ULX tutorial

Search for ULX sources and X-ray binaries in nearby galaxies (developed by M.A. Perez-Torres, P. Padovani, E. Hatziminaoglou)

Uses Aladin, and TOPCAT


ULXs (Ultra-Luminous X-ray sources) are X-ray sources that are less luminous than AGN but more luminous than any known stellar process (Lx > 1039 erg/sec). We start from a list of nearby galaxies, estimate their angular size from information found in the hyperLeda catalogue, then find all possible ULXs by cross-matching with the 2XMMi catalogue. A diagram of the workflow can be found here.

    • launch Aladin, TOPCAT


       launch the tools with a larger memory buffer; e.g. 

java -Xmx512M -jar topcat-full.jar

  • in TOPCAT, load the NED-1D galaxy catalogue (a .csv [coma-separated values] version of the catalogue can be found here: NED-1D.csv)
  • the RA and Dec are in sexagecimals; convert RA, Dec to decimal: select Display Column Metadata and then Add new sky coordinate column based on existing one; get rid of the multiple entries using the option Internal match, under Joins, selecting the Action Eliminate All But First of Each Group, using as a matching algorithm either Sky, or Exact Value with the "Name" column as reference
  • optional: apply a filter in distance (e.g. D<10 Mpc) using TOPCAT (the stricter the criterion, the smaller the number of objects!)
  • send the catalogue to Aladin
  • this catalogue does not contain any information about the size of the objets; such information can be found in the hyperLeda catalogue: load the catalogue [(VII/237) from the VizieR Catalogue Service] (in theVO menu), requesting All rows, an unlimited Maximum Row Count and all Output Columns (this might take a while as it contains about a million objects); the catalogue is also available here in a .gz VOTable format (unzip the table before uploading it)
  • cross-match the two catalogues using a 5" radius; add new column (under Display Column Metadata) with the radius, in arcmin: pow(10.,$diameter_column)*0.1/2 (where "diameter_column" is the ID of the Aparent diameter column, expressed in log and units of 0.1 arcmin - check the table metadata); send the catalogue to Aladin
  • in Aladin, load the 2XMMi catalogue (3rd incremental release) [(IX/41)], which gives the calibrated X-ray fluxes in various bands for about 260,000 sources; a copy of the 2XMMi catalogue in gzipped VOTable format can be obtained from here: 2xmmiDR3.vot.gz (unzip the table before uploading it); zoom out (1x) and see how the two catalogues overlay on the Allsky DSS image (it can be loaded from the Server Selector, Allskyoptical)
  • send the 2XMMi catalogue to TOPCAT (you can also load it directly in TOPCAT by-passing the previous step, using again the VizieR Catalogue Service) and cross-correlate; we want to find all X-ray sources that lie within the galaxies: Joins; Pair Match; "Sky with Errors"; "1&2"; "All Matches" the 2XMMi with the nearby galaxies catalogue using the new radius and the X-ray position uncertainty (ePOS) as errors
  • the definition of a ULX is based on its X-ray luminosity, which we need to compute: add new column with the luminosity Lx for the point like sources from the calibrated flux in the 0.2-12 keV band (Flux8) and distance (D) to each galaxy, i.e. Lx = 4*π*D2*fx = (do the math!) 50.078+2*log10($distance_column)+log10($fx_column)
  • filter those sources with Lx ≥ 1039 erg/s (these will be the ULX candidates)
  • plot Lx vs cross-correlation separation: some objects are at very low offsets and large Lx, therefore X-ray emission most probably comes from the nucleus; some objects are clearly off; keep those above a given separation which you can select based on the separation distribution
  • send the catalogue to Aladin keeping only a few columns: name, distance, X-ray coordinates, Lx (select Display Column Metadata and then Make all table columns invisible)
  • cross-match with X-ray binaries table(s) (for example J/ApJS/157/59) to find the non-matches
  • cross-match with quasars and/or AGN catalogue(s) (example: Veron & Veron 13th edition, VII/258) to find the non-ULX sources
  • find existing ULX catalogues to confirm candidates (e.g. J/ApJ/452/739); cross-correlate and compare the X-ray power in these catalogues with the one in our list of ULX candidates (tip: the Lx column in the ULX catalogue is in units of 1032 W; transform to log10(Lx) and add 39 to get it in erg/sec)
  • find images of the galaxies, plot the positions of the AGN and ULX candidates; verify their separation as well as from the galaxy centre (try M 84, NGC 3310, NGC 253, NGC 5128, NGC 6946)