Low Luminosity IPs
Pretorius & Mukai (2014) pointed out that, while the hard X-ray luminosity of most IPs are at about 1033 ergs s-1 or above, there is a hint of a separate population of low-luminosity IPs (LLIPs). Note that the bolometric X-ray luminosity is a factor of a few higher than the values tabulated by Pretorius & Mukai (2014), who used the Swift BAT 14-195 keV luminosity.
The table below is the list of confirmed or ironclad IPs with X-ray (in the traditional band, measured mostly with XMM-Newton or with Swift XRT) below 2.5x1032 ergs s-1. This is based on the Gaia EDR3 distance estimate, as well as the cataloged X-ray fluxes. In the vast majority of cases, these objects are undetected in the Swift BAT hard X-ray survey; in the few cases with BAT detection, the hard X-ray luminosity is also given.
I chose 2.5x1032 ergs s-1 as the cut-off here, because that's just allows DO Dra, with BAT luminosity of 8.2x1031 ergs s-1 and was one of the original LLIPs of Pretorius & Mukai (2014), to be included. First IP to be excluded by this criterion is 1RXS J211336.1+542226 at 4.5x1032 (with a BAT-band luminosity of 3.4x1032).
Several objects in this table deserve special comments:
The X-ray luminosities of 6 confirmed or ironclad IPs are unknown, either because the X-ray fluxes are unknown, or because there are no Gaia EDR3 parallaxes. Of these, XY Ari may well qualify as an LLIP. Although there is no Gaia parallax (because of the intervening molecular cloud), Littlefair, Dhillon & Marsh 2001 used IR data to estimate a distance of 270+/-100 pc. Using 270 pc, its X-ray luminosity is 1.2x1032 ergs s-1 (hard X-ray luminosity: 2.6x1032 ergs s-1).
A List of Low-Luminosity IPs
A List of Regular IPs
For comparison, I continue the above table with the rest of (regular) IPs. Note that the listed X-ray luminosities are not corrected for intrinsic absorption. While this is probably not a major issue for LLIPs (with low intrinsic absorption), it may have a major effect on these, regular, IPs. The true, intrinsic, unabsorbed luminosities are higher than those listed in this table. This is not an issue with the BAT-band hard X-ray luminosity, where any absorption correction is likely to be minimal. I find that 46 of the 71 confirmed or ironclad IPs have X-ray luminosity in the range 9.6x1032 to 9.6x1033 ergs s-1, with three outliers with even higher X-ray luminosity.
Please send your comments, suggestions etc. to Koji.Mukai@nasa.gov and/or Koji.Mukai@umbc.edu