Four X-ray-selected cataclysmic variables in the Galactic plane

Hertz, P.; Bailyn, Charles D.; Grindlay, J.; Garcia, M. R.; Cohn, H. N.; Lugger, P. M.

doi:10.1086/169408

Cited by 28 publications

(40 citation statements)

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“…However, Howel and Szkody (1990) also point out an alternative possibility that these are nearby members of a distribution of intrinsically very faint systems (M v > 10), which would also imply { 13 { that these objects have a considerable space density. This idea coincides with the conclusion of Hertz et al (1990), and the inferred optical luminosity is indeed consistent with a low-_ M CV (see discussion above). Howell and Szkody (1990) also nd that the faint CVs show preference for orbital periods below the period gap, and point out that the signi cantly lower mean orbital period implies that these systems are relatively old.…”

Section: Intrinsicly Faint Cataclysmic Variablessupporting

confidence: 89%

“…Two of them are nearby (d<200 pc), and have a very low inferred mass accretion rate ( _ M < 2 10 12 M yr 1 ). Hertz et al (1990) argue that these CVs belong to a large population of low-luminosity CVs that release the majority of their accretion energy as x-rays (F x =F opt >5). They emphasize that it would be extremely di cult to discover these objects through traditional means (except through their x-ray emission or very blue colors).…”

Section: Intrinsicly Faint Cataclysmic Variablesmentioning

confidence: 99%

“…Surprisingly, this ts nicely as well. Hertz et al (1990) report on four spectroscopically identi ed CVs, originally discovered and thus selected by an x-ray survey (Hertz & Grindlay 1984), with L x < 10 31 erg/s, and which are optically fainter then other known CVs (M v > 11:5). Two of them are nearby (d<200 pc), and have a very low inferred mass accretion rate ( _ M < 2 10 12 M yr 1 ).…”

Section: Intrinsicly Faint Cataclysmic Variablesmentioning

confidence: 99%

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An extended galactic population of low-luminosity x-ray sources (CVs?) and the diffuse x-ray background

Maoz¹,

Grindlay²

1995

ApJ

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The incompatibility of the properties of the x-ray background (XRB) with AGNs contributing > 60% at energies of a few KeV has often been interpreted as being due to a substantial contribution of a new population of yet unrecognized x-ray sources. The existence of such population has been recently suggested also by an analysis of very deep ROSAT observations (Hasinger et al. 1993) which revealed a considerable excess of faint x-ray sources over that expected from QSO evolution models, and that the average spectrum of the resolved sources becomes harder with decreasing ux limit. These sources could be extragalactic in origin, but if they make a substantial contribution to the XRB then they must exhibit much weaker clustering than galaxies or QSOs in order to be consistent with the stringent constraints on source clustering imposed by autocorrelation analyses of the unresolved XRB.We investigate the possibility that the indicated new population of x-ray sources is Galactic in origin. Examining spherical halo and thick disk distributions, we derive the allowed properties of such population which would resolve the discrepancy found in the number counts of faint sources, and be consistent with observational constraints on the total background intensity, the XRB anisotropy, the number of unidenti ed bright sources, the Galaxy's total x-ray luminosity, and with the results of uctuation analyses of the unresolved XRB.We nd that a attened Galactic halo (or a thick disk) distribution with a scale height of a few Kpc is consistent with all the above requirements. The typical x-ray luminosity of the sources is 10 30 31 erg/s in the 0:5-2 KeV band, the number density of sources in the Solar vicinity is 10 4:5 pc 3 , their total number in the Galaxy is 10 8:5 , and their total contribution to the Galaxy's x-ray luminosity is 10 39 erg/s. We discuss the possible nature of these sources, including them being subdwarfs, LMXBs, and old neutron stars. We argue that the inferred x-ray and optical luminosities of the sources, their 2-4 KeV spectrum, and the derived local number density and spatial distribution are all consistent with them being intrinsicly faint cataclysmic variables with low accretion rates. We suggest a few possibilities for the origin of such population, including an origin from disrupted globular clusters or dark clusters. We make predictions and suggest tests that could either con rm or rule out our proposal in the near future.

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Section: Intrinsicly Faint Cataclysmic Variablessupporting

confidence: 89%

Section: Intrinsicly Faint Cataclysmic Variablesmentioning

confidence: 99%

Section: Intrinsicly Faint Cataclysmic Variablesmentioning

confidence: 99%

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An extended galactic population of low-luminosity x-ray sources (CVs?) and the diffuse x-ray background

Maoz¹,

Grindlay²

1995

ApJ

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“…∼10 −5 pc −3 (Patterson 1998) and ∼3 × 10 −6 pc −3 (Warner 2001). A particularly relevant study to compare to is Hertz et al (1990), since it was also based on a sample of four X‐ray selected CVs. Hertz et al find ρ 0 ≃ (2–3) × 10 −5 pc −3 (where the uncertainty includes only the possible incompleteness of the sample), again consistent with the estimate presented here.…”

Section: Discussionmentioning

confidence: 99%

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

Pretorius¹,

Knigge²,

O’Donoghue³

et al. 2007

Monthly Notices of the Royal Astronomical Society

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We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X‐ray flux‐limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time‐resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is 1.1+2.3−0.7× 10−5 pc−3. We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as 2 × 10−4 pc−3 (as has been predicted theoretically), the vast majority of CVs must be fainter than LX≃ 2 × 1029 erg s−1.

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“…Observational estimates are typically lower, but have a large range; values from ≤5 × 10 −7 pc −3 to ρ∼ 10 −4 pc −3 have been reported (e.g. Ritter & Burkert 1986; Ritter & Özkan 1986; Hertz et al 1990; Shara et al 1993; Patterson 1998; Warner 2001; Cieslinski et al 2003; Schreiber & Gänsicke 2003; Araujo‐Betancor et al 2005; Aungwerojwit et al 2006; Rogel, Cohn & Lugger 2008). Fewer estimates of the CV luminosity function are available, but X‐ray (Sazonov et al 2006; Byckling et al 2010) and hard X‐ray (Revnivtsev et al 2008) luminosity functions have been published (see also Ak et al 2008, for near‐IR and optical luminosity functions).…”

Section: Introductionmentioning

confidence: 99%

The space density and X-ray luminosity function of non-magnetic cataclysmic variables

Pretorius

Knigge

2011

Monthly Notices of the Royal Astronomical Society

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We combine two complete, X‐ray flux‐limited surveys, the ROSAT Bright Survey (RBS) and the ROSAT North Ecliptic Pole (NEP) survey, to measure the space density (ρ) and X‐ray luminosity function (Φ) of non‐magnetic cataclysmic variables (CVs). The combined survey has a flux limit of FX≳ 1.1 × 10−12 erg cm−2 s−1 over most of its solid angle of just over , but is as deep as ≃10−14 erg cm−2 s−1 over a small area. The CV sample that we construct from these two surveys contains 20 non‐magnetic systems. We carefully include all sources of statistical error in calculating ρ and Φ by using Monte Carlo simulations; the most important uncertainty proves to be the often large errors in distances estimates. If we assume that the 20 CVs in the combined RBS and NEP survey sample are representative of the intrinsic population, the space density of non‐magnetic CVs is . We discuss the difficulty in measuring Φ in some detail – in order to account for biases in the measurement, we have to adopt a functional form for Φ. Assuming that the X‐ray luminosity function of non‐magnetic CVs is a truncated power law, we constrain the power‐law index to −0.80 ± 0.05. It seems likely that the two surveys have failed to detect a large, faint population of short‐period CVs, and that the true space density may well be a factor of 2 or 3 larger than what we have measured; this is possible, even if we only allow for undetected CVs to have X‐ray luminosities in the narrow range 28.7 < log(LX/erg s−1) < 29.7. However, ρ as high as 2 × 10−4 pc−3 would require that the majority of CVs has X‐ray luminosities below LX= 4 × 1028 erg s−1 in the 0.5–2.0 keV band.

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Four X-ray-selected cataclysmic variables in the Galactic plane

Cited by 28 publications

References 0 publications

An extended galactic population of low-luminosity x-ray sources (CVs?) and the diffuse x-ray background

An extended galactic population of low-luminosity x-ray sources (CVs?) and the diffuse x-ray background

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

The space density and X-ray luminosity function of non-magnetic cataclysmic variables

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