- Participants: A. Giménez, Elisa Nespoli, Damya Souami, Olivera Latkovic
VO tutorial: H-alpha emitters in X-ray surveys
H-alpha emitters in X-ray surveys
Tutorial instruction
by Ivan Zolotukhin (Sternberg Astronomical Institute / Observatoire de Paris)
0. Download jar files of Aladin and TOPCAT if you do not have them. One can easily find them
using Google. To be on the safe side, here is the links:
1. Aladin: http://aladin.u-strasbg.fr/java/nph-aladin.pl?frame=get&id=Aladin.jar
2. TOPCAT: http://www.star.bris.ac.uk/~mbt/topcat/topcat-lite.jar
1. Launch them on Linux (in terminal): java -jar topcat-lite.jar. On Windows and Mac they can be
launched by double-click on a jar-file.
2. Load first catalog. In TOPCAT do: File → Load table → DataSources → VizieR Catalogue
Service → All rows → By Keyword: Sugizaki (first author of the faint X-ray sources in the Galactic
plane catalog, obtained with ASCA space observatory) → Search Catalogues. Then take the only
found catalog in the field below and press OK to load it as a whole since it is not too large. Please
make sure you loaded the catalog by choosing Graphics → Sky in the menu. You should see a
bunch of points along the Galactic plane around Galactic center.
3. Load second catalog. In TOPCAT: File → Load table → DataSources → Cone Search →
Keyword: Witham (first author of H-alpha emitters catalog, obtained using IPHAS survey data). We
need this to get an idea of IPHAS footprint. You can download the whole catalog by emulating cone
search request with big radius: RA = 0 deg, Dec = 0 deg, Radius = 200 deg → OK
4. Graphics → Sky. Witham catalog is displayed on a sphere. Click on Add new data set icon and
add second catalog on a sphere (by choosing corresponding position in drop-down Table menu).
You can zoom and unzoom with mouse wheel. By holding left mouse button you can move the
sphere. Study carefully the region where 2 catalogs intersect.
5. Choose Draw a region on the plot to define a row subset button on a toolbar at the top of the plot
window (you can also select it from the menu: Subsets → Draw Subset Region) and manually draw
a region around X-ray sources from ASCA catalog that fall within IPHAS (i.e. Witham catalog in
our case) footprint. Then press this toolbar button one more time and type New Subset Name of a
subset we have just defined, e.g. asca+iphas, then press Add Subset button. Points of new subset are
designated with a new color. These actions are needed to continue our study of selected X-ray
objects in H-alpha survey, since if our X-ray objects do not fall within its footprint, we cannot apply
this method obviously.
6. Plot hardness vs. line of sight extinction diagram with ASCA data. To do this, select ASCA table
in the main TOPCAT window (one that has 163 rows), then press Scatter Plot on a toolbar (or using
menu Graphics → Plot). For X axis choose NH (number of hydrogen atoms on the line of sight; it is
obtained from the model which was used to fit X-ray spectrum of a source), for Y axies choose
Gamma, slope of the power law of the spectrum. The more NH is the more distant source we look
at (in average). The less Gamma is the more hard spectrum we have (since spectrum less decreases
to the hard side). We are interested in a distant hard sources to discriminate various X-ray active
stars (that behave much like the Sun with its coronal activity). On the right in Row Subset area turn
off All checkbox to look at asca+iphas subset only, that we are going to study further. Draw by hand
a subset of sources with Gamma < 3 and NH > 2e22 cm^-2, there should be 4-5 of them. Let us
start our study from the highest source with Gamma = 2.61, NH = 3.39. Copy its coordinates or
write them down. One can conveniently do this by highlighting the source on a plot and at the same
time looking at the highlighted row in a Table Browser (open it using Views → Table Data while
given table being selected in the Table List).
7. Request all stars with H-alpha narrow band photometry existing in IPHAS around the coordinates
we obtained at previous step. File → Load Table → DataSources → Cone Search → Keywords:
IPHAS → Submit Query. Among the catalogs found choose IPHAS IDR service and yet below —
IDR, PhotoObjBest table with measurements in all survey filters. Then enter the coordinates and
error box of our X-ray source: RA = 297.416, Dec = 26.53 deg, radius = 1.2 arcmin (this is the
positional uncertainty of ASCA coordinates). Then press OK. There should be the table with ~400
rows loaded.
8. Add 2 new columns with colors in the table to be able to plot color-color diagram from IPHAS
data easily. Views → Column Info → Columns → New Synthetic Column. Fill Name (R - I and R -
Halpha) and Expression (coremag_r - coremag_i and coremag_r - coremag_ha, correspondingly)
only, it is enough.
9. Plot color-color diagram for these sources (see Drew et al. 2005 about the details of this method,
http://adsabs.harvard.edu/abs/2005MNRAS.362..753D). Graphics → Plot. X axis: R - I column, Y
axis: R - Halpha column. First, zoom to the valid measurements area by holding left mouse button
and dragging a
rectangle around central cloud of points. The wider and brighter H-alpha emission in object's
spectrum is, the higher is its position on this diagram. Press Add auxiliary axis button in the plot
window and add brightness as a third axis, e.g. coremag_r column (do not forget to tick axis
Flip checkbox, so that brighter stars are in red which seems to be more intuitive). Add uncertainties
to the diagram: press Toggle X error bars on/off, and same for Y axis. Either choose
coremagerr_r column, or more honest, add expression coremagerr_r + coremagerr_i and the same
for H-alpha as an error column. Note that sources on the very top are violet (i.e. very faint) and one
can hardly trust their measurements, their errors are quite big as well. Then we must say, that here
we have not IPHAS sources but IPHAS detections. It means that 1 point corresponds to 1
measurement of a source and for a single object there can be several measurements. Click on Edit
style for row subset All in Row Subsets area, then click Linear Correlation in the new window and
press OK. This is just for clearness because this straight line roughly corresponds to main sequence
stars which do not exhibit H-alpha emission. At the same time outlier points correspond to H-alpha
emitters. We need to understand which points at this diagram represent our X-ray object. Do not
close this window since we will use it later. Even now we can notice 3 green points with relatively
small error bars in X=0.5-0.7, Y=0.6-0.8 region, which outlie from the main cloud to the top. One
can notice that these are just 3 measurements of a single object (for example, highlight them one by
one by clicking on the points at the diagram and look simultaneously at the Table Data window you
should open before; these all have almost identical coordinates).
10. Now let us load metadata of IPHAS images in this regions. To do this go to File → Load Table
→ DataSources → SIA Query (Simple Image Access Query) → Keywords: IPHAS → Submit
Query. One service will be found and we specify our coordinates to search images in it: RA =
297.416 deg, Dec = 26.53 deg, Radius = 0 deg. A new table containing links to FITS files will be
loaded in TOPCAT. One can look at this SIAP reply table using Views → Table Data.
10. Since TOPCAT is not able to show images, launch Aladin (keep TOPCAT running). In the main
TOPCAT window highlight IPHAS images table on the left and press Interop → Broadcast Table. A
Server Selector window will raise in Aladin after that, listing all IPHAS images grouped by filters.
Choose H-alpha filter, tick the box next to one of the images (e.g. CCD2) and press Submit. Aladin
starts to load FITS file with IPHAS image displaying some progress information on the right of the
main window (be patient, it can take some time, it is 16MB image). Tune cuts of the image by
moving mouse to the left and right while holding the right button. Note nice H-alpha nebula at the
lower left corner. Zoom and unzoom with mouse wheel, double-click to center the image.
11. In TOPCAT click on the IPHAS sources table on the left of the main window and then broadcast
it. Note that Aladin plotted these sources over IPHAS image.
12. Check if there are deep X-ray observations of this field in Aladin. Unzoom it first to see the
whole image and then choose File → Load catalog → Missions in VizieR → logChandra. There is
one observation available for this field in Chandra archive. Click on this point overplotted on
IPHAS image and check that this is observation with ObsID 8236 (will be shown in the table below
the image in Aladin window). Press High button there to load high resolution X-ray imaging dataset
from Chandra archive. Wait a bit and there will be X-ray image loaded with the single black object
in the center. Tick the box in front of H-alpha image in Aladin data stack (on the right in the main
window) and note that there is small stick appeared in front of the IMAGE[0] X-ray image. You can
move it by holding left mouse button to change transparency of this layer. Note that the only X-ray
object visible corresponds to a single star from H-alpha image. Turn on IPHAS catalog layer in
Aladin (tick the box next to it) and click on the markers plotted over our H-alpha and X-ray object.
When you click on a marker in Aladin, it is highlighted in TOPCAT color-color diagram window,
which is very convenient, you need not to match sources manually. Note that our X-ray source is Halpha
emitter since it lies higher than main sequence linear regression. You can discover that these 3
points correspond to a single object which do exhibit H-alpha emission, probably even variable.
Therefore, using Chandra and IPHAS data we can suspect this optical object to be unknown ASCA
source counterpart indeed.
13. Write down all the information available in the Virtual Observatory about this source. Estimate
H-alpha equivalent width using narrow band photometry (see fig. 2 in Witham et al. 2006,
http://adsabs.harvard.edu/abs/2006MNRAS.369..581W).
14. Check ADS, are there any papers about this source? Try putting ASCA object name into title
and abstract fields. Seems this source is not known yet. There will be Zolotukhin et al. (2010)
however soon.
15. Determine possible object type from color-color diagram and USNO B-1.0 colors. The correct
answer is either Be-transient or cataclysmic variable. One cannot say for sure without additional
data.
16. If you are willing to go further, try to check the remaining hard and distant ASCA sources in
IPHAS.
For more 'adql' information:
http://docs.g-vo.org/adql
The Zolotukhin article using VO:
I. Yu. Zolotukhin and I. V. Chilingarian, 2011, A&A, "Virtual Observatory based identification of AX J194939+2631
as a new cataclysmic variable"
