## Friday, May 23, 2014

### Plotting images with axis coordinates in Octave

It is remarkable that I've struggled to find any examples in the Octave documentation or other people's tutorials or examples on how to plot an image (or matrix) with axis coordinates that aren't the pixel or cell numbers.

function image_with_axes()
clear all;
more off;

% These are our eventual axis coordinates.
x = linspace(-10,10,100);
y = x - 2;

% Generate an Airy disk
[xg,yg]=meshgrid(x,y);
rad = sqrt( xg.^2 + yg.^2);
airyintensity = abs(airy).^2;

% The normal imagesc invocation. Note axis ratio.
figure(1);
imagesc(airyintensity)
axis image
%colorbar

% Now invoke with axis vectors before the image/matrix
figure(2);
imagesc(x,y,airyintensity);
axis image
%colorbar

% Demonstrate the coordinates a little more clearly
% Coordinate axes are taken to be the center of the
% image pixel.
a=rand(4);
x=[3:6];
y=[-1:2];
figure(3);
imagesc(x,y,a);

% What about non-linear axes. Not handled correctly like this.
% This case is dealt with another time,
figure(4);
x2=sqrt(x)
y2=x.^2
imagesc(x2,y2,a);

endfunction

## Tuesday, May 13, 2014

### SN 2014J update

I've been exceptionally busy lately, and haven't had any time to follow up on what observations of SN 2014J have told us. A google search didn't show very much, so I ended up searching Arxiv.org for papers associated with SN 2014J returns the following list (search http://arxiv.org/find/all/1/all:+2014J/0/1/0/all/0/1):

1.
No X-rays from the very nearby Type Ia SN2014J: constraints on its environment
Comments: 9 pages, Submitted to ApJ
2.
Diffuse Interstellar Bands vs. Known Atomic and Molecular Species in the Interstellar Medium of M82 toward SN 2014J
Daniel E. Welty (1), Adam M. Ritchey (2), Julie A. Dahlstrom (3), Donald G. York (1) ((1) Univ. of Chicago, (2) Univ. of Washington, (3) Carthage College)
3.
Supernova 2014J at maximum light
Comments: 10 pages, 4 figures, submitted to Contrib. Astron. Obs. Skalnate Pleso
4.
Constraints on the Progenitor System of the Type Ia Supernova 2014J from Pre-Explosion Hubble Space Telescope Imaging
5.
Expectations for the Hard X-ray Continuum and Gamma-ray Line Fluxes from the Type Ia supernova SN 2014J in M82
Comments: Accepted to the Astrophysical Journal (March 31, 2014), with one additional figure and further discussion of technique
6.
Specific predictions for SN 2014J in the context of the Quark Nova Ia model
Rachid Ouyed (1), Nico Koning (1), Denis Leahy (1), Jan E. Staff (2) ((1) Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada, (2) Department of Physics and Astronomy, Macquarie University, Australia)
7.
Estimating the First-Light Time of the Type Ia Supernova 2014J in M82
Comments: Accepted for publication in ApJ Letters
This is quite a varied set of papers. I quite like the Margutti et al paper, not just because it is on X-ray emission but because of the impressive figures.

Otherwise, the most interesting of the bunch is Zheng et al. Not only did they uncover some earlier but unrecognized observations of the supernova, but the managed to estimate the explosion time (*) at Jan 14.75 UT (2014), about a week before Fossey's UCL undergraduates first noticed it. Tsvetkov et al present UVBRI light and color curves and conclude that SN 2014J belongs the the "normal" [Quotation marks in the original] subset of Type 1a SNe. How this matches up with the unusual early-time powerl-law light curve Zheng discovered doesn't seem to be discussed in their article, and as the subject is out of my (former) expertise I don't have enough information to make an informed speculation.

(*) Technically the time that any light from the explosion would have passed Earth. At a distance of D~3.5 Megaparsecs the actual explosion occurred ~11 million years ago (i.e. well before our ancestors and the ancestors of chimpanzees split off from each other).