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)

EN

EN

EN

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’

EN

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/