Astronomers have found a peculiar active galaxy that is exciting for several reasons. Despite being active – it's had two bouts of activity in the last century alone – its jets are not that powerful. More importantly, observations in radio waves make it look like a TIE fighter from Star Wars.
The object known as TXS 0128+554, or TXS 0128 for short, is located roughly 500 million light-years away in the constellation Cassiopea. At its core lies a supermassive black hole 1 billion times the mass of our Sun that releases jets of material, the cause behind the galaxy's activity.
In The Astrophysical Journal, the team reports that radio observations were key to understanding that TXS 0128 is a young source of gamma-rays, and revealed the emissions are episodic rather than continuous. The galaxy was first discovered by NASA’s Fermi gamma-ray telescope in 2015, but its jets are so weak it took five years of data to confirm it as a gamma-ray source. Using the Very Long Baseline Array (VLBA) researchers were able to probe the closest 35 light-years around the supermassive black hole, which showed that it has been recently active.
“After the Fermi announcement, we zoomed in a million times closer on the galaxy using the VLBA’s radio antennas and charted its shape over time,” lead author Professor Matthew Lister from Purdue University said in a statement. “The first time I saw the results, I immediately thought it looked like Darth Vader’s TIE fighter spacecraft from Star Wars: Episode IV – A New Hope. That was a fun surprise, but its appearance at different radio frequencies also helped us learn more about how active galaxies can change dramatically on decade time scales.”
In gamma-rays, the galaxy appears to be 100,000 times weaker than similarly active galaxies. The team believes this is because the jets are at an angle with respect to our line of sight, compared to most active galaxies which have one jet pointing almost directly towards Earth. For TXS 0128, we are not staring down the barrel of the gun, but are seeing the emission skewed by 50 degrees.
Given the three-dimensional geometry of the system and the fact that the speed of light is finite, there’s a time delay between the two lobes. One side is closer to us so we are seeing it at a later point in time compared to the other, providing some important insights into its evolution. The jets appeared to start around 90 years ago, then stopped about 50 years later. Then, a decade ago they started up again, producing the emission seen closer to its core. What caused these two periods of activity, however, remains unclear.
6