J/ApJ/770/98 Chandra study of NGC 6791 (van den Berg+, 2013)
A Chandra X-ray study of the interacting binaries in the old open cluster
NGC 6791.
van den Berg M., Verbunt F., Tagliaferri G., Belloni T., Bedin L.R.,
Platais I.
<Astrophys. J., 770, 98 (2013)>
=2013ApJ...770...98V 2013ApJ...770...98V
ADC_Keywords: Clusters, open ; Binaries, X-ray ; Cross identifications ;
Photometry, UBVRI ; Stars, variable
Keywords: binaries: close; novae, cataclysmic variables;
open clusters and associations: individual: NGC 6791;
stars: activity; X-rays: binaries
Abstract:
We present the first X-ray study of NGC 6791, one of the oldest open
clusters known (8Gyr). Our Chandra observation is aimed at uncovering
the population of close interacting binaries down to
LX∼1x1030erg/s (0.3-7keV). We detect 86 sources within 8' of the
cluster center, including 59 inside the half-mass radius. We identify
20 sources with proper-motion cluster members, which are a mix of
cataclysmic variables (CVs), active binaries (ABs), and binaries
containing sub-subgiants. With follow-up optical spectroscopy, we
confirm the nature of one CV. We discover one new, X-ray variable
candidate CV with Balmer and He II emission lines in its optical
spectrum; this is the first X-ray-selected CV in an open cluster. The
number of CVs per unit mass is consistent with the field, suggesting
that the 3-4 CVs observed in NGC 6791 are primordial. We compare the
X-ray properties of NGC 6791 with those of a few old open (NGC 6819,
M67) and globular clusters (47 Tuc, NGC 6397). It is puzzling that the
number of ABs brighter than 1x1030erg/s normalized by cluster mass
is lower in NGC 6791 than in M 67 by a factor ∼3-7. CVs, ABs, and
sub-subgiants brighter than 1x1030erg/s are under-represented per
unit mass in the globular clusters compared to the oldest open
clusters, and this accounts for the lower total X-ray luminosity per
unit mass of the former. This indicates that the net effect of
dynamical encounters may be the destruction of even some of the
hardest (i.e., X-ray-emitting) binaries.
Description:
We observed NGC 6791 with the Advanced CCD Imaging Spectrometer (ACIS)
on Chandra from 2004 July 1 20:51 UTC until July 2 10:49 UTC for a
total exposure time of 48.2ks (ObsID 4510).
We obtained low-resolution spectra of candidate optical counterparts
to guide the classification of the X-ray sources. A total of 16
candidate counterparts brighter than V∼18.3 were observed with the
FAST long-slit spectrograph on the 1.5m Tillinghast telescope on
Mt. Hopkins on nine nights between 2005 June 7 to September 2
(coverage from 3480 to 7400Å and a 3Å resolution).
Candidate optical counterparts fainter than V∼17 were observed with
the fiber-fed multi-object spectrograph Hectospec on the 6.5m
Multi-Mirror Telescope. A total of 16 candidate counterparts were
observed on the nights of 2005 May 13 and July 4-6 (spectra that cover
3700 to 9150Å with a resolution of 6Å).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 85 86 Chandra sources within 8' of the center of NGC 6791
table2.dat 168 60 Optical properties of candidate counterparts
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See also:
B/chandra : The Chandra Archive Log (CXC, 1999-2014)
B/cb : Cataclysmic Binaries, LMXBs, and related objects (Ritter+, 2014)
B/hst : HST Archived Exposures Catalog (STScI, 2007)
IX/15 : Einstein EMSS Survey (Gioia+ 1990, Stocke+ 1991)
J/AJ/148/61 : WIYN open cluster. LIX. NGC 6791 RVs (Tofflemire+, 2014)
J/AJ/147/4 : HST photometry of 5 globular clusters (Ross+, 2014)
J/A+A/548/A97 : Catalogue of variable stars in open clusters (Zejda+, 2012)
J/A+A/543/A106 : UBVRI photometry in NGC6791 (Brogaard+, 2012)
J/ApJ/758/110 : CN and CH indexes in stars of NGC 6791 (Carrera, 2012)
J/A+A/525/A2 : Eclipsing binaries in NGC6791 (Brogaard+, 2011)
J/ApJ/729/L10 : KIC stars properties in NGC 6791 and NGC 6819 (Basu+, 2011)
J/A+A/493/339 : XMM-Newton serendipitous Survey. V. (Watson+, 2009)
J/A+A/484/609 : BVI photometry and proper motions in M67 (Yadav+, 2008)
J/AJ/136/2107 : Distances of cataclysmic variables (Thorstensen+, 2008)
J/A+A/471/515 : V light curves of NGC 6791 variable stars (De Marchi+, 2007)
J/AJ/133/1585 : Stromgren photometry in NGC 6791 (Anthony-Twarog+, 2007)
J/AJ/129/2856 : Variable stars in NGC 6791 (Mochejska+, 2005)
J/AJ/129/656 : Infrared photometry of NGC 6791 (Carney+, 2005)
J/A+A/418/509 : Chandra observation of M67 (Van Den Berg+, 2004)
J/PASP/115/413 : BVI photometry in the open cluster NGC 6791 (Stetson+, 2003)
J/AJ/126/3017 : Distances of Cataclysmic variables (Thorstensen, 2003)
J/AJ/123/3460 : Low-amplitude variables in NGC 6791 (Mochejska+, 2002)
J/A+A/333/231 : O-M stars model atmospheres (Bessell+ 1998)
J/ApJ/469/355 : Teff, B-V and BC relation (Flower, 1996)
J/A+AS/114/1 : CCD UBVI photometry of NGC 6791 (Kaluzny+, 1995)
J/AJ/108/585 : UBVI photometry of NGC 6791 (Montgomery+, 1994)
J/MNRAS/243/492 : BV Photometry of NGC 6791 (Kaluzny, 1990)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- Loc Inside/Outside source location (1)
9- 10 I2 --- Seq [1/86] Source number (2)
11 A1 --- f_Seq [*] Limit of detection (3)
13- 27 A15 --- CXOU Source name (HHMMSS.s+DDMMSS; J2000)
29- 38 F10.6 deg RAdeg Right Ascension; decimal degrees (J2000) (4)
40- 49 F10.6 deg DEdeg Declination; decimal degrees (J2000) (4)
51- 54 F4.2 arcsec r95 [0.3/4] The 95% positional uncertainty radius
56- 59 F4.2 arcmin Offset [0.3/8] Angular offset from cluster center
61- 63 I3 ct Cts [3/213] Net 0.3-7 keV band counts
65- 66 I2 ct e_Cts Uncertainty in Cts
67 A1 --- f_Cts [bc] Unreliable flux (5)
69- 73 F5.1 10-19W/m2 FX [4/318] Unabsorbed 0.3-7keV flux (6)
75- 78 F4.2 keV E50 [0.8/3.3]? Median 0.3-7keV band energy (7)
80- 83 F4.2 keV e_E50 ? Uncertainty in E50
85 A1 --- Opt Optical candidate counterpart flag
(+: counterpart detected)
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Note (1): Location is:
Inside = Source inside the half-mass radius;
Outside = Source outside half-mass radius but less than 8' away from the
cluster center.
Note (2): Ordered by brightest for sources inside and outside the half-mass
radius separately.
Note (3): * = source not detected by wavdetect for a threshold of 10-7.
Note (4): After applying the boresight correction of
ΔRA = -0.06"±0.06",
ΔDE = -0.21"±0.04" (Chandra minus optical).
Note (5): Flag as follows:
b = A source is detected but its proximity to the CCD edge could make the
estimates for net counts and flux unreliable.
c = CX 67 and CX 71 are separated by 2.4". Counts were extracted from a
region enclosing 39% of the PSF at 1.5 keV (compared to a PSF fraction
of 90% for the other sources) to minimize contamination.
Note (6): In units of 1e-16 erg/s/cm2. In the 0.3-7 keV band for the
assumption that the source spectrum is a 2-keV Mekal plasma seen
through a neutral-hydrogen column density 7.8e20 cm-2.
Note (7): Only for sources with >5 counts.
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 A2 --- Mm Group membership (1)
4- 5 I2 --- Seq [1/86] Source number (as in table 1)
7 A1 --- f_Seq a: this source has another candidate counterpart
(see table 3) (2)
9- 13 I5 --- OID [553/15561]? Number of candidate optical
counterpart in the S03 catalog (3)
15- 18 F4.2 arcsec dOX [0/3] Angular separation between the X-ray
and optical source after correcting for boresight
20- 21 I2 --- pmu [0/99]? Probability for cluster membership (4)
23- 24 A2 --- p Membership probability from HST/ACS (5)
26- 30 F5.2 mag Vmag [11.5/28.5]? V-band magnitude (6)
32 A1 --- f_Vmag [:b] Uncertain or previoulsy unknown (7)
34- 38 F5.2 mag B-V ? B-V color index (6)
39 A1 --- u_B-V B-V uncertainty flag
41- 44 F4.2 mag V-I ? V-I color index (6)
45 A1 --- u_V-I Uncertainty flag on V-I
47- 50 F4.1 [10-7W] LX [1/53]? Unabsorbed X-ray (0.3-7keV) luminosity (8)
52 A1 --- l_FX/FV Limit flag on FX/FV
54- 58 F5.2 --- FX/FV [-5.1/2.1] Ratio of X-ray (0.3-7keV) to V band
fluxes after correcting for extinction
59 A1 --- u_FX/FV Uncertainty flag on FX/FV
61- 64 F4.2 --- e_FX/FV [0.03/0.3]? FX/FV uncertainty
66- 73 A8 --- Var Name of the associated optical variable
75- 80 F6.3 d Per [0.2/50.5]? Period of variability
82 A1 --- n_Per [s] another period value is 23.947
84-116 A33 --- Opt Spectroscopic properties of the candidate optical
counterpart
118-120 A3 --- Class Source class (9)
122-168 A47 --- Comm Comments (10)
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Note (1): Code as follows:
pm = Proper-motion cluster member
? = Uncertain or no membership information
n? = Likely not associated with the cluster
Note (2): Table 3 (alternative optical counterparts):
-----------------------------------------------------------------
Seq OID dOX Vmag B-V V-I Comment
(arcsec)
-----------------------------------------------------------------
21 11098 0.78 18.48 0.93 0.97 pmu = 90
27 11365 0.68 16.58 0.76 0.94 pmu = 0, F star
34 0.68 Extended
57 7395 0.76 21.02 1.47 1.96 Non-member, K star
78 569 3.12 16.50 0.95 1.03 pmu = 0
82 13395 0.70 14.66 0.96 0.98
84 15442 3.01 20.64 1.7
-----------------------------------------------------------------
Note (3): S03 = Stetson et al. 2003, J/PASP/115/413;
<Cl* NGC 6791 SBG NNNNN> in Simbad. Sources without a number were
found in the HST/ACS images or (for CX 55 only) by visual inspection
of the S03 cluster image.
Note (4): Probability based on the proper-motion studies of Platais et al.
(2011ApJ...733L...1P 2011ApJ...733L...1P) (the case of CX 3 is described in more detail
in Section 4.2) and K. Cudworth et al. (private communication).
Note (5): Probability for cluster membership based on the HST/ACS study
described in Bedin et al. (2006A&A...460L..27B 2006A&A...460L..27B);
m=member or nm=non member.
Note (6): Optical magnitudes and colors from Stetson et al. 2003,
J/PASP/115/413, except for the (unnumbered) HST counterparts where
we list the F606W magnitude and the F606W-F814W color if available.
Note (7): Flag on Vmag as follows:
: = Uncertainty flag
b = The counterpart is not included in the S03 (Stetson et al. 2003,
J/PASP/115/413) catalog. The listed value is the magnitude in the
g' band (Platais et al. 2011ApJ...733L...1P 2011ApJ...733L...1P).
Note (8): Unabsorbed X-ray luminosity (0.3-7keV) in units of 1030erg/s
assuming a distance of 4.1kpc and the fluxes in Table 1.
Note (9): Source class as follows:
CV = cataclysmic variable
CV? = candidate cataclysmic variable
AB = likely active binary,
AB? = candidate active binary,
SSG = sub-subgiant,
RG = red giant,
S = likely foreground star not associated with the cluster without any
indication of binarity,
EG = extra-galactic source,
FB = Faint object to the Blue of the main sequence.
Note (10): Information on optical variability was gathered from the
following references:
* Kaluzny & Rucinski (1993MNRAS.265...34K 1993MNRAS.265...34K),
* Rucinski et al. (1996MNRAS.282..705R 1996MNRAS.282..705R),
* Mochejska et al. 2002 (J/AJ/123/3460); 2003AJ....125.3175M 2003AJ....125.3175M;
2005 (J/AJ/129/2856),
* Bruntt et al. (2003A&A...410..323B 2003A&A...410..323B),
* Hartman et al. (2005AJ....130.2241H 2005AJ....130.2241H),
* de Marchi et al. 2007 (J/A+A/471/515)
* Brogaard et al. (2011, J/A+A/525/A2).
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History:
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(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 12-Jan-2015