A Missing-Link in the Supernova-GRB Connection

Engine Driven Supernovae

A diverse range of engines power supernovae.

National Radio Astronomy Observatory (NRAO) and Harvard-Smithsonian Center for Astrophysics (CfA) have released a joint press release about our resent work on SN 2012ap. It is the first supernova with relativistic decelerating ejecta but with no GRB associated with it.

Astronomers using the National Science Foundation’s Very Large Array (VLA) have found a long-sought “missing link” between supernova explosions that generate gamma-ray bursts (GRBs) and those that don’t.

Read more about it at:


Or read our paper at:


An SGR near Sgr A*

The centre of our galaxy, in the constellation of Sagittarius, harbours a gigantic black hole. Astronomers refer to it, in shorthand, as Sgr A* (http://en.wikipedia.org/wiki/Sagittarius_A*). Last year an international team of astronomers (http://arxiv.org/abs/1112.3264) spotted a gas cloud, called G2, on its way towards this supermassive black hole. A smaller team from Harvard and Arizona (http://arxiv.org/abs/1207.4215) went on to predict that the a bow shock around the G2 on its collision course, will accelerate electrons which will then produce radio synchrotron emission. G2 may be fly-weight compared to beast at the galactic centre, but the collision has the potential to teach us about our backyard supermassive black hole. This prompted a large number of observatories to initiate observational campaigns to monitor the galactic centre over the year.

In the midst of all this eager anticipation, the Swift X-ray Telescope satellite (http://www.astronomerstelegram.org/?read=5006) detected a large flare coming from the direction of Sgr A*. This discovery was made possible because,

Swift is carrying out a daily monitoring campaign throughout 2013 to study the evolution of the X-ray properties of Sgr A* as it interacts with the G2 cloud.

In fact the Sgr A* — Swift Monitoring Program 2013, has its own dedicated website (http://swift-sgra.com/). But this flare made me wonder. A gas cloud is a large and tenuous mass. It takes time to heat it up. For example if you try to heat up a gas cloud, of size R with a shockwave which can never travel faster than c, you cannot do it in a time shorter than R/c. Any flare from such a cloud which rises (or falls) in a time shorter than R/c would violate causality. This seemed to rule out an interaction with G2 as the origin of such a flare.

Astronomers using the NUSTAR (http://www.astronomerstelegram.org/?read=5020) telescope found pulsations with a period of 3.76 seconds in the hard X-rays coming from this flare. Flares like this, with pulsations from them, do not usually come from supermassive black holes, but from Soft Gamma Repeaters (abbreviated as SGR http://en.wikipedia.org/wiki/Soft_gamma_repeater; no connection to Sagittarius) and not from supermassive black holes. So, astronomers decided to use Chandra, the X-ray telescope with the highest resolution available to us. The Chandra X-ray Observatory, named after Subrahmanyan Chandrasekhar, can resolve two X-ray sources less than an arcsecond away from each other. To put that into perspective; it is like being able to distinguish two strands of hair, half a millimeter away from each other, from a distance of 100 meters. This is what the team of astronomers (http://www.astronomerstelegram.org/?read=5032) saw using the Chandra.


The right panel shows emission from Sgr A* seen in earlier observations. The left panel shows the source of the new outburst, merely 3 arcseconds away. This small separation could not have been resolved with the other X-ray telescopes.

Radio observations (http://www.astronomerstelegram.org/?read=5035http://www.astronomerstelegram.org/?read=5040) have now confirmed that this source is also a radio pulsar, consistent with the origin of SGRs from highly magentized neutron stars. Pulsars are neutron stars which emit radio and/or X-rays into an often narrow cone away from them. They also rotate very fast. This one rotates once every 3.76 seconds. Compare this to the 24 hours that the Earth takes to rotate. The Earth would actually rip itself apart if it was rotating this fast! This rotation points the cone of emission towards us like a lighthouse, once every rotation period, causing the pulsations. The line-of-sight distance to this object is still not certain, but this might well be the closest pulsar known to the galactic centre.

So, what did I learn from all this? You’ll find new stuff if you are looking hard. But, you may not find what you were looking for. So, its important to keep an open mind for serendipitous discoveries.