J/ApJS/177/465 X-ray variability in NGC 6946, NGC 4485/90 (Fridriksson+, 2008)
The long-term variability of the X-ray sources in NGC 6946 and NGC 4485/4490.
Fridriksson J.K., Homan J., Lewin W.H.G., Kong A.K.H., Pooley D.
<Astrophys. J. Suppl. Ser., 177, 465-492 (2008)>
=2008ApJS..177..465F 2008ApJS..177..465F
ADC_Keywords: X-ray sources ; Binaries, X-ray ; Galaxies, nearby
Keywords: galaxies: individual (NGC 6946, NGC 4485, NGC 4490) - X-rays: binaries
X-rays: galaxies
Abstract:
We analyze data from five Chandra observations of the spiral galaxy
NGC 6946 and from three Chandra observations of the irregular/spiral
interacting galaxy pair NGC 4485/4490, with an emphasis on
investigating the long-term variability exhibited by the source
populations. We detect 90 point sources coincident with NGC 6946 down
to luminosities of a few times 1036ergs/s, and 38 sources coincident
with NGC 4485/90 down to a luminosity of ∼1x1037ergs/s. Twenty-five
(15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks
to years) variability in luminosity; 11 (4) are transient candidates.
The single ultraluminous X-ray source (ULX) in NGC 6946 and all but
one of the eight ULXs in NGC 4485/90 exhibit long-term flux
variability. Two of the ULXs in NGC 4485/90 have not been identified
before as ultraluminous sources. The widespread variability in both
systems is indicative of the populations being dominated by X-ray
binaries, and this is supported by the X-ray colors of the sources.
The distribution of colors among the sources indicates a large
fraction of high-mass X-ray binaries in both systems. The shapes of
the X-ray luminosity functions of the galaxies do not change
significantly between observations and can be described by power laws
with cumulative slopes ∼0.6-0.7 (NGC 6946) and ∼0.4 (NGC 4485/90).
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 107 450 Basic Chandra source properties for NGC 6946
table6.dat 103 114 Basic Chandra source properties for NGC 4485/4490
table9.dat 84 90 Chandra variability properties for NGC 6946 sources
table10.dat 70 38 Chandra variability properties for NGC 4485/4490
sources
spec.dat 68 450 Chandra spectral properties for NGC 6946 sources
(table 11 and 12 of the paper)
table13.dat 68 114 Chandra spectral properties for NGC 4485/4490 sources
--------------------------------------------------------------------------------
See also:
B/chandra : The Chandra Archive Log (CXC, 1999-)
Byte-by-byte Description of file: table[26].dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- [FHL2008] Number identification
4- 18 A15 --- CXOU Source designation (HHMMSS.s+DDMMSS)
20- 28 F9.5 deg RAdeg Right ascension in decimal degrees
(J2000)
30- 37 F8.5 deg DEdeg Declination in decimal degrees (J2000)
39- 43 F5.1 --- SN Signal-to-noise ratio (1)
45- 48 I4 --- ObsID Observation ID number
50 A1 --- l_Flux Limit flag on Flux (2)
51- 56 F6.1 ph/ks/m2 Flux ?=- Photon flux in the 0.3-7keV band (2)
58- 61 F4.1 ph/ks/m2 E_Flux ? Positive error of Flux (2)
63- 66 F4.1 ph/ks/m2 e_Flux ? Negative error of Flux (2)
68- 73 F6.1 ph/ks/m2 Fco Photon flux in the co-added exposure (2)
75- 78 F4.1 ph/ks/m2 e_Fco Error on Fco (2)
80- 84 F5.1 10+30W XLum Average X-ray luminosity in the
0.3-7keV band (3)
86- 90 F5.1 10+30W XLumax Maximum of X-ray luminosity in the
0.3-7keV band (4)
92-107 A16 --- Comm Comments (5)
--------------------------------------------------------------------------------
Note (1): Photometric signal-to-noise ratio based on all observations, as
computed by ACIS Extract.
Note (2): In 10-7ph/s/cm2.
For sources with fewer than five net counts in an observation
an upper limit with 3σ Gaussian confidence is given.
Details of obs. ID:
-----------------------------------------------------
Start Date Instrument Obs. ID Exp. (ks) Data Mode
-----------------------------------------------------
2001/09/07 ACIS-S 1043 58.29 Faint
2002/11/25 ACIS-S 4404 29.92 Faint
2004/10/22 ACIS-S 4631 29.70 Faint
2004/11/06 ACIS-S 4632 27.94 Faint
2004/12/03 ACIS-S 4633 26.62 Faint
2000/11/03 ACIS-S 1579 19.52 Faint
2004/07/27 ACIS-S 4725 38.41 Very Faint
2004/11/20 ACIS-S 4726 39.59 Very Faint
----------------------------------------------------
Note (3): Luminosity calculated from a co-added exposure spanning all the
observations.
Note (4): Luminosity calculated from the observation in which the source is
brightest.
Note (5): Symbols used in comments as follows:
T = transient candidate;
N1 = source not detected by Holt et al. (2003ApJ...588..792H 2003ApJ...588..792H);
N2 = source not detected by Roberts et al. (2002MNRAS.337..677R 2002MNRAS.337..677R);
SSS = supersoft source candidate;
S = source is most likely a foreground star;
LD = source coincides with a supernova remnant in the list of Lacey & Duric
(2001ApJ...560..719L 2001ApJ...560..719L);
MF = source coincides with a supernova remnant in the list of Matonick &
Fesen (1997ApJS..112...49M 1997ApJS..112...49M).
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table9.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- [FHL2008] Number identification
4- 18 A15 --- CXOU Source designation (HHMMSS.s+DDMMSS)
20- 26 E7.2 --- K-S1043 ?=- K-S test for short-term flux variability
for obs. ID 1043 (G1)
28- 33 E6.2 --- K-S4404 ?=- K-S test for short-term flux variability
for obs. ID 4404 (G1)
35- 40 E6.2 --- K-S4631 ?=- K-S test for short-term flux variability
for obs. ID 4631 (G1)
43- 48 E6.2 --- K-S4632 ?=- K-S test for short-term flux variability
for obs. ID 4632 (G1)
50- 53 F4.2 --- K-S4633 ?=- K-S test for short-term flux variability
for obs. ID 4633 (G1)
55- 58 F4.1 --- Svar Significance parameter for long-term flux
variability (G2)
60 A1 --- l_R Limit flag on R
61- 65 F5.1 --- R Long-term flux variability (G3)
67- 69 F3.1 --- SHR1 Significance parameter for the variability in
the Hardness Ratio 1 (G4)
71- 73 F3.1 --- SHR2 Significance parameter for the variability in
the Hardness Ratio 2 (G5)
75- 84 A10 --- Comm Comments (G6)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table10.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- [FHL2008] Number identification
4- 18 A15 --- CXOU Source designation (HHMMSS.s+DDMMSS)
20- 25 E6.2 --- K-S1579 ?=- K-S test for short-term flux variability
for obs. ID 1579 (G1)
27- 33 E7.2 --- K-S4725 ?=- K-S test for short-term flux variability
for obs. ID 4725 (G1)
35- 41 E7.2 --- K-S4726 K-S test for short-term flux variability for
obs. ID 4726 (G1)
43- 46 F4.1 --- Svar Significance parameter for long-term flux
variability (G2)
48 A1 --- l_R Limit flag on R
49- 53 F5.1 --- R Long-term flux variability (G3)
55- 57 F3.1 --- SHR1 Significance parameter for the variability in
the Hardness Ratio 1 (G4)
59- 61 F3.1 --- SHR2 Significance parameter for the variability in
the Hardness Ratio (G5)
63- 70 A8 --- Comm Comments (G6)
--------------------------------------------------------------------------------
Byte-by-byte Description of files: spec.dat table13.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- [FHL2008] Number identification
4- 18 A15 --- CXOU Source designation (HHMMSS.s+DDMMSS)
20- 23 I4 --- ObsID Observation ID number
25- 29 F5.2 --- HR1 ?=- HR1 (1)
31- 34 F4.2 --- e_HR1 ? Error on HR1
36- 40 F5.2 --- HR2 ?=- HR2 (2)
42- 46 F5.2 --- e_HR2 ? Error on HR2
48- 52 F5.2 --- HR1co ?=- HR1 for combined observations (3)
54- 57 F4.2 --- e_HR1co ? Error on HR1co
59- 63 F5.2 --- HR2co ?=- HR2 for combined observations
65- 68 F4.2 --- e_HR2co ? Error on HR2co
--------------------------------------------------------------------------------
Note (1): Soft color, HR1=(M-S)/T where M, S, and T are the net counts in
the 1-2keV (medium), 0.3-1keV (soft), and 0.3-7keV (total) bands,
respectively.
Note (2): Hard color, HR2=(H-M)/T where H is the net counts in the 2-7keV
Note (3): Hardness ratios calculated from the co-added exposure.
--------------------------------------------------------------------------------
Global Notes:
--------------------------------------------------------------------------------
Note (G1): Significance level of a one-sided Kolmogorov-Smirnov test
statistic with respect to a uniform count-rate model. We define a
source as showing evidence for variability if the significance of the
computed K-S statistic is less than 3x10-3.
Note (G2): Svar=maxi,j|Fi-Fj|/(σ2Fi+σ2Fj)0.5 where Fi
denotes the photon flux of a source in the ith Chandra observation. We
define a source as variable if Svar>3.
Note (G3): R=Fmax/Fmin. For sources with no detected flux in at least one
of the observations a lower limit with 3σ Gaussian confidence is
given.
Note (G4): SHR1=maxi,j|HR1i-HR1j|/(σ2HR1,i+σ2HR1,j)0.5
Note (G5): SHR2=maxi,j|HR2i-HR2j|/(σ2HR2,i+σ2HR2,j)0.5
Note (G6): Flags in "comments" as follows:
s = Short-term flux variability
l = long-term flux variability
sp = spectral variability
star = Foreground stars
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Emmanuelle Perret [CDS] 01-Sep-2009