supervisor.
Our new object, ASKAPJ173608.2-321635 does share some properties with the GCRTs, but there are also differences. We don't know the sources so it adds to the mystery.
This composite infrared photo shows the Milky Ways galactic centre. This image shows complex structures within the hot ionized gases swirling around the central 300-light-years. Image Credit: NASA, ESA and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech) NASA Image of the Day, Public Domain, https://commons.wikimedia.org/w/index.php?curid=9378571
The new object is still a mystery for now. Future facilities will be more sensitive and powerful. The Australian SKA is only one part of the Square Kilometer Array, which will include thousands of dishes from around the globe. It will be available online within the next ten years.
The transcontinental Square Kilometre Array radio telescope (SKA), will be online within the next ten years. Professor Murphy stated that it will be capable of making sensitive maps of the sky each day. The telescope's power will allow us to solve many mysteries, such as the latest one. However, it will also provide vast new areas of the cosmos for exploration in the radio spectrum.
Future research will reveal more information about this object and other similar objects. It could be a Galactic Center Transient.
ASKAP J173608.2?321635 is also notable for its position towards the GC. However, we don't know if this is a coincidence or if it is related to its nature. Similar questions could be raised regarding the GCRT sources. The authors note that future comprehensive searches will determine the exact number and locations of these sources in the sky.
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