I (Megan Whewell) began my PhD at MSSL last October, and have written before on this blog about why I decided this was the right path for me. This post is focussed on my work since arriving at MSSL and the publication last week of the first peer-reviewed research I have been involved with, meaning that I am a co-author on an academic paper for the first time!
As well as being part of the MSSL Astrophysics group, I am part of an international consortium of astronomers (along with my supervisor). As a consortium, we have been working on a large, multiwavelength campaign observing the active galaxy NGC 5548, and analysing data taken from six space observatories (XMM-Newton, Hubble, Swift, NuSTAR, INTEGRAL and Chandra) as well as two ground based locations (in Israel and Chile). Active galaxies have a supermassive black hole in their centre, pulling in gas and dust from its surroundings. As the gas and dust falls towards and travels around the black hole, it heats up and emits radiation over the whole electromagnetic spectrum. As well as material falling in, some material can be pushed outwards from the black hole through this messy ‘feeding’ process. We intended to calculate distances from the central black hole of these outflows, called warm absorbers, previously observed in NGC 5548.
As with many stories of scientific discovery, this one starts with unexpected results. Over six months (June 2013- Feb 2014), we observed NGC 5548 over mainly ultraviolet (UV) and X-ray energies, and expected to see very similar relative brightnesses of each as have been observed in the past. Surprisingly, NGC 5548 appeared to be 25 times dimmer in the low energy X-rays (called soft X-rays) than in observations from 2002, but the higher energy X-rays (called hard X-rays) were the same brightness as in the 2002 data (see Fig. 1, left). It is very unusual to see this huge variation of brightness in one energy range without variation in the other ranges as well, especially in an object so constant over the last few decades, so we knew something strange was going on.
The advantage of a multiwavelength campaign is that we can look at the simultaneous UV and X-ray data together to get a clearer picture of what is going on. The UV data showed us that the warm absorbers (those outflows we expected to study) were much less ionised than had been seen before. This means they must be being illuminated by less light from the accreting material near the black hole, but the hard X-ray data indicated that the material around the central black hole was still emitting just as much light as it had been in the past. The UV data also showed evidence of another outflowing gas stream, travelling much faster than the warm absorbers in this system.
Analysing the combination of such a decrease in soft X-rays reaching us and the indications of new material from the UV led us to conclude that a new stream of outflowing gas, which I will call the obscurer, has flowed into our line of sight (the direction we look at this system from Earth). This new stream is travelling at about 5000 km per second (over ten million miles per hour) and must be closer to the central black hole than the warm absorber outflows are. This is because the lower ionisation of the warm absorbers can be explained if the new obscurer is absorbing some of the light from around the black hole before it gets to the warm absorbers, so therefore the obscurer is closer to the source of this light (the material around the central black hole).
All of these different components can be very hard to visualise when trying to analyse data like this, so we were lucky to work with Renaud Person, one of the directors of the (world famous) Assassin’s Creed video game, who developed a fantastic animation for us to show the inner regions of NGC 5548.
I feel very lucky to have been part of this research, especially as everyone at MSSL and in the consortium has been constructive, encouraging and supportive as I begin my own research career.
For more information on this research, visit:
- UCL’s news story (for a more general summary)
- Animated journey through the central regions of NGC 5548, with voiceover and without voiceover
- The NGC 5548 consortium’s webpage
- Published paper: “A fast and long-lived outflow from the supermassive black hole in NGC 5548″, J. S. Kaastra et al, Science, 19 June 2014 (10.1126/science.1253787)
- Free to access version of this paper
- Six minute YouTube video summarising this paper (aimed at experts)