Presentation on theme: "Basic Principles of X-ray Source Detection Or Who Stole All Our Photons?....."— Presentation transcript:
Basic Principles of X-ray Source Detection Or Who Stole All Our Photons?.....
Overview Optical sources Comparison to X-ray images Why so few X-rays? What can we do about it? The theory of low count levels Application to X-ray sources Calculating flux and luminosity.
Why so few X-rays? - Mirrors & PSF Mirrors Made from Zerodur Coated with Iridium 100% reflectivity at 0.1keV 10% reflectivity at 10keV 10m focal length Optical/UV filters remove 20% of low energy X-rays. PSF Angular resolution of 0.5” on axis PSF increases rapidly off axis Also a function of energy
99.7% of detections are or could be background – that is of a non cosmic origin. Why so few X-rays? - Background Distant Sources Solar wind – 11 year cycle Solar flares – short time scale Radiation belt (Cosmic rays) Types of Particle Mostly protons – magnets deflect those with lowest momentum Heavy ions Neutrons Gamma rays X-rays In a normal image, these would completely swamp the sources, but in X-ray astronomy we have one major advantage……………… Local Sources Flaring pixels Detector radiation – gold, aluminium, silicon Trailing (only for bright sources)
Can obtain x, y, E, t and “grade” Why so few X-rays? – CCD’s Position - x, y - obvious Energy -1 electron per 3.7eV -Typically ~1000 electrons -Read noise ~ 2 electrons -E/ E =~ 50 -Energy is summed over the surrounding pixels which share the signal. Time of arrival Low count rate 3.2 sec frame time 0.04 sec readout time Fine for most sources (no pileup unless very bright) Removes flickering pixels Grade -Triggering one pixel will effect those around it in a way that depends on the type of particle and its incident angle. -Therefore looking at the 3x3 region around a peak can distinguish between true X-rays and background events.
The 3x3 region around a peak can predict the cause of the event. If all pixels with a value above a given threshold are included, the resulting shape determines the grade. Rejecting certain grades greatly increases the signal to noise ratio. Why so few X-rays? – Grades and Filters Rejected grades > Accepted grades > Probable background events are removed by filtering for grade. Events received during times of particularly high background are removed. Events with E 8keV are removed.
Source Detection 1.Detect using Wavelets 2.Apertures and annulus defined by wavelet 3.Calculate counts and background 4.Find errors
Calculating Flux and Luminosity Select all significant sources Find source counts and error Assume a source spectrum Convert counts to ergs using assumed spectrum Create Exposure Map allowing for Quantum Efficiency and source spectrum Convert ergs to ergs/cm 2 /sec using exposure map Convert to luminosity using distance (requires optical astronomers!) Finished!