observation of the crab nebula
DESCRIPTION
Lee Clement PHYS 2070 Thursday, 7 April, 2011. Observation of the Crab Nebula. Image Credit: NASA (Hubble). M1: The Crab Nebula. Plerionic type remnant of Supernova SN 1054 Filled shell structure powered by pulsar wind (Green) Located in Taurus - PowerPoint PPT PresentationTRANSCRIPT
Observation of the Crab NebulaLee Clement
PHYS 2070Thursday, 7 April, 2011
Image Credit: NASA (Hubble)
M1: The Crab Nebula Plerionic type remnant of Supernova SN 1054
Filled shell structure powered by pulsar wind (Green) Located in Taurus
RA: 05h 34m 31.95s Dec: +22° 58’ 54.4” (J2000) (Cutri et al.)
First observed by John Bevis (1731) Independently rediscovered by Charles Messier (1758)
Distance: 1930 pc (Trimble, 1973) Angular size: 6’ x 4’ (Dyer) Magnitude: 8.4 (Dyer)
Image Credit: Stellarium
GAO Data Collection 20-21 January, 2011
Conditions▪ Cold! (-27 C)
▪ Sunset: 17:03▪ Moonrise: 18:38▪ Waning gibbous with 99% of the visible disc illuminated▪ Hoped to get data before moon got too high
▪ Cloud: Initially none▪ Worsened over the course of the evening
▪ Sky glow: Initially some to the North (Winnipeg)▪ SQM: 19.61▪ Worsened as moon rose and clouds rolled in
SQM: 17.15 around 20:45 SQM: 15.63 around 21:30
Data Collection Challenges Series of unfortunate events
Deadbolt on dome side door frozen shut▪ Possibly inadequate temperature equalization inside dome
Telescope would only focus in one direction▪ Had to wait for Jennifer▪ Wire had become disconnected in dome▪ Rheostat adjustment needed in fuse panel
Dome froze stuck▪ M1 had drifted past the edge of the aperture▪ Spent ~30 mins trying to assist dome motor to rotate dome
End result Not able to start collecting data until 21:20
▪ Bright moon▪ Partial cloud cover
Raw Data
30 x 60-sec exposures
Large variations in brightness Variable
clouds Bright moon
Calibration Frames: Master Bias
To sample readout noise of the CCD chip
Average of 10 x 0-sec exposures
Calibration Frames: Master Dark To correct for thermal noise
in the CCD chip
5 x 60-sec exposures with the shutter closed
Master bias subtracted then frames averaged
Interesting feature: periodic noise Unknown source
Calibration Frames: Master Flat To correct for illumination variations,
dust, etc.
30 x 60-sec exposures of sky regions near M1
Master bias and master dark subtracted
Normalized to mean value of 1
Median taken
Also exhibits periodic noise Unknown source
Image Registration
Master bias and master dark subtracted from raw data
Result divided by master flat
Registered images in 3 parts Brightness variations Discarded 5 frames
Reduced Data
Average of registered calibrated images
Problem: Persistent
gradient from northeast to southwest
EN
Gradient Removal
Used ImageJ’s ‘Subtract Background’ function Radius: 300.0 px Create background Sliding paraboloid
Produced a model of the gradient
Gradient Removal
Subtracted gradient model from reduced data
EN
Measurement of Expansion Rate
Expansion Rate Nugent (1998) measured the expansion of the
Crab Nebula Scans of 4 high-resolution photographs Measured the motion of several filaments of the Crab’s
structure
Measuring filaments not feasible with GAO data Too blurry Needed to look at larger scale features instead
Compared GAO image to POSS I (1951) and POSS II (1991) red filter images
Expansion Rate
Took the square root of the gradient-subtracted image to enhance contrast around the edges Vertically flipped to match
orientation
Resized POSS I and POSS II images to be a similar scale to GAO image Also stretched brightness
so that only portions visible in GAO image were visible▪ Some guesswork
GAO (2011)
POSS I (1951) POSS II (1991)
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EN
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Calibration of Image Scale
Made use of the bottom two stars of the southwest trapezoid formation In Aladin, measured
separation on POSS I and POSS II images▪ Distance: 1.09’
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1.09’
Measurement Methodology
Initially intended to measure distance from central pulsar to various features Problem: Not
obvious where the centre is
GAO (2011)
POSS I (1951) POSS II (1991)
EN
EN
EN
? ?
Measurement Methodology Alternative
method Measured
distance between two sets of easily distinguishable features
Tried to approximate the major and minor axes
GAO (2011)
Measurements
Dataset Julian Date Major “Axis” (arcmin) Minor “Axis” (arcmin)
POSS I 2433957.916667 4.870 3.839
POSS II 2448546.958333 5.034 3.914
GAO 2455583.645833 5.409 4.353
All measurements made using line segments in ImageJ
Analysis Plotted measured
“axis” distances against Julian dates of observations Calculated slopes of
lines of best fit to approximate rate of expansion▪ Similar for both
“axes”▪ Uniformly expanding
2430000 2435000 2440000 2445000 2450000 2455000 24600003.503.703.904.104.304.504.704.905.105.305.50
Expansion of the Crab NebulaMajor Axis Linear (Major Axis)Minor Axis Linear (Minor Axis)
Julian Date
Leng
th (
arcm
in)
Results Rate of expansion was found to be 0.5 arcsec/yr
Inconsistent with published value of 0.15 arcsec/yr (Bietenholz)
Extrapolated to point of zero size to determine original date of supernova Found CE 1405 ± 29▪ Inconsistent with result of Nugent (1998): CE 1130 ± 16▪ Inconsistent with result of Trimble (1968): CE 1140 ± 15▪ Closer to result of Bietenholz (1991): CE 1245 ± 92▪ Measured only the synchrotron component
However, known date of supernova is CE 1054▪ Confirms result that expansion of the Crab Nebula is accelerating▪ Exact expansion function unknown (Nugent)
Sources of Error Very limited data set
Low detail of GAO data precluded measuring more than a few features
Random errors would average out with more measurements
Guesswork involved in determining what parts of POSS I and POSS II images were visible in GAO image
Guesswork involved in matching edges of features between images
Summary Despite unfortunate
observing conditions, managed to acquire data that shows some large-scale structure of the Crab Nebula
Data not so good for astrometry! Calculated expansion rate
inconsistent with previous results
Calculated date of supernova inconsistent with previous results▪ Still later than actual date
▪ Confirms acceleration of expansion
EN
ReferencesBietenholz, M. F., Kronberg, P. P., Hogg, D. E. and Wilson, A. S. The Expansion
of the Crab Nebula 1991
Cutri, et al. 2MASS All-Sky Catalog of Point Sources 2003
Dyer, Alan. The Messier Catalogue 2010
Green, Dave. G184.6-5.8 2009
Trimble, Virginia. The Distance to the Crab Nebula and NP 0532 1973
Trimble, Virginia. Motions and Structure of the Filamentary Envelope of the Crab Nebula 1968
Images:http://hubblesite.org/gallery/album/pr2005037a/http://archive.stsci.edu/dss/