Authors: Y. Maan, B.C. Joshi, M. Surnis, M. Bagchi, P. K. Manoharan
First Author’s Institution: ASTRON (Netherlands Institute for Radio Astronomy)
Status: Published in ApJ Letters, open access on arxiv
Pulsars. Fast Radio Bursts. Magnetars. The world of high-energy stellar astrophysics has no shortage of weird objects that do not always behave like we think they should. From the mysterious workings inside a neutron star to the unknown reason behind why some fast radio bursts repeat, these sources continue to surprise and mystify us. Now, the world of magnetars, stars with incredibly high magnetic fields, just got a little more interesting.
Magnetars: Not Your Average Stellar Objects
Magnetars (short for “magnetic stars”) are neutron stars with some of the strongest magnetic fields in the universe. Their magnetic field strengths are on the order of ~1015 Gauss; to put this in perspective, the magnetic field of the Earth that shields us from the Sun’s rays and produces auroras is about 0.5 Gauss. If a magnetar was at a distance from Earth equal to that of the moon, it could strip the information off of all of the credit cards on the planet. Magnetars also very young stars and emit variable X-ray radiation and transient radio emission and that was all, until XTE J1810-197 came along.
In 2006, magnetar XTE J1810-197 (which is also classified as an X-ray pulsar because in addition to having a very strong magnetic field, it intermittently emits X-rays) was found to be emitting radio pulses after a very strong outburst of energy in the radio frequency regime. At the beginning of this outburst, the pulsar had a nearly flat spectral index. The spectral index tells you how much the total power from the source is dependent on frequency, so if the spectral index was flat, it means that the power emitted was about the same at all frequencies. During that burst, radio emission came in spikes that lasted about 10 milliseconds. After the outburst, the source faded in power and essentially went off before it was re-observed 13 years later and a second radio burst was detected by the authors of today’s work. In Figure 1, you can see how the power emitted by the source declined over time but increased over frequency. Similar “spiky” short-duration radio pulses have been seen in high-energy phenomena such as giant pulses (essentially really bright radio pulses that occasionally come from some sources) and fast radio bursts (FRBs). With these similarities, the bursts from this magnetar could suggest a common origin for these phenomena. Let’s look a bit further into what the authors found from this mysterious source!
What Are These “Spiky” Bursts?
Magnetar XTE J1810-197 was observed by the team in December of 2018 and February of 2019 with the Giant Metrewave Radio Telescope (GMRT) in India. During that time, they found four spiky bursts. When pulsars are observed, we see what is called an average profile, which maps the energy emitted over the time period that the beam crosses our sightline (the pulse phase window), and we make similar diagrams for a magnetar. The first panel in Figure 2 shows the regular pulse profile of the magnetar. In their observations, the authors saw that the position of the bursts roughly match that of the average profile. Unlike giant pulses, this emission is not confined to a small area of the pulse phase, which differentiates it from the similar phenomenon. However, these bursts occur on a much smaller timescale than the regular emission, and occur with different rates at different frequencies with a width of 1-4 ms at lower frequencies and < 1 ms at higher frequencies (see second panel of Figure 2). Most of these variations, however, are most likely caused by propagation effects in the interstellar medium.
The spectral index of the bursts also seems to vary quite a bit with frequency. Using models of the electron density of the galaxy, the authors calculate that these variations must be intrinsic to the source and not due to the interstellar medium. The variations are also closer to the onset of the outburst, meaning something happens to the spectral index right before a burst happens. This only happens before bursts in earlier sessions, though, not in later observing sessions.
So What Does All of This Mean?
When the first burst was observed in 2003, it was found that the radio spectrum was nearly flat, but after the onset of the current outburst, it was found to be much higher. It is possible that the spectrum is higher at lower frequencies. The power emitted by the object has decreased rapidly since the onset of the burst, but there are no clear physical links between spindown rate (how fast the magnetar slows down because it loses rotational energy) and emitted power.
While the outbursts have similarities to some known objects, it is not quite clear exactly what they are yet. Giant pulses are characterized by an average energy 10x greater than the average energy emitted by the source. Though the energies are large in these bursts, they’re not at that limit. The pulses are actually more similar to the giant micropulses in the Vela pulsar, as the widths are similar, so it is possible that these are giant micropulses coming from this magnetar.
It has also been suggested by multiple teams that these magnetar pulses could be connected to fast radio bursts. In January, a team led by AstroBites author Aaron Pearlman looked at magnetar J1745-2900, a magnetar that’s very close to the supermassive black hole in the center of our galaxy, and found that the magnetar pulses, just like the FRBs, show frequency structure in single pulses. This was the first time this kind of behavior had ever been observed from a magnetar. The authors of today’s paper found frequency variations of the spiky emission in XTE J1810-197. This spiky emission, which appears in both FRBs and these magnetar pulse, cannot be caused by the interstellar medium. The spikes look similar to the repeating FRB’s pulses, but the long-term frequency drift (essentially when the power emitted is detected at different frequencies) of the FRB’s pulses do not appear in the magnetar’s bursts. Though the magnetar’s brightest burst is ~5x more powerful, the FRB is nearly 10,000x farther away implying that it is intrinsically brighter than the magnetar bursts.
The fact that magnetars are the third object, after the Crab Pulsar and the repeating FRBs, to exhibit frequency structure in its bursts points to a similar emission mechanism for all three. In addition, the fact that the frequency structure is intrinsic to the star points to some kind of process similar to that of other unexplained high-energy phenomena points to something interesting here. What will happen when the star bursts again? Will we find more bursts from other magnetars like this, or repeating FRBs that look similar? Only time will tell!