Tuesday, December 10, 2013

I've neglected this blog after saying I was going to write it over the summer—my apologies. Life has been crazy.  Or rather, having a life has been crazy. For once in my life I've been taking time off to enjoy friends and take off on ill-advised weekend trips where I pick the only day in the year that it's pouring in the damned desert and then the term started and work happened. So without further ado, here's what I've really been up to:

This past summer I've was working for Prof. H on characterising spectra from ultraluminous X-ray source NGC 5204 X-1. Back up, now.  What is an ultraluminous X-ray source?

Short answer: we don't know.
Helpful answer: see below (or if all this is obvious, click here)

Ultraluminous X-ray Sources
So how do we define an ultraluminous X-ray source if we don't know what it is? (Note: I'm going to assume we know what an X-ray binary is, and if not, Wikipedia's treatment is actually quite good) In astrophysics we have a concept called the Eddington Limit, named for Sir Arthur Stanley Eddington who brought general relativity to the English-speaking world. This limit puts an upper bound on the luminosity of an accreting system, in this case a black hole X-ray binary. For our purposes, this requires: $$L_X \leq 3 \times 10^4 \left(\frac{M}{M_{sun}}\right) L_{sun}$$ Our so-called ultraluminous objects usually radiate in excess of \(10^{37} \text{erg/s}\).  For reference, the Sun's luminosity is right around \(4 \times 10^{33} \text{erg/s}\). So an ultraluminous X-ray source  (ULX) is an X-ray binary so bright that it should be 100+ times more massive than the sun in order to satisfy the Eddington Limit.

The problem with this is, of course, that a black hole binary that size technically doesn't exist. We've got stellar mass black holes, which get as large as a few times 10 solar masses and supermassive black holes such as the one at the centre of our galaxy. We have two major possible explanations for this behaviour. The first, which is looking most probable is that ULXs are a new type of intermediate mass (100-100,000 solar masses) black hole. The strongest evidence lies in the physics of known black hole binaries. The temperature at the inner edge of black hole accretion disks scales inversely with mass. Modelling ULX observations with a disk indicates intermediate mass black holes of approximately 1,000 solar masses. However, prior research also indicates that this does not describe the majority of ULXs.1 

Artist's impression of black hole binary with accretion disk (Image credit: ESA, NASA)

An alternate possibility is that they are governed by an entirely new accretion regime that either breaks the rules or appears to do from our perspective. One such mechanism is an aptly named "ultraluminous state" for stellar mass black holes.2

NGC 5204
Now that we've answered in a roundabout way what a ULX is, what is my job exactly? I'm looking at one particular ultraluminous source in the galaxy NGC 5204. Using new data, I have extended the known spectrum for this source to higher energies than we have previously observed. This has two major goals:

  1. Once we have a good idea of the spectral shape, we can compare it to systems we do understand. For instance, stellar mass black hole binaries. This can help us work out if it is the same system only bigger or something different.
  2. By analysing a large group of ULXs with our new data, we can start figuring out whether ULXs are all the same type of source or if we're actually looking at a diverse population with a variety of physical mechanisms.
The second point is a bit tricky: So far we have only seen these sources up to energies of 10 keV and they look quite similar to one another, but technological limitations have meant we're seeing a truncated picture. Just because we haven't seen differences before doesn't mean they're not there! Imagine trying to decide whether this horse is an appaloosa based only on his head:

Because we don't understand the physics of how these sources work, right now my model fitting is entirely empirical—we change parameters to try and fit the shape of the data without minimal regard to what these parameters mean. Hopefully the end result of this is to be able to draw physical conclusions that explain the best fitting models and help us differentiate the various hypotheses of ULX physics.

1 Roberts, T. P.: 2007, AP&SS 311, 203
2 Gladstone, J. C., Roberts, T. P., and Done, C.: 2009, MNRAS 397, 1836