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:

AE Aqr is a known propeller system, of course.
The lowest X-ray luminosity systems, V455 And (with a spin period of 67.619685 s) and V1460 Her (38.87126 s), may well be unrecognized propellers.
DQ Her is a deeply eclipsing system in which the direct line of sight to the white dwarf proper is blocked by the accretion disk. Thus, the intrinsic X-ray luminosity is potentially much higher than the observed value. It is therefore not clear if DQ Her should be classified as an LLIP.

The rest of the systems listed definitly qualify as LLIPs.

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

Rank	Name	L_X	L_HS	G_abs	P_o
1	V455 And	5.4x10²⁸		11.6	1.351421
2	V1460 Her	1.6x10³⁰		7.9	4.9884535
3	DQ Her	3.5x10³⁰		6.1	4.646902
4	EX Hya	6.6x10³⁰	1.0x10³¹	9.4	1.6376123
5	V1025 Cen	1.1x10³¹	4.2x10³¹	10.7	1.410
6	AE Aqr	1.4x10³¹		6.1	9.8797346
7	CTCV J2056-3014	1.8x10³¹		9.9	1.76
8	AX J1853.3-0128	3.7x10³¹		10.7	1.45
9	V598 Peg	4.9x10³¹		9.8	1.3854
10	CC Scl	7.9x10³¹		10.3	1.4056136
11	HT Cam	8.7x10³¹		8.8	1.43309
12	DW Cnc	8.9x10³¹		9.0	1.435024
13	DO Dra	2.4x10³²	8.2x10³¹	8.8	3.96898

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.

Rank	Name	L_X	L_HS	G_abs	P_o
14	1RXS J211336.1+542226	4.5x10³²	3.4x10³²	9.1	4.17
15	GK Per	7.1x10³²	1.3x10³³	4.4	47.9249
16	V1323 Her	8.0x10³²		6.1	4.4021
17	PBC J0927.8-6945	9.7x10³²	1.5x10³³	5.2	4.790
18	FO Aqr	9.8x10³²	1.8x10³³	5.3	4.849430
19	RX J2015.6+3711	1.0x10³³	<4.9x10³³	6.7	12.7611
20t	WX Pyx	1.1x10³³		5.7	~5.3
20t	IGR J17014-4306	1.1x10³³	1.3x10³³	5.6	12.8166
20t	IGR J19267+1325	1.1x10³³		8.3	3.449
20t	MU Cam	1.1x10³³	1.7x10³³	5.0	4.7186
24t	V405 Aur	1.2x10³³	1.8x10³³	4.8	4.143012
24t	AO Psc	1.2x10³³	7.9x10³²	4.9	3.591012
26	V349 Aqr	1.3x10³³		5.5	3.23
27	Swift J183920.1-045350	1.4x10³³		6.6	~5.6
28	V2069 Cyg	1.5x10³³	2.9x10³³	5.3	7.48039
29	IGR J16500-3307	1.7x10³³	3.3x10³³	5.7	3.617
30t	TX Col	1.8x10³³	1.2x10³³	5.8	5.691
30t	V3037 Oph	1.8x10³³		7.2	5.722
30t	PQ Gem	1.8x10³³	2.1x10³³	4.8	5.1926
30t	TV Col	1.8x10³³	1.8x10³³	5.5	5.486398
34	IGR J17195-4100	1.9x10³³	1.9x10³³	6.2	4.006
35	IGR J15094-6649	2.1x10³³	3.4x10³³	4.5	5.88
36t	BG CMi	2.2x10³³	2.2x10³³	5.2	3.2339650
36t	AX J1832.3-0840	2.2x10³³	5.0x10³³	8.3
36t	UU Col	2.2x10³³		5.6	3.45
36t	HZ Pup	2.2x10³³		4.6	5.09
36t	IGR J16547-1916	2.2x10³³	2.9x10³³	5.5	3.715
41t	V1033 Cas	2.3x10³³	4.1x10³³	5.6	4.033
41t	AX J1740.1-2847	2.3x10³³		7.5	2.1 ?
43	V709 Cas	2.5x10³³	4.8x10³³	5.1	5.332898
44t	V515 And	2.7x10³³	2.0x10³³	5.0	2.731087
44t	IGR J08390-4833	2.7x10³³		5.8	~8
46	V2306 Cyg	2.8x10³³	2.6x10³³	5.2	4.371490
47	V2400 Oph	2.9x10³³	2.8x10³³	4.9	3.43
48	IGR J04571+4527	3.0x10³³	5.1x10³³	6.2	6.2 ? 4.8?
49	Swift J0717.8-2156	3.3x10³³	5.1x10³³	5.9	5.515
50	V1223 Sgr	3.4x10³³	4.9x10³³	4.3	3.365856
51	1RXS J230645.0+550816	3.6x10³³		4.8	3.26
52	V647 Aur	3.7x10³³	6.0x10³³	4.5	3.46565
53	Swift J2138.8+5544	4.1x10³³		5.6	4.426
54t	IGR J18308-1232	4.6x10³³	1.0x10³⁴	5.9	5.374
54t	V1062 Tau	4.6x10³³	4.5x10³³	5.7	9.98119
56	V4743 Sgr	4.8x10³³		3.6	6.7176
57	V667 Pup	5.1x10³³	1.0x10³⁴	3.0	5.611
58	EI UMa	5.4x10³³	4.4x10³³	4.6	6.434
59	1RXS J213344.1+510725	6.1x10³³	1.2x10³⁴	4.3	7.13834
60	V418 Gem	6.2x10³³	8.0x10³³	4.5	4.36925
61	NY Lup	7.8x10³³	1,8x10³⁴	2.8	9.87
62	Swift J2006.4+3645	9.5x10³³	3.3x10³⁴	4.6	17.2842
63	V2731 Oph	1.1x10³⁴	3.1x10³⁴	4.4	15.421
64	IGR J18173-2509	2.9x10³⁴	3.9x10³⁴	3.5	1.53
65	1RXS J180431.1-273932	4.4x10³⁴		2.9	4.99

Please send your comments, suggestions etc. to Koji.Mukai@nasa.gov and/or Koji.Mukai@umbc.edu

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