Is it a supernova? Check transients with Aladin
Here are a few simple steps you can use to check if transients are real and unreported. But first:
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Sometimes we find things in images that look like supernovae and we want to check if they are real unreported transients.
- Why call them transients and not supernovae?
Because to be certain it is a supernova you need to confirm this with observed light-curves and/or spectra taken form the transient, submit the data and get an official designation by the International Astronomical Union. Before that they are not officially supernovae. Ofcourse if you find a real and unreported transient it is very likely to be a supernova, but there are also some objects and processes in the universe that merely mimic the appearance of a supernova.
-What questions do I need to answer before I know for certain it is a real and unreported transient?
Well to be honest many times it is not possible to be 100% sure if something is an artifact or transient without follow-up observations. (or other types of object for that matter). That is the reason I usually use words as ''likely'' ''maybe'' ''probably'' etc. The more things you can verify or exclude the more confident you can be about what something in the images is, but sadly most of the time they are just artifacts.
The questions you need to answer as best as possible: Is it not an artifact such as a cosmic ray hit? Is it not another type of object such as an asteroid? Is it really a transient and not a permanent object such as a Milky Way star? Is it unreported or an already known transient / supernova?
Here are a few simple steps anyone can do to check for themselves (not in any particular order);
1. Check other surveys & databases
2. Check other data in the survey
3. Check the individual bands of the image (Aladin Desktop)
4. Check the Transient Name Server
1. CHECK OTHER SURVEYS & DATABASES
There are several surveys in the (mostly) optical bands you can use to check if the object is present in multiple surveys. There will be more surveys available in the future.
If it is in there you can usually assume it is real but not a transient event. Ofcourse it is not impossible for a transient / supernova to appear in multiple surveys but the odds of that are so small that it is as good as impossible. If you think an object is a transient in two different surveys you would have to compare the dates and times when the images were taken to be 100% sure.
For all surveys you simply need to fill in the right RA Dec coordinates and check.
Or search for the latest Data Release (DR)
DECaLS you can see the latest DECaLS Data Release in the same webbrowser as multiple other surveys including SDSS.
Aladin Lite Combines multiple surveys in one application, browser version of Aladin Desktop.
GAMA Panchromatic Swarp Imager for Greyscale bands choose 'All', also possible to create RGB combined images
The Hubble Legacy Archive (HLA), get lucky!
The Dark Energy Survey (DES) DR1, very deep images of a Southern Hemisphere area. Same telescope as DECaLS. Need a free-for-all registration to use.
IRSA Finder Chart combines several surveys in one application; DSS1, DSS2 and SDSS (optical) and 2MASS and WISE (Infrared).
In IRSA Finder Chart you can also view the individual bands of SDSS DR7 without the need to download software.
VizieR Database that searches through available catalogs. Use the second location box that says ''Search by Position across xxxxx tables'' to find all catalogs that have objects near your search position and press go! Set finding box to ~5 arcsec to minimize number of catalogs + entries you get.
NASA/IPAC EXTRAGALACTIC DATABASE or NED for short.
The Supernova Pages
Select 'Archive' and search on coordinates in the right archive year.
IAU Search page
2. CHECK OTHER DATA IN THE SURVEY
Most surveys have a lot more data in store then can be seen from the images displayed or object explorer page. For example DECaLS has this information stored in downloadable FITS files like many surveys.
Within SDSS it is also possible to see these images with a 'Coverage Check' search. I will give an example of how this works below using subject AGZ000dcge with coordinates for the central galaxy RA/Dec: 355.22058 -2.02813
Because of the way the images are stored it is best to have an additional window open that shows the area (in DECaLS or SDSS), here is a link to that area: http://cas.sdss.org/dr9/en/tools/chart/navi.asp?ra=355.22058&dec=-2.02813
Then open the SDSS Coverage Check page here: https://dr9.sdss.org/coverageCheck
Clear away all the coordinates that are already in there, paste these coordinates: 355.22058 -2.02813 and press Submit so we get this page:
To find other images of our galaxy we will only use the most right colomn that says 'Image Coverage'. That column contains all the images nearby our search coordinates. What can make this time-consuming is that;
- Not all images contain our galaxy
- The images are rotated in comparison to the object explorer images
So that means you have to visually find the target galaxy in these images using nearby objects and shapes to locate it.
Right-click the top link that says '7778 / 4 / 284' and open it in a new window:
Here we see the jpg image we need to check if it contains the galaxy, use the other page with the entire area to locate it, or to be sure it is not in this image. To better zoom in and out you can click on the image to enlarge it.
Because it is a relative large galaxy it is easy to find that it is only visible in two images: 8038 / 5 / 187 and 7907 / 5 / 40 so now we have two images to compare. Other areas may contain much more images to compare. Also notice that the green object in our galaxy is only visible in one of the images and looks more like an artifact than in the Galaxy Zoo image and the SDSS Object Explorer image.
3. CHECK THE INDIVIDUAL BANDS OF THE IMAGE
Artifacts are usually very square and only visible in one band of an image, and a colour image is built up from three different bands. These images are usually kept in FITS format in astronomy you need to download, and you need the right software to open and see them. There are several programs that can do this, and those that were recommended to me were FITS Liberator, Aladin Desktop and SAOImage DS9.
I tried FITS Liberator first, but after crashing many times on the first, second and third attempt I will not be using it anymore. This may be different for you so you'll just have to try it yourself.
Then I tried Aladin Desktop and it worked very good for me, especially because for checking out SDSS images you don't need to download the FITS files, just open them through a menu. I do love simple and time-saving methods! So I will use this for the example.
SOAImage DS9 is considered the scientific / professional option. I haven't really tried using it but I might in the future for some other things I have 'planned' on doing that I can't with Aladin Desktop.
UPDATE I have made my first color image with SOAImage DS9 using this excellent guide by Melina (moderator at The Milky Way Project): https://melinasworldblog.wordpress.com/2017/07/21/how-to-create-an-image-from-fits-files/amp/
So here is a simple step-by-step example of how to check for transients / artifacts in Aladin Desktop. For this example I'll be using subject AGZ000dcge, which is an SDSS image. SDSS has five bands called u, g, r, i and z. The colour images are made from the g r i bands.
This particular image has a green object that we want to check out.
The coordinates of the central galaxy are RA/Dec 355.22058 -2.02813
STEP 1 Download, install and open Aladin Desktop. You can download the program from here: http://aladin.u-strasbg.fr/java/nph-aladin.pl?frame=downloading
Your screen will look like this:
On the top of the screen you can already spot ''SDSS'', it is in a pre-selected row of surveys so that's very convenient.
STEP 2 Press the ''SDSS'' button to get a screen that is similar to this (probably other coordinates but no problem):
You can now see the SDSS (colour) layer in the bottom right where all layers will put, it is called the stack. Next to it is a tick box. What we want to do is keep this SDSS color layer on the bottom of the stack, so below all other layers, and keep the tickbox of this layer checked. We will use it as a 'Base Layer'' that is always visible when all other layers are 'off'.
STEP 3 now we want to insert the individual layers of SDSS9. So we go to FILE -> Progressive sky (HiPS) -> Image -> Optical -> SDSS -> SDSS9 u and click on SDSS9 u. This layer is now put on top of the SDSS colour base layer, also it has it's tick box checked but we will fix that later. One tip: Aladin Desktop seems to zoom in after every new layer that is inserted to the point that it can freeze up. So I just zoom out before adding another layer to prevent this from happening. Repeat this selection step for the other SDSS layers g, r, i and z (and if needed zoom out in between selecting them).
After inserting all the layers, again select the tickbox of the bottom SDSS colour layer so it is the 'active'' layer, your screen should now look similar like this (it might be on different coordinates because of my previous searches):
With SDSS9 colour at the bottom and it's tickbox checked, and all the other layers have little red sliders that are all the way to the left.
STEP 4 insert the coordinates 355.22058 -2.02813 in the top white bar that has 'Location' in front of it and press Enter. Now zoom in until we get a good view of our target galaxy and the green feature that looks like a transient:
STEP 5 Drag the red slider of the layer 'SDSS9 u' all the way to the right, it might take a few seconds to fully load. Then drag it all the way to the left again and repeat for the other layers so they are all loaded for these coordinates.
Then use the sliders to compare all layers one by one with the green feature in the SDSS9 colour layer (and slide it all the way back to the left if you want to check another layer). For me it only matches with the r-band layer and also looks pretty square to me:
Now this doesn't mean it is a 100% sure artifact. I am not a scientist and there can also be a few other things that are in play. For instance there is a small possibilty that the r-band was the last image taken and that was also the moment a supernova just went boom. We would have to check the date and time the images were taken (MJD) to make sure. But for me just by the way it looks here, squarish and only in r-band, I think this is very likely an artifact and I'm finished with research.
I might be wrong in this case, well then you just have to dig deeper and get more compelling evidence 😃 (such as checking out the MJD dates of the different bands, or if it is present in other images such as described in 2. Check other data in the survey
Good luck and have fun!
PS1 FYI Aladin can open any FITS files you download. Just use File -> Open local file and select the file. But it has to look like xxxx.FITS for Aladin Desktop to open. Sometimes the files look like xxxxx.FITS.GZ or something similar, which means you have to unpack the file. I have used the program 7-ZIP a couple of times to do this.
PS2 It is also possible to see the individual SDSS DR7 bands in IRSA Finder Chart, see 1. CHECK OTHER SURVEYS & DATABASES
4. CHECK THE TRANSIENT NAME SERVER
If you want to know if the object you have found is an already known transient / supernova you can check this in the Transient Name Server. As of January 1, 2016 the Transient Name Server (TNS) is the official IAU mechanism for reporting new astronomical transients such as supernova candidates. So if it is known it should be in there.
Just fill in the right RA Dec coordinates and set the search radius to 1 or 2 arcminute and press 'Submit'. This is to make sure we catch everything in the local area, sometimes transients have not been reported with the right exact coordinates so can be a little off to the coordinates you are using.
If the system finds any nearby transients check them out to see if they are your object or not. To help there is usually a little SDSS image at the top of the page:
EDIT: it seems there are a lot of supernova teams that don't necassarily report their supernovae to the TNS, so to be sure if it's unreported or not it is really necessary to also search in: NED, SIMBAD, The supernova pages, IAU Search page, ADS, VizieR and also google on combinations of: ATel, coordinates, host galaxy name, supernova etc.
Well that about wraps it up for now.
If anyone has any comments / improvements / questions etc. just post them below, hope this helps some of you with doing some research yourself 😃
Updated with links for SIMBAD, VizieR, IRSA Finder Chart (SDSS DR7 individual bands view), NED and Aladin Lite.
thx i'll try it