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X-ray variation in Einstein Cross (Q2237+0305) JI X In collaboration with Dr. Robert W. Schmidt

Astronomisches Rechen-Institut

Abstract

Einstein Cross (Q2237+0305)

Collection of photons from Einstein Cross

Generating PHA spectrum

Fitting a model

Conclusion

reference

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This research focuses on the gravitationally lensed system QSO2237+0305, also known as the Einstein Cross. Three Observations are performed by NASA’S Chandra X-ray Observatory, using the ACIS system (Advanced CCD Imaging Spectrometer) onboard the observatory, covering around 200 days of the variation of this quasar. Analysis of X-ray photons from the quasar is performed on CIAO 4.7, the analytical platform for Chandra X-ray Observatory. Variation of Einstein Cross’s luminosity and profiles of its spectrum are being studied.

Einstein Cross (Q2237+0305) is a gravitationally lensed quasar with a redshift parameter of 1.695 sitting behind the lensing galaxy ZW 2237+030, Huchra’s Lens which has z of 0.0394. Due to the gravitational lensing effect, light paths of photons from the quasar are bent, resulting in four apparent images (marked as A, B, C and D in the observation. Optical image from Hubble Space Telescope of Einstein Cross:

B

A C

D

Gravitational lensing effect

Giant mass (in this case the lensing galaxy) generates a gravitational field, which is strong enough to bend light paths of photons. As human eyes and observation instruments assume that light travels in straight paths, what

we observe is the deformed image of the quasar, being four images of the same distant quasar appearing around a foreground galaxy.

Instead of visible light with wavelengths between 400nm and 600nm, Chandra X-ray Observatory is collecting X-ray photons with wavelengths around 1nm, which is energetic enough to penetrate ordinary mirrors. Therefore, a specially designed photon collection system is designed to collect those high energy photons. Four pairs nested mirrors coated with Iridium are used to let high energy photons ‘bounce off’ instead of reflecting them in a normal way.

Photons collected are then detected by ACIS (Advanced CCD Imaging Spectrometer) sensor onboard, recording energy flux and the position of each photon. Observations of X-ray photons from the observatory:

X-ray images of the quasar are displayed in DS9 (an extraction tool incorporated inside CIAO 4.7). Four circles represent the four images A, B, C and D in the optical observation. Circle regions are extracted from the X-ray image to reduce background noise level in the later analysis. Also, since region A is much more luminous than the other regions, it is processed separately from the other regions. This same process is done for all of the three observations (14516, 14517 and 14518).

After running the analysis script, one PHA spectrum is generated for region A and region BCD separately in each of the observations. Energy level of X-ray photons is limited between 0.8keV to 7.0keV since this is the most sensitive range for ACIS sensor onboard. Spectrum for region A in observation 14517:

Spectrum displays photon counts against energy level of them. Most of the data counts are between 0.8keV and 2keV.

We fitted a one dimensional power-law model and an Xspec photoelectric absorption model to the X-ray photon spectrum.

Light curve describes the variation iof the quasar during the 200 days of observation. Data counts of photons is plotted against MJD (Julian date, cosmological time scale). Energy level of the quasar varied dramatically during the observation. To exclude the possibility of microlensing effect resulting in this, we also plotted ratio of energy flux between region A and region BCD in each observation.

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Data$sum$

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Light curve

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Ra#o

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Since the ratio of energy flux are consistent over 200 days of observation, we can exclude the possibility of the variation due to result of microlensing and hence make sure that it’s due to the quasar’s intrinsic variation.

Photon mission from quasar

Quasars are extremely luminous and are moving very fast away from us. Luminosity of Einstein Cross varied dramatically during the 200 days of observation. In particular, its luminosity dropped by a half between the last two observations, which were only 24 days apart from each other.

Quasars are the most energetic and distant members of a class called active galactic nuclei(AGN). They are extremely luminous and were first identified as being highly redshift sources of electromagnetic energy, including radio waves and visible light.

Picture of the theoretical model of a quasar

1.http://www.fromearthtotheuniverse.org/Enlarged_images/einstein_cross.php 2. http://davidjarvis.ca/dave/gallery/lg/gravitational-lens-01.jpg 3. http://chandra.si.edu/graphics/resources/illustrations/cxcmirrors-72.jpg 4. http://www.nature.com/nature/journal/v495/n7440/images/495165a-i2.0.jpg

There’s a central black hole inside the quasar whose gravitational potential energy is the source of the quasar’s luminosity. Outside the event horizon of the black hole, an accretion disk contains the matter which is pulled towards the black hole and loses angular momentum during viscous and turbulent processes. It is thought to be the origin of the continuum emission in optical, UV and X-ray regime.