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. =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 ~10^38^erg/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: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- 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 {Gamma}, 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 ?=- {chi}^2^ 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 {Gamma}, 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 ?=- {chi}^2^ 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) -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- Byte-by-byte Description of file: table3.dat -------------------------------------------------------------------------------- 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- 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 kT_in_ 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 ?=- {chi}^2^ 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 -------------------------------------------------------------------------------- Byte-by-byte Description of file: table4.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- 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. -------------------------------------------------------------------------------- Byte-by-byte Description of file: table5.dat -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- 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 2{sigma} level, i.e. |C2-1|/{sigma}_C2_>2. -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Patricia Vannier [CDS] 30-Apr-2015