X-ray and Observational Astronomy

News May 2002: Long XMM-Newton observation of the BL Lac Mkn 421 shows previous studies underestimate jet properties

The Palomar sky survey image of Mkn 421 taken in blue light. Mkn 421 is the small fuzzy blob in the centre of the picture. The image is about 0.13 degrees across.

Some galaxies have very bright centres, they are known as active galaxies. Some of them are bright radio sources, and a few of these radio galaxies are BL Lac objects, rare galaxies which look more like stars than galaxies in optical images. They are thought to contain a black hole millions of times more massive than the sun at their centre which is sucking in gas from the galaxy. They are bright X-ray sources, having luminosities so high that the emission must be beamed in our direction so as not to blow away all the in-falling gas which powers this emission. The X-rays from BL Lacs are thought to come from a jet of particles interacting with a magnetic field, giving rise to synchrotron radiation. This jet is also so bright at optical wavelengths that it out-shines the rest of its galaxy, this is why BL Lac galaxies look star-like. You can see some radio images of the jets in more 'normal' radio galaxies here.

The measurement of the way in which the X-ray spectrum varies in BL Lac objects can be used to constrain the magnetic field in the X-ray emitting jet. This is done be looking for 'spectral lags' in which the lower energy X-rays vary later than those at higher energies, due to the more rapid cooling of higher energy electrons when emitting synchrotron radiation.

Recent X-ray observations of the bright BL Lac object Markarian 421 made with XMM-Newton by Steve Sembay, Rick Edelson, Alex Markowitz, Gareth Griffiths, and Martin Turner from the Universities of Leicester and California. Four observations were made, each uninterrupted and up to 12 hours long. It is the long unbroken nature of these observations which make them so valuable. Earlier observations have been made by X-ray satellites in low earth orbit. Because such satellites have to stop observing when they pass through areas of high electron density (such as those near the auroral regions) and when the earth gets in the way of the source, observations made with these satellites suffer regular interruptions. It can be difficult to interpret non-continuous data unambiguously, hence the value of observations made with XMM-Newton, which is in a deep 48 hour orbit taking it far from the earth. The panel below summaries the X-ray intensity variations seen from Mkn 421 during these observations.

The top panels show the X-ray intensity as a function of time in the 4 XMM-Newton observations. Lower panels show the ratio of high energy to low energy X-ray intensities, the normalised high energy (dashed) and low energy (dotted) X-ray intensities, and the difference between these normalised curves. A wide range of behaviours was seen from Mkn 421.

Previous workers have witnessed apparent lags in both the high energy variations compared to the lower energy variations, and vice versa. Because these carry implications for the physics of the X-ray emitting jet, and because they were based on observations made by low earth satellites, it is important to try to confirm them with the high quality observations possible with XMM-Newton.

The panel below shows that such claims are not confirmed. No lag greater than 5 minutes was seen.

Cross-correlation plots from the XMM observations of Mkn 421. These show the degree of similarity between the X-ray intensity variations in two different energy bands for various delays ('lags') between them. Soft X-rays have low energies, hard X-rays have higher energies. The maximum degree of similarity varies from one observations to another, but always occurs at zero lag. There are no delays between the high and low energy X-ray variations.

The lack of high energy - low energy lag implies a higher magnetic field and/or faster electrons than has previously been inferred from the older and poorer quality data, and this result puts some pressure on the most common X-ray emission models in the scientific literature. These results are also consistent with those obtained recently by the same team on another BL Lac object: PKS 2155-304.

The full results of the work on Mkn 421 and PKS 2155-304 are available from these links. The PKS 2155-304 result is already published in the Astrophysical Journal, the Mkn 421 paper has just recently been accepted for publication there.

The XMM-Newton EPIC camera used to obtain this result was built by a team led by Dr Martin Turner of the University of Leicester.

Other news items are available here.

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Last updated: 2002 May 13 by Julian Osborne