NEXXUS 2 - The database for Nearby X-ray and extreme UV emitting Stars

How to search in NEXXUS 2 and interpret the available data

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Star data
Einstein data
EUVE data
ROSAT data
XMM-Newton data

Search in NEXXUS

There are several possibilities to search in NEXXUS 2, either by Coordinates, Color Index and/or Distance. Most common will be direct search in the star catalogs.

You can query several star catalogs. NEXXUS 2 is based on the NStars database, containing stars within a distance of 25 pc to the Sun. NStars designations are based on the J2000.0 positions, following the scheme NS HHMM+DDMM with optional component identifiers.
Many of the stars included in the NStars database are listed in previous nearby-star or high-proper motion catalogs, like e.g. the Catalog of Nearby Stars. The CNS underwent several changes and additions over the decades (W. Gliese, [First] Catalogue of Nearby Stars, 1957; [Second] Catalogue of Nearby Stars, Edition 1969; W. Gliese and H. Jahreiss, Supplement to the CNS2, 1979; H. Jahreiss, 3rd version from 1991).
The Luyten Half Second Catalog (LHS) in its second edition (W.J. Luyten, 1979) lists stars with annual proper motions exceeding 0.5". Stars with a proper motion larger than 2 "/yr were numbered 1 to 100, between 1 and 2 "/yr 101 to 1000, numbers 1001 through 5000 are assigned to those stars with proper motion between 0.5 and 1 "/yr. The stars with motions between 0.480 and 0.499 "/yr have been assigned numbers larger than 5000. Numbers larger than 6000 refer to stars for which at one time or another a value larger than 0.49 "/yr was published.
The so-called Giclas numbers indicate stars included in the Lowell Proper Motion Survey (H.L. Giclas, R. Burnham Jr., N.G. Thomas, Northern Hemisphere 1971, Southern Hemisphere 1978).
Additional to these catalogs, lists with Henry Draper (HD), Hipparcos (HIP), HR, and Durchmusterung (BD, CD, CPD) numbers are provided. The catalog lists contain the whole sample of screened stars (where numbers in the corresponding catalogs available); links to a star page were provided for those stars identified as X-ray or EUV sources. Thus, stars that are not linked were checked, but refused or not detected.

For a search by coordinates just enter coordinates in the given form to check if there is a star identified with a NEXXUS 2 source at the selected position. Search by coordinates will provide a list of stars with its designations and coordinates (or a message if no star is found). the link "more" will guide you to the star page with more information about the star and the identified x-ray and/or extreme UV source(s).

Searches by proper motion, color index or distance produce a list of detected stars with additional star information containing proper motion, color index and/or distance within the chosen values. Note that the input of the full interval for the color indices will not provide you with the full sample of detected stars because the color indices are not available for any star. Searches by coordinates, proper motion, color indices and distance can also be combined.

Interpreting the Data from the Star Pages

The star pages are divided into sections containing information about the star and the assigned X-ray and extreme UV sources. Observations include Einstein detections (with the Imaging Proportional Counter IPC and the High Resolution Imager HRI), EUVE detections (from the All-Sky survey, the Deep Survey of half the ecliptic plane, sources detected in other ways, joint EUVE-ROSAT Detections, and the Right Angle Program RAP), the ROSAT All-Sky Survey (RASS in X-rays, and with the Wide Field Camera WFC in the EUV), ROSAT pointed observations (with the Position Sensitive Proportional Counter with (PSPCF) and without boron filter (PSPC), with the High Resolution Imager HRI and with the WFC), as well as X-ray sources included in the pre-release of the 2XMM Serendipitous EPIC Source Catalog and the XMM Slew Survey EPIC Source Catalog.

The ROSAT X-Ray All-Sky Survey Image Browser provides a smoothed false-color X-ray image from the ROSAT All-Sky Survey of the region close to the star's position. The link to SIMBAD leads either directly to the individual page of the star in SIMBAD or to a list of objects in 10' around the star's coordinates in SIMBAD. Additionally, the corresponding entry for the star in the NStars database is linked.

Star Information
Right ascension and declination of the stars are calculated for Equinox and Epoch J2000.0. Note that the coordinates and especially the proper motion values are sometimes not very precise.
Proper motion is provided as μ and θ, i.e. the total proper motion in "/yr and its angle relative to the north direction. Radial velocity (taken from SIMBAD) is given in km/sec. When no information about these values is available, they are not listed.
Where available, the spectral type and a list of magnitudes (U, B, V, Rc, Ic, J, Ks, H) is given. While B and V magnitudes are given for most stars, the other magnitudes, especially U, are not that common.
For each star, the distance in pc resulting from either trigonometric or photometric parallaxes as included in NStars (partly Hipparcos data) is listed.
Addidtional information about some of the stars may be found in the online version of the CNS (ARICNS).

Einstein data

Einstein Imaging Proportional Counter
NEXXUS 2 includes the 2E Catalog of Einstein IPC X-ray sources (Harris et al. 1994) as it is incorporated in catalog IX/13 at the CDS, additional information on the observed fields is taken from catalog IX/25. The source name of Einstein IPC sources can either be of the form 2E consecutive_number, which is not listed in NEXXUS, or based on 1950 positions (format 2E HHMM.M+DDMM). J2000.0 coordinates of the source are additionally given.
A sequence ID lists the sequence number of the observation and a unique source ID. A preview image and links to corresponding FITS files at the Einstein archive at HEASARC are also provided.
The source count rate and its error given in counts/sec, off-axis angle (in arcminutes) and the signal-to-noise ratio for the source are also listed. Finally, the observation date and exposure time are given.

Einstein HRI
Columns for sources observed with the Einstein High Resolution Imager (operating from approximately 0.15-3.0 keV) are analogous to the Einstein IPC, however, no coordinate-based source identifiers have been assigned. Catalog information has been taken from CDS catalog entry IX/24 (Giacconi et al. 1979).

EUVE data

EUVE All-Sky Survey
The EUVE Source Catalog (Bowyer et al. 1996) consists three source lists, the first being an all-sky survey with 514 detected sources. Observations have been performed in the 4 bands in the wavelength range from 100-600 Å (corresponding to an energy range of 20-120 eV): The 100 Å band with Lexan/Boron filter ranges from 58-174 Å, with the peak at 89 Å. The 200 Å Al/Ti/C band ranges from 156-234 Å with 171 Å peak, the 400 Å Ti/Sb/Ti/Al band from 368-590 Aring (405 Å peak), and the 600 Å Sn/SiO band from 519-742 Å and the peak at 555 Å. NEXXUS 2 lists the count rates observed in these four bands, together with coordinates and a catalog designation based on J2000 position (format EUVE JHHMM+DDM)

EUVE Deep Survey of half the ecliptic plane
A few of the 35 sources detected by the EUVE deep survey of half the ecliptic plane are NEXXUS 2 sources. The deep survey has been performed only in the 100 Å and 200 Å bands, and with slightly modified filter configurations, resulting in different band passes compared to the all-sky survey: The 100 Å Lexan/Boron band now ranges from 67-178 Å with the peak at 91 Å, the 200 Å Al/C band ranges from 157-364 Å with the peak at 171 Å

EUVE Sources detected in other ways
The third list contains 188 sources that have been detected in other ways, basically by deep exposures with the scanner telescopes as part of the Right Angle Program through 24 December 1994. In principle, detectiona in all four filter bands from the all-sky survey plus the two deep survey bands are possible, but no detections of NEXXUS sources were possible in the two long-wavelength bands of the all-sky survey, and and combined detections the the all-sky survey and deep survey bands are not possible.

Joint EUVE-ROSAT Detections
The data from 58-174 Å band included in the EUVE All-Sky Survey and early Right Angle Program have been reprocessed and crosscorrelated with ROSAT WFC sources (Lampton et al. 1997). The reprocessed count rates are given here.

EUVE Right Angle Program
The second part of the EUVE Right Angle Program lists the detections made from the observations since 1994 January and covers 17% of the sky (Christian et al. 1999). The coordinate-based source designation is slightly changed, now with a leading digit (2 meaning normal scanning data, 3 meaning long wavelength scanner data, and 4 meaning deep survey data). Count rates are available for the 100 Å and 200 Å all-sky survey bands, together with the corresponding exposure times.

The EUVE archive provides further information about EUVE and data retrieval.

ROSAT data

ROSAT All-Sky Survey
The ROSAT PSPC performed an All-Sky survey during the first half year (1990/91) of the ROSAT mission. X-ray sources in the ROSAT All-Sky Survey (RASS) are derived from ROSAT All-Sky Survey Bright Source Catalogue (Voges et al. 1999) or ROSAT All-Sky Survey Faint Source Catalogue (Voges et al. 2000). RASS sources are unique, a star can only have one assigned RASS source. RASS detection numbers may also be entered in Simbad. The name includes the '1RXS' acronym, and is based on the J2000.0 position (format JHHMMSS.S+DDMMSS) of the source.
The source count rate and its error are vignetting corrected and given in counts/sec. The vignetting corrected background count rate is given in counts/sec/arcmin2.
RASS exposure times vary with the sky position.
The likelihood of the source detection algorithm is set to 9999 where >9999.9
Two hardness ratios characterize the spectral distribution in the power spectrum of the star. They are defined as: HR1=(B-A)/(B+A), where A is the count rate in the PHA range 11-41 (0.1-0.4 keV) and B is the count rate in the PHA range 52-201 (0.5-2.0 keV); HR2=(D-C)/(D+C), where C is the count rate in the PHA range 52-90 (0.5-0.9 keV) and D is the count rate in the PHA range 91 - 201 (0.9-2.0 keV). PHA is the PSPC Pulse Height Amplitude. The true error of the hardness ratios is probably larger than the formal values given. Values for the hardness ratios and errors are sometimes not available.
The X-ray luminosity (in ergs/sec) is derived from the stars' distance and the source count rate, a conversion factor of 6 x 10-12 ergs/cm2/counts was used.

ROSAT Position Sensitive Proportional Counter
The ROSAT PSPC data were taken from the Second ROSAT Source Catalog of Pointed Observations (ROSAT Consortium, ROSAT News No. 72, 25-May-2000). The PSPC data available is similar to RASS but with further information.
PSPC detection numbers are similar to RASS detection numbers but they are not unique. Detections with the same position (e.g. from different observations) will have the same detection name.
Individual sources in specific observations can be identified by a Sequence ID, which contains a six-digit sequence number, and a source number. The first digit of the sequence number indicates the target of the observation, see ROSAT Observation categories for the meaning of the digits. Most of the sequences in NEXXUS 2 begin with 2, thus the target was a normal star or white dwarf, indicating the identified star was probably also target of the observation. When other digits appear, the star was just coincidently in the field of view of the observation.
The off-axis angle OAX (in arcminutes) is a sign for quality of coordinates and point spread function.
The observation dates for a sequence number were presented as year-month-day numbers.
Pointed observations also contain links to false-color images from the X-ray data similar to All-Sky Survey Image Browser. In these images, the sources are numbered according to the last 3 digits of the source numbers from the sequence ID.

ROSAT Position Sensitive Proportional Counter with Filter
The data from Pointed Observations with the PSPC with boron filter were taken from the Second ROSAT Source Catalog of Pointed Observations with the Position Sensitive Proportional Counter with Filter. (The ROSAT Consortium, ROSAT NEWS No. 74)
PSPCF data are similar to PSPC data, but the conversion factor for the X-ray luminosity is 3 x 10-12 ergs/cm2/counts.

ROSAT High Resolution Imager
The ROSAT HRI data were obtained from the ROSAT Source Catalog of Pointed Observations with the High Resolution Imager, 3rd Release. (The ROSAT Consortium, ROSAT NEWS No. 71).
HRI data differ only little from RASS and PSPC data: Instead of the likelihood a signal-to-noise ratio (SNR) is given. The conversion factor for the X-ray luminosity is 2.4 x 10-12 ergs/cm2/counts.

ROSAT Wide Field Camera Survey
The ROSAT Wide Field Camera (WFC) performed an All-sky survey at EUV wavelengths, simultaneously to the X-ray All-Sky Survey. The survey was conducted in two filter bands: S1 covers 90-206 eV (60-140 Å) with a mean of 125 eV (100 Å), and S2 between 62-110 eV (110-200 Å) with a mean of 90 eV (140 Å)
ROSAT WFC Survey sources have detection numbers similar to the X-ray sources, but again with a less precise format (JHHMM+DDM) for position. Thus, right ascension and declination for the source were additionally given. WFC Survey sources are unique like RASS sources.
The count rate is splitted into S1 and S2 and given in counts/sec. Instead of likelihood or signal-to-noise ratio, a significance is given for both filter bands and in total (as square root(significance of S12 + significance of S22)).

ROSAT Wide Field Camera Pointed Observations
Pointed Observations with the WFC have been obtained simultaneously to the X-ray observations, they have corresponding sequence numbers and Sequence IDs. Sources from WFC pointed observations have no coordinate-based detection number, only an integer-like source number. J2000 coordinates for the sources are therefore given too.
Pointed observations were taken in one filter band only, so only one count rate and the used filter are given.

For more information about ROSAT see also ROSAT Home Page at MPE.

XMM-Newton data

XMM-Newton slew survey Source Catalog (XMMSL1)
The XMM-Newton slew survey Source Catalog has been generated from 219 slews between regular XMM observations, executed between revolutions 314 and 978. Not all slews made in this period have been used; slews with particularly high background or which gave processing problems have been rejected. The slews have been processed separately in three energy bands (total band 0.2-12 keV hard band 2-12 keV, and soft band 0.2-2 keV) to form three distinct surveys which have been subsequently combined to form the catalog. NEXXUS 2 includes the full catalog, containing 5200 detections of sources found with a likelihood of DET_ML>8. Exposure times for a singe source are typically a few seconds. There is a small overlap between the slew paths (a few percent), which has led to 100 sources being observed two or more times. The full catalog has an expected spurious fraction of about 20%, determined from simulations of blank fields. The number of sources and sky area for the two catalogues is 3882 int the total band, 694 in the hard band, and 2619 in the soft band, for a sky area of 6240 square degrees.
The detection number again encodes the J2000 sky position, but due to possible slew overlaps they are only unique in combination with an observation number, which is not given here. NEXXUS 2 lists the source count rate in the total band, exposure time, observation dates, and the detection likelihood. A hardness ratio, defined by HR1 = (hard-soft)/(hard+soft) has been calculated for the sources which have a positive detection in both bands, an X-ray luminosity has been calculated from the EPIC fluxes, which have been calculated for an absorbed power-law spectrum of slope 1.7 and NH = 3.0 x 1020 cm2, resulting in energy conversion factors between the source count rate (c/s) to flux in units of 10-12 ergs/s/cm2 of 3.159 for the total band, 9.144 for the hard band, and 1.436 for the soft band.
The documentation of the XMM-Newton slew survey Source Catalog can be found here.

2XMM Serendipitous EPIC Source Catalog (pre-release)
NEXXUS 2 includes the pre-release of the 2XMM Serendipitous EPIC Source Catalog (this replaces the First XMM-Newton Serendipitous Source Catalogue included in the previous version of NEXXUS). Entries from XMM observations in NEXXUS 2 consist of several rows for each observation. The first row provides general information about the XMM observation: A source name in the format 2XMMp JHHMMSS.S+DDMMSS, a source identification number which contains of the observation number and a source number in this observation. Then there is the observation date, time is given in UT, and a combined detection likelihood of the three cameras as well as an X-ray luminosity computed from the mean total band flux (0.2-12 keV) of the three detectors (see below).
In addition to the catalog information, links to the XMM-Newton observation logs and to the XMM postcard server, provide information about the observation itself, quick-look images and the posibility to download the whole dataset.
Detector-specific information is provided in self-contained lines for the PN, MOS1, and MOS2 camera each. The detector-specific lines contain the name of the detector, the filter and window mode for this observation, the count rate (0.2-12 keV) and its error, the source detection likelihood, the off-axis angle, and three hardness ratios, which are defined like the ROSAT hardness ratios. HR1 is computed from the bands 1 and 2, ranging from 0.2-0.5 keV and 0.5-1.0 keV respectively. HR2 is computed from the bands 2 and 3 (1.0-2.0 keV), HR3 from bands 3 and 4 (2.0-4.5 keV), and HR4 from bands 4 and 5 (4.5-12.0 keV). When comparing the values of different instrumental setups, keep in mind that the hardness ratios depend on spectral sensitivity of the detector and the blocking filter in use. Note that the definition of the XMM spectral bands changed from 1XMM to 2XMMp.
The catalog provides fluxes for each of the five spectral bands. They have been computed using different energy conversion factors (see here for details) that have been calculated assuming an absorbed powerlaw with an absorbing column density NH = 3.0 x 1020 cm2 and with a continuum slope γ = 1.7 as spectral model, which is in general not an appropriate model for stellar coronal plasmas. To verify that the fluxes accomplish to stellar sources, individual fluxes have been computed from fits with a hot thermal plasma (VAPEC) for a small sample of stars. The computed fluxes agree quite well with the fluxes given in the catalog. Thus the given total band flux has been used to compute an X-ray luminosity.
Sometimes the entries for one or two of the detectors are missing. There are several reasons for this, which do not always mean that there is no data: 1. The detector was working in Fast Uncompressed or Timing mode. 2. The quality the observation is not sufficient, mostly due to cosmic rays or saturation. In that case, the data for the detector was not included in the catalog. 3. PN only: If the PN camera was working in Small Window mode, the data is also not included, because the field of view is too small for optimate background fitting. Unfortunately, this case appears quite often for the NEXXUS 2 objects.
The documentation of the 2XMM Serendipitous EPIC Source Catalog can be found here.
For further information about XMM-Newton go to the XMM-Newton Science Operations Center at ESAC.

NEXXUS 2 provides much more information than is available through the web interface! Ask for special information or other cross-correlations. And last but not least: If you have any questions or find mistakes in the data please contact me.

Last modified on Thursday, 12 April 2007.   C. Liefke | Impressum