Introduction to X-Rays thanks to Anna Wolter

Anna Wolter INAF-Osservatorio Astronomico di Brera

What are exactly the X-ray?

People are used to think about X-rays as a diagnostic tool, when they go to the doctor to see if a bone is broken. Actually X-rays are just a different form of light. Light can have many colours, as we see in the rainbow, for instance. Each colour of the rainbow is defined by its wavelength, the length of the wave of light that we see with that colour. Going towards the colour red you go towards the longer wavelengths (and further to the InfraRed and the Radio band). Going towards the blue you go to the shorter wavelengths (and further to the UltraViolet, or UV, the X-rays and the Gamma-rays). X-rays have been discovered in 1895 by W. Roentgen, when he realised that a photographic plate had been exposed to some kind of light (or, to be precise, some kind of unknown radiation, that took the name 'x-rays') produced by its experiment even when covered by a carton box. He got the Nobel prize in Physics in 1901 for his discoveries. X-rays are then light with a wavelength of about 0.01 to 10 nanometers. This makes them very useful to study the shape of crystals, for instance. An interesting result that used X-rays is the discovery of the helix of the DNA, made by Rosalind Franklin and her colleagues in 1952, see e.g.:

https://www.dnalc.org/view/15014-Franklin-s-X-ray-diffraction-explanation-of-X-ray-pattern-.html

Crick, Watson and Wilkins later received the Nobel prize for the explanation of DNA.

Another Nobel prize that cannot be ignored about X-rays is that of our colleague Riccardo Giacconi, "for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources", assigned in 2002 for his work of forty year earlier.

[A non-complete list of Nobel prizes linked to the discovery and use of X-rays can be found here:

http://www.nobelprize.org/educational/physics/x-rays/discoveries-1.html]

What cause the X-ray? Why are they so interesting in astronomy?

X-ray can be produced in different ways. In the original experiment by Roentgen, the X-rays were produced by electrons striking a metal target (in a "X-ray tube"). The electrons were liberated from a heated filament and accelerated by a high voltage towards the metal target. The X-rays are produced when the electrons collide with the atoms and nuclei of the metal target.  This is basically how X-rays are produced even today for most medical applications. There are other ways of producing X-ray like bremsstrahlung, which is the deceleration of a charged particle deflected by another charged particle, typically an electron deflected by a ion in a hot plasma. Another way of making X-rays is through the synchrotron radiation: the bending of the path of the electron is due, in this case, to the presence of a magnetic field. A third way is called inverse Compton scattering: it is due to a very rapidly moving (relativistic: the velocity is close to that of the light) electron impinging on a photon (the quantum of light) of lower energy: the collision slows the electron and gives energy to the photon that can reach X-ray energies.

The interest for astronomy is due to the fact that these phenomena occur in the presence of very high temperatures and high density of matter. Therefore X-rays help in finding hot bodies (like the hot plasma surrounding cluster of galaxies) and very compact ones (like Neutron Stars or Black Holes).

What is the farthest object discovered which sends out X-rays? And the oldest one?

Most of the astronomical objects produce X-rays. The most distant objects observed are galaxies (a z > 8, which correspond to a few hundreds million years after the Big Bang). Possibly the farthest object with a measured redshift is a Gamma Ray Burst, which has been observed in the InfraRed to derived the redshift, called GRB 090423. Also a quasar has a very high redshift measured: ULAS J1120+0641. This quasar emitted the light observed on Earth today less than 770 million years after the Big Bang, i.e. about 13 billion years ago. The farthest objects are also the oldest, since the speed of light is finite, and therefore we see them now as they were at the beginning of the life of the Universe.  

Does the universe send out X-rays in the same way as the cosmic microwave background?

The cosmic microwave background is the echo of the Big Bang, and it is seen as a wave of lower and lower temperature (today it measures 2.7 K) in every direction in the sky. There is a similarly called background in X-rays which is however of a different origin. This is one of the discoveries of the 1962 rocket flight that merited Giacconi his Nobel prize. In every direction of the sky there were X-rays coming. The Moon, as observed by ROSAT in the '90s, for instance, is shading part of this background. We now know that this background is made by the unresolved emission of many many sources, mostly galaxies and quasars.

How are we able to place black holes thanks to X-ray when these do not send out any form of energy?

Black holes are "black" because also light is attracted by them in such a way that it cannot escape the "event horizon" and reach us. However, their gravitational influence can reach out to larger distances: the effect is of attracting all the material that reaches their surroundings. Usually the matter settles in a disk, called accretion disk, in which friction has a role in slowing down the particles so that they fall on the hole. This process in turn produces a very high quantity of energy which is emitted primarily as X-rays. The first source discovered in 1962, Sco X-1 is one of these very bright sources of X-rays, it is powered by a Neutron Star in a binary system. Other sources are binaries powered by a Black Hole and seen via their X-ray emissions (Cyg X-1 for instance).

Cosmic dust is able to absorb X-rays. If energy can be neither created nor be destroyed, in which type does the energy absorbed by X-rays change?

Cosmic dust and gas absorb most of the wavelengths, each in its own way (it depends on the density of the gas, for instance, and in the size of the dust particles). The net effect of absorption is an increase of energy of the absorber. Most of the dust is therefore heated and re-radiates this heat in the InfraRed band. Many galaxies are observed to have a peak of emission in the IR, just due to re-radiation of higher energy absorption.

X-rays are dangerous even though we are safe thanks to our atmosphere. How far should X-rays be produced to be dangerous to life on Earth?

The atmosphere of the Earth is fundamental for the development of life. Not only X-rays but also other radiation such as UV are dangerous for life. X-ray have the power, for instance, to destroy the DNA molecules. The atmosphere absorbs mainly due to its content of water, via the photoelectric effect: a photon is absorbed by an atom and an electron is removed. An X-ray photon passing through the atmosphere will encounter as many atoms as it would in passing through a 5 meter thick wall of concrete! The Sun emits many X-rays (although its intensity in the X-ray band is only 1 millionth of the visible light intensity) and does not pose a problem for life. A Gamma Ray Burst, the most powerful events known in our Universe, could very well cause damage to the ozone layer and the life on Earth if it occurred in our half of the Galaxy, and beamed directly at Earth. However the likelihood of the event taking place is very very small, even if difficult to estimate exactly.

How do X-rays help to the deduction of dark matter?

Dark matter is matter that we do not see (does not emit "light", at any of the wavelength observed) but has a gravitational effect that we detect. Not only there are various evidences for the presence of this anomalous kind of matter, but they all point to the fact that these matter constitutes about 85% of the total matter of the Universe. One of the confirmation of the dark matter is the presence of hot gas in clusters of galaxies. This hot gas is detected through its X-ray emission due to the hot temperature (millions of degrees). A careful analysis of the X-ray emission of many clusters allows us to derive the total gravitational potential needed to keep the gas bound to the cluster. This results on average to be due to a mass about 6-7 times greater than the visible mass (due to both the stars in the galaxies and the hot gas itself). The "missing mass" is what we call dark matter. We do not know yet what is dark matter made of, but there are intriguing observations, again in the X-ray domain, of a strong emission line that hint to the presence of the so-called sterile neutrinos, hypothetical particles with a very small mass predicted to interact with normal matter only via gravity. Some scientist have proposed that sterile neutrinos might explain at least partially the dark matter.  Further observations are needed to confirm this.

What does this image represent?

This image represent the total area of sky covered by the X-ray satellite XMM-Newton of ESA during part of its life when moving from one target to the next. The lighter the colour, the longer is the time spent by the satellite on that area of sky. The data have been used by a team of scientists to extract a catalog of X-ray sources. The catalog contains bright sources (a few tens of thousands) over a large area of sky.

Details can be found here:

http://xmm.esac.esa.int/external/xmm_products/slew_survey/xmmsl1d_ug.shtml

A more significant image that I would like to suggest is that of the distribution of the detected sources in the whole sky made by the German satellite ROSAT in the '90s: the colours represent the spectra of the sources (soft (or steep) to flat (or hard) as described in the caption), which on first order can divide the sources in classes (e.g. galactic sources, like Supernova Remnants or X-ray binaries, vs. extra-galactic sources like quasars or clusters of galaxies).

See e.g. for details: http://www.xray.mpe.mpg.de/rosat/survey/rass-bsc/sup/

These were the questions about astronomy. Although, as the end of the interview, we would like to know more about your investigation, the progress made and whatever you may want to emphasise for its importance or we might have forgiven about it.

My work is indeed mostly in the X-ray band. However I believe that a multi-wavelength approach is the best way to learn the insight on the different classes of astronomical objects. Currently I am studying the Ultra-Luminous X-ray sources, or ULXs, which are very bright sources detected mostly in spiral galaxies, that are non-nuclear and brighter than expected for "normal" X-ray binaries seen in the Milky Way. These sources could host the long-sought InterMediate Mass Black Holes (IMBHs) which have a mass between 100 and 105 solar masses and are proposed to exist but not yet observed. My most recent investigation are: 1) a sample of Collisional Ring galaxies which I have observed in the X-ray band. They seem to have a small number of very bright sources that could indeed be IMBHs. and 2) the collection of multi wavelength measures for a dozen of ULXs in order to study a possible interpretative model for their multi-band emission.