J/other/RAA/14.1251 Variability of bright X-ray point sources (Jithesh+, 2014)
Long term optical variability of bright X-ray point sources in elliptical
galaxies.
Jithesh V., Misra R., Shalima P., Jeena K., Ravikumar C.D., Babu B.R.S.
<Res. Astron. Astrophys., 14, 1251-1263 (2014)>
=2014RAA....14.1251J 2014RAA....14.1251J (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; X-ray sources
Keywords: (Galaxy:) globular clusters: general - galaxies: photometry -
X-rays: galaxies
Abstract:
We present long term optical variability studies of bright X-ray
sources in four nearby elliptical galaxies with the Chandra Advanced
CCD Imaging Spectrometer array (ACIS-S) and observations from the
Hubble Space Telescope (HST) Advanced Camera for Surveys. Out of the
46 bright (X-ray counts >60) sources that are in the common field of
view of the Chandra and HST observations, 34 of them have potential
optical counterparts, while the rest of them are optically dark. After
taking into account systematic errors, estimated using optical sources
in the field as a reference, we find that four of the X-ray sources
(three in NGC 1399 and one in NGC 1427) have variable optical
counterparts at a high level of significance. The X-ray luminosities
of these sources are ∼1038erg/s and are also variable on similar
time scales. The optical variability implies that the optical emission
is associated with the X-ray source itself rather than being the
integrated light from a host globular cluster. For one source, the
change in optical magnitude is >0.3, which is one of the highest
reported for this class of X-ray sources and this suggests that the
optical variability is induced by the X-ray activity. However, the
optically variable sources in NGC 1399 have been reported to have blue
colors (g-z>1). All four sources have been detected in the infrared
(IR) by Spitzer as point sources, and their ratios of 5.8 to 3.6um
flux are >0.63, indicating that their IR spectra are like those of
Active Galactic Nuclei (AGNs). While spectroscopic confirmation is
required, it is likely that all four sources are background AGNs. We
find none of the X-ray sources having optical/IR colors different from
AGNs to be optically variable.
Description:
The samples were selected based on three criteria. (1) The distance to
the host galaxy is ∼20 Mpc, (2) the galaxy has a Chandra observation
and (3) it has more than one epoch of HST observations in the same
filter. Based on these criteria, we have selected five galaxies which
are listed in Table 1.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 75 10 Sample galaxy properties
table2.dat 153 46 Spectral properties of point sources and
best-fit models for the first and second epoch
table3.dat 142 23 Combined spectral properties of point sources
fitted with best-fit model
table4.dat 126 48 Variability of optical counterparts in the sample
table5.dat 106 4 Properties of optically varying sources in
the Sample
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- --- [NGC]
5- 8 I4 --- NGC Host galaxy NGC name
10- 13 F4.1 Mpc Dist Distance to the host galaxy from NED
15- 19 I5 --- IDC ?=- Chandra observation ID
21- 31 A11 "date" ObsC Chandra Observation Date
33- 38 F6.2 ks Texp ?=- Exposure time for Chandra observation
40- 48 A9 --- IDH HST observation ID
50- 55 A6 --- Filt HST Filter used
57- 67 A11 "date" ObsH HST Observation date
69- 70 I2 --- Nxo [1/18]? Number of common sources in the field
of view of X-ray and optical images
72- 75 F4.2 10+20cm-2 nH ? Column density from galactic absorption
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 A1 --- Note [+*d] Note on source (1)
2- 4 A3 --- --- [NGC]
6- 9 I4 --- NGC Host galaxy NGC name
11- 12 I2 h RAh X-ray Right ascension (J2000)
14- 15 I2 min RAm X-ray Right ascension (J2000)
17- 21 F5.2 s RAs X-ray Right ascension (J2000)
23 A1 --- DE- X-ray Declination sign (J2000)
24- 25 I2 deg DEd X-ray Declination (J2000)
27- 28 I2 arcmin DEm X-ray Declination (J2000)
30- 34 F5.2 arcsec DEs X-ray Declination (J2000)
36- 39 F4.2 10+22cm-2 nH1 ? Equivalent hydrogen column density
for the first observation
41- 45 F5.2 10+22cm-2 E_nH1 ? Error on nH1 (upper value)
47- 51 F5.2 10+22cm-2 e_nH1 ? Error on nH1 (lower value)
53- 56 F4.2 --- Gam1 ?=- Photon power law index Γ, or inner
disk temperature (keV) for first observation
58- 61 F4.2 --- E_Gam1 ?=- Error on Gam1 (upper value)
63- 67 F5.2 --- e_Gam1 ?=- Error on Gam1 (lower value)
68 A1 --- l_logLX1 [<] Limit flag on logLX1
69- 73 F5.2 [10-7W] logLX1 Unabsorbed X-ray luminosity in the energy
range 0.3-8.0keV for the first observation
75- 79 F5.2 [10-7W] E_logLX1 ? Error on logLX1 (upper value)
81- 84 F4.2 [10-7W] e_logLX1 ? Error on logLX1 (lower value)
86- 91 F6.2 --- chi21 ?=- χ2 value in the first observation
92 A1 --- --- [/]
93- 94 I2 --- dof1 [2/84]? Degrees of freedom of the first
observation
96 A1 --- Mod1 [DP-] Best fit model for the first
observation (G2)
98-101 F4.2 10+22cm-2 nH2 ? Equivalent hydrogen column density for
the second observation
103-106 F4.2 10+22cm-2 E_nH2 ? Error on nH2 (upper value)
108-111 F4.2 10+22cm-2 e_nH2 ? Error on nH2 (lower value)
113-116 F4.2 --- Gam2 ?=- Photon power law index Γ, or inner
disk temperature (keV) for second observation
118-121 F4.2 --- E_Gam2 ?=- Error on E_Gam (upper value)
123-126 F4.2 --- e_Gam2 ?=- Error on E_Gam (lower value)
127 A1 --- l_logLX2 Limit flag on logLX2
128-132 F5.2 [10-7W] logLX2 ?=- Unabsorbed X-ray luminosity in the
energy range 0.3-8.0keV for the second
observation
134-137 F4.2 [10-7W] E_logLX2 ? Error on logLX2 (upper value)
139-142 F4.2 [10-7W] e_logLX2 ? Error on logLX2 (lower value)
144-148 F5.2 --- chi22 ?=- χ2 value for the second observation
149 A1 --- --- [/]
150-151 I2 --- dof2 [2/44]? Degrees of freedom of second
observation
153 A1 --- Mod2 [DP] Best fit model in the second
observation (G2)
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Note (1): Notes as follows:
d = source only present in the first observation
* = source only present in the second observation
+ = an additional MEKAL model added to get a better fit for this source
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- --- [NGC]
5- 8 I4 --- NGC Host galaxy NGC name
10- 11 I2 h RAh Right ascension (J2000)
13- 14 I2 min RAm Right ascension (J2000)
16- 20 F5.2 s RAs Right ascension (J2000)
22 A1 --- DE- Declination sign (J2000)
23- 24 I2 deg DEd Declination (J2000)
26- 27 I2 arcmin DEm Declination (J2000)
29- 33 F5.2 arcsec DEs Declination (J2000)
35- 38 F4.2 10+22cm-2 nH1 ? Equivalent hydrogen column density for
the first observation
40- 43 F4.2 10+22cm-2 E_nH1 ? Error on nH1 (upper value)
45- 48 F4.2 10+22cm-2 e_nH1 ? Error on nH1 (lower value)
50- 53 F4.2 --- Gamma [1.2/3.5]?=- Photon power law index
55- 58 F4.2 --- E_Gamma [0.1/2.4]?=- Error on Gamma (upper value)
60- 63 F4.2 --- e_Gamma [0.1/1.3]?=- Error on Gamma (lower value)
65- 68 F4.2 10-5 Norm1 ?=- Power law normalization
70- 74 F5.2 10-5 E_Norm1 ? Error on Norm1 (upper value)
76- 79 F4.2 10-5 e_Norm1 ? Error on Norm1 (upper value)
81- 84 F4.2 keV kTin ?=- Inner disk temperature kTin
86- 89 F4.2 keV E_kTin ?=- Error on E_kTin (upper value)
91- 94 F4.2 keV e_kTin ?=- Error on E_kTin (lower value)
96- 99 F4.2 10-5 Norm2 ?=- Disk black body normalization
101-104 F4.2 10-5 E_Norm2 ? Error on Norm2 (upper value)
106-109 F4.2 10-5 e_Norm2 ? Error on Norm2 (lower value)
111-114 F4.2 0.1 C2 ?=- Constant C2, variability indicator (G3)
116-119 F4.2 0.1 E_C2 ? Error on C2 (upper value)
121-124 F4.2 0.1 e_C2 ? Error on C2 (lower value)
126-131 F6.2 --- chi2 ?=- χ2 value
132 A1 --- --- [/]
133-135 I3 --- dof [8/129]? Degrees of freedom
137 A1 --- Var [Y/N] X-ray variable (Y-Yes, N-No)
139-142 F4.2 --- Sig X-ray variability significance
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1 A1 --- Note [*] 2 possible optical counterparts (1)
2- 4 A3 --- --- [NGC]
6- 9 I4 --- NGC Host galaxy NGC name
11- 12 I2 h RAh X-ray Right ascension (J2000)
14- 15 I2 min RAm X-ray Right ascension (J2000)
17- 21 F5.2 s RAs X-ray Right ascension (J2000)
23 A1 --- DE- X-ray Declination sign (J2000)
24- 25 I2 deg DEd X-ray Declination (J2000)
27- 28 I2 arcmin DEm X-ray Declination (J2000)
30- 34 F5.2 arcsec DEs X-ray Declination (J2000)
36 A1 --- l_logLX1 Limit flag on logLX1
37- 41 F5.2 [10-7W] logLX1 Unabsorbed X-ray luminosity in the energy
range 0.3-8.0keV for the first observation
43- 47 F5.2 [10-7W] E_logLX1 ? Error on logLX1 (upper value)
49- 52 F4.2 [10-7W] e_logLX1 ? Error on logLX1 (lower value)
54 A1 --- Mod [DP-] Best fit model (G2)
56- 59 F4.2 0.1 C2 ?=- Constant C2, variability indicator (G3)
61- 64 F4.2 0.1 E_C2 ? Error on C2 (upper value)
66- 69 F4.2 0.1 e_C2 ? Error on C2 (lower value)
71 A1 --- Var [Y/N-] X-ray variable (Y-Yes, N-No)
73- 76 F4.2 --- Sig ?=- X-ray variability significance
79 A1 --- l_m1 Limit flag on m1
80- 85 F6.3 mag m1 Aperture corrected magnitude in the
first observation
87- 91 F5.3 mag e_m1 ? rms uncertainty on m1
93 A1 --- l_m2 Limit flag on m2
94- 99 F6.3 mag m2 Aperture corrected magnitude in the
second observation
101-105 F5.3 mag e_m2 ? rms uncertainty on m2
107-112 F6.3 mag Dm ? Difference in magnitude (m1-m2)
114-118 F5.3 mag e_Dm ? rms uncertainty on Dm
120-126 F7.3 --- chi2 ?=- Significance of the optical variability
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Note (1): *: In the sample, two sources (03:38:33.09, -35:27:31.53 in NGC 1399
and 12:30:47.15, 12:24"15.91 in NGC 4486) have two possible optical
counterparts. Hence we report the magnitude of each counterpart.
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 A3 --- --- [NGC]
5- 8 I4 --- NGC Host galaxy NGC name
10- 11 I2 h RAh Right ascension (J2000)
13- 14 I2 min RAm Right ascension (J2000)
16- 20 F5.2 s RAs Right ascension (J2000)
22 A1 --- DE- Declination sign (J2000)
23- 24 I2 deg DEd Declination (J2000)
26- 27 I2 arcmin DEm Declination (J2000)
29- 33 F5.2 arcsec DEs Declination (J2000)
35- 39 F5.2 [10-7W] logLX1 Unabsorbed X-ray luminosity in the energy
range 0.3-8.0keV for the first observation
41- 44 F4.2 [10-7W] E_logLX1 ? Error on logLX1 (upper value)
46- 49 F4.2 [10-7W] e_logLX1 ? Error on logLX1 (lower value)
51- 56 F6.3 mag Dm ? Difference in magnitude (m1-m2)
58- 62 F5.3 mag e_Dm ? rms uncertainty on Dm
64- 70 F7.3 --- chi2 ?=- Significance of the optical variability
72- 76 F5.3 mag g-z g-z colour index derived from Vega magnitude
78 A1 --- l_F3.6 Limit flag on F3.6
79- 83 F5.2 mJy F3.6 Infrared Spitzer/IRAC 3.6um flux
85- 88 F4.2 mJy e_F3.6 ? rms uncertainty on F3.6
90 A1 --- l_F5.8 Limit flag on F5.8
91- 95 F5.2 mJy F5.8 Infrared Spitzer/IRAC 5.8um flux
97-100 F4.2 mJy e_F5.8 ? rms uncertainty on F5.8
102 A1 --- l_Fratio Limit flag on Fratio
103-106 F4.2 --- Fratio ? Flux ratio F5.8/F3.6
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Global notes:
Note (G2): Model is:
P = Power law
D = Disk black body (thermal accretion disk)
Note (G3): To quantify the long term variability of the X-ray sources, we
consider sources that are in the field of view of both observations.
We jointly fit the spectra using the same model parameters, except that
we introduce a constant factor which multiplies the later observation.
In other words, we keep the absorption and the spectral parameters
(i.e. either the temperature or the power-law index) the same for both
data sets, but allow for variation in the relative normalization. If
the constant is unity, then the source has not varied. We consider a
source to be X-ray variable only if the constant C2 is inconsistent
with unity at the 2σ level, i.e. |C2-1|/σC2>2.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 30-Apr-2015