ROSAT North Ecliptic Pole Survey Research Articles

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

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 × 10 29 erg s −1 .

Unobscured QSO 2: a new class of objects?

We present in this paper optical and X-ray follow up observations for three X-ray selected objects extracted from the ROSAT North Ecliptic Pole survey which is a flux-limited, completely identified survey. All three objects have X-ray luminosities in the 1044 erg s-1 regime and show narrow emission lines in their optical discovery spectra, typical of QSO 2 type objects. Spectroscopic data for the three QSO 2 candidates, obtained with the Telescopio Nazionale Galileo, confirm the widths of the Hα or Hβ emission lines are less than 750 km s-1. On the other hand XMM-Newton data do not show any sign of obscuration as expected for this class of objects. The X-ray spectra of the three objects are all well fit by a single power law model with with low energy absorption fixed to the Galactic value along the line of sight to each object. Most observational evidence supports the scenario where optical and X-ray obscurations are linked, contrary to our findings. We discuss the unanticipated results of these observations, and compute the space density in soft X-ray surveys of this possibly new class of objects. Their spatial density in the ROSAT NEP survey is 2.8 h3 Mpc-3 in a ΛCDM model with . Unobscured QSO 2 candidates could go unrecognized in current X-ray surveys where the low hydrogen column density is inferred by a hardness ratio rather than a more precise X-ray spectrum measurement.

RX J1821.6+6827: A cool cluster at z = 0.81 from the ROSAT NEP survey

We present an analysis of the properties of the cluster of galaxies RX J1821.6+6827, or NEP 5281, at a redshift . RX J1821.6+6827 was discovered during the optical identification of the X-ray sources in the North Ecliptic Pole (NEP) region of the ROSAT All-Sky Survey and it is the highest redshift cluster of galaxies of the NEP survey. We have measured spectroscopic redshifts for twenty cluster galaxies using the Keck-I and the Canada-France-Hawai'i (CFH) telescopes. The value for the cluster velocity dispersion is km s-1 . The cluster was also observed by XMM-Newton . Both the optical and X-ray data are presented in this paper. The cluster has an unabsorbed X-ray flux in the 2-10 keV energy band of erg cm-2 s-1 and a K-corrected luminosity in the same band of erg s-1 (90% confidence level). The cluster X-ray bolometric luminosity is erg s-1 ( erg s-1 in the concordance cosmology). The data do not allow fitting both metal abundance and temperature at the same time. The abundance is unconstrained and can vary in the range 0.28-1.42 while the best fit X-ray temperature is keV. This emission weighted X-ray temperature is a little lower, barely within the uncertainties, than the predicted temperature, keV, from the relation of local clusters published in the literature. The optically measured velocity dispersion is consistent with the velocity dispersion expected from the relationship. We also examine the point X-ray source RX J1821.9+6818, or NEP 5330, located to the south east of the cluster which was identified as a QSO at in the ROSAT NEP survey. The X-ray source is well fitted by an absorbed power law model with atoms cm-2 and a photon index typical of an active galactic nucleus.

The ROSAT North Ecliptic Pole Survey

In the ROSAT All-Sky Survey, an 80.7 deg region around the north ecliptic pole (NEP) constitutes the deepest observation of the X-ray sky ever achieved with such a large, contiguous solid angle (W. Voges et al. 2001, ApJ, 553, L119). Here, 445 unique sources are detected with fluxes measured at greater than 4 j significance. The median and maximum exposure times are approximately 5 and 38 ks, respectively, and the minimum flux limit is ∼ ergs s cm 2 (0.5–2.0 keV). 14 1 2 # 10 We have identified the physical nature of 443 (99.6%) of the ROSAT NEP X-ray sources through a comprehensive program of imaging and spectroscopy primarily with the University of Hawaii 2.2 m, Canada-France-Hawaii 3.6 m, and Keck II 10 m telescopes (J. P. Henry et al. 2001, ApJ, 553, L109). The main constituents of the resulting catalog are 217 active galactic nuclei (AGNs; 49.0%), 152 stars (34.3%), and 64 groups and clusters of galaxies (14.4%). Minor components include eight BL Lacertae objects, one isolated elliptical galaxy, and one planetary nebula. We have obtained spectroscopic redshifts for all of the extragalactic objects. The median and maximum redshifts of the cluster sample are 0.205 and 0.811, whereas for the AGNs the corresponding values are 0.408 and 3.889. We have used the complete samples of extragalactic X-ray sources to examine the evolution and distribution of large-scale structure in the universe (I. M. Gioia et al. 2001, ApJ, 553, L105). The low-redshift X-ray luminosity function (XLF; 0.02 ! z ! ) of the NEP clusters is consistent with previous determi0.3 nations of the local XLF. Conversely, the high-redshift XLF ( ) shows significant deviations from the local XLF 0.3 ! z ! 0.85 at high luminosities. Similarly, a comparison of the number of observed clusters versus the number expected assuming noevolution models indicates a deficit of clusters at high redshift and high luminosity. Thus, the NEP data show negative evolution in the cluster population at ergs s and 44 1 L 1.5 # 10 z 1 X at greater than approximately 4.2 j. This result agrees with 0.3 the most simple form of hierarchical, bottom-up structure formation that predicts fewer luminous/massive objects in the past as compared to the present. Using the ROSAT NEP survey data, we have detected a supercluster at (C. R. Mullis et al. 2001, ApJ, 553, z p 0.087 L115). The X-ray data greatly improve our understanding of this supercluster’s characteristics, approximately doubling our knowledge of the structure’s spatial extent and tripling the cluster/group membership compared to the optical discovery data. The supercluster is a rich structure consisting of at least 21 galaxy clusters and groups, 12 AGNs, 61 IRAS galaxies, and various other objects. A majority of these components were discovered with the X-ray data, but the supercluster is also robustly detected in optical, IR, and UV wavebands. Extending ( Mpc), the 1 129 # 102 # 67 h50 NEP supercluster has a flattened shape oriented nearly edge-on to our line of sight. Owing to the softness of the ROSAT X-ray passband and the deep exposure over a large solid angle, we have detected for the first time a significant population of X-ray–emitting galaxy groups in a supercluster. Also relevant to large-scale structure, and in particular cluster formation, is the distant cluster RX J1716.6 6708 ( ), z p 0.81 which we discovered during the ROSAT NEP survey (J. P. Henry et al. 1997, AJ, 114, 1293; I. M. Gioia et al. 1999, AJ, 117, 2608). The member galaxies have a filamentary distribution, with the X-rays coming from the midpoint of the filament. The X-ray contours have an elongated shape that coincides roughly with weak gravitational lensing contours. This distant system of galaxies could be an example of a protocluster where matter is flowing along filaments and the X-ray flux is maximal at the impact point of the colliding streams of matter. Working beyond coherent structures, we have measured the degree of clustering in the ROSAT NEP data by calculating twopoint spatial correlation functions. Accounting for the effects of the NEP supercluster, the correlation function of galaxy clusters is consistent with previous measurements, albeit with large uncertainties. More importantly, our measurement of the spatial correlation function for X-ray–selected AGNs provides the best constraints (3.8 j) reported so far. The maximum likelihood determination of the power-law fit indicates best-fit values of Mpc and . If we set , the 2.5 1 1.40 r p 6.6 h g p 1.65 g p 1.8 0 5.4 1.07 correlation length is Mpc. These results suggest 1.5 1 r p 6.8 h 0 1.6 that X-ray–luminous AGNs are spatially clustered in a manner similar to that of normal galaxies and optically selected AGNs. Additional details concerning the ROSAT NEP survey can be found at http://www.ifa.hawaii.edu/∼mullis/nep.html.

Groups of Galaxies in the ROSAT North Ecliptic Pole Survey

The X-ray properties of groups of galaxies are presented. Their distribution of luminosity and temperature appears to be associated with the extrapolation of these distributions from rich clusters of galaxies. The properties of the ensemble of groups of galaxies are almost totally unknown. Only a few X-ray observations of groups that were selected by optical methods were published so far. A sample of eight groups with 'z' inferior to 0.04, of which three have 'z' inferior to 0.03 was investigated. The temperature and the luminosity functions at one point were determined.