J/ApJ/825/132 NuSTAR hard X-ray survey of the Galactic Center. II. (Hong+, 2016)
NuSTAR hard X-ray survey of the Galactic Center region.
II. X-ray point sources.
Hong J., Mori K., Hailey C.J., Nynka M., Zhang S., Gotthelf E.,
Fornasini F.M., Krivonos R., Bauer F., Perez K., Tomsick J.A., Bodaghee A.,
Chiu J.-L., Clavel M., Stern D., Grindlay J.E., Alexander D.M., Aramaki T.,
Baganoff F.K., Barret D., Barriere N., Boggs S.E., Canipe A.M.,
Christensen F.E., Craig W.W., Desai M.A., Forster K., Giommi P.,
Grefenstette B.W., Harrison F.A., Hong D., Hornstrup A., Kitaguchi T.,
Koglin J.E., Madsen K.K., Mao P.H., Miyasaka H., Perri M., Pivovaroff M.J.,
Puccetti S., Rana V., Westergaard N.J., Zhang W.W., Zoglauer A.
<Astrophys. J., 825, 132-132 (2016)>
=2016ApJ...825..132H 2016ApJ...825..132H (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Galactic center ; X-ray sources ; Binaries, X-ray ;
Photometry
Keywords: Galaxy: center - X-rays: binaries - X-rays: diffuse background -
X-rays: general
Abstract:
We present the first survey results of hard X-ray point sources in the
Galactic Center (GC) region by NuSTAR. We have discovered 70 hard
(3-79 keV) X-ray point sources in a 0.6 deg2 region around Sgr A*
with a total exposure of 1.7 Ms, and 7 sources in the Sgr B2 field
with 300 ks. We identify clear Chandra counterparts for 58 NuSTAR
sources and assign candidate counterparts for the remaining 19. The
NuSTAR survey reaches X-ray luminosities of ∼4x and ∼8x1032 erg/s at
the GC (8 kpc) in the 3-10 and 10-40 keV bands, respectively. The
source list includes three persistent luminous X-ray binaries (XBs)
and the likely run-away pulsar called the Cannonball. New
source-detection significance maps reveal a cluster of hard (>10 keV)
X-ray sources near the Sgr A diffuse complex with no clear soft X-ray
counterparts. The severe extinction observed in the Chandra spectra
indicates that all the NuSTAR sources are in the central bulge or are
of extragalactic origin. Spectral analysis of relatively bright NuSTAR
sources suggests that magnetic cataclysmic variables constitute a
large fraction (>40%-60%). Both spectral analysis and logN-logS
distributions of the NuSTAR sources indicate that the X-ray spectra of
the NuSTAR sources should have kT>20 keV on average for a single
temperature thermal plasma model or an average photon index of
Γ=1.5-2 for a power-law model. These findings suggest that the
GC X-ray source population may contain a larger fraction of XBs with
high plasma temperatures than the field population.
Description:
Observations of the GC region with NuSTAR began in 2012 July, shortly
after launch. The original survey strategy for the GC region was to
match the central 2°x0.7° region covered by the Chandra X-ray
Observatory (Wang et al. 2002Natur.415..148W 2002Natur.415..148W; Muno et al. 2009,
J/ApJS/181/110). The field of views (FOVs) of neighboring NuSTAR
observations in the survey were designed to overlap with each other by
∼40%. Multiple observations of the same region with relatively large
FOV offsets tend to average out the vignetting effects of each
observation, enabling a more uniform coverage of the region. Multiple
observations are also suitable for monitoring long term X-ray
variability of sources in the region. Even when observing a single
target, the NuSTAR observation is often broken up into two or more
segments with relatively large pointing offsets to allow an efficient
subtraction of a detector coordinate-dependent background component
(e.g., Mori et al. 2013ApJ...770L..23M 2013ApJ...770L..23M).
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 123 70 NuSTAR Galactic Point (NGP) Source List
table3.dat 123 7 NuSTAR Point Sources in the Sgr B2 Field
table4.dat 162 70 NuSTAR Aperture Photometry Results of NuSTAR
Sources
table5.dat 162 7 NuSTAR Aperture Photometry Results of Sources
in the Sgr B2 Field
--------------------------------------------------------------------------------
See also:
B/chandra : The Chandra Archive Log (CXC, 1999-2014)
J/A+A/368/835 : Soft X-ray sources in the Galactic Center region
(Sidoli+, 2001)
J/ApJ/589/225 : Chandra X-ray sources toward Galactic Center (Muno+, 2003)
J/ApJ/613/1179 : X-ray sources in Galactic Center (Muno+, 2004)
J/PAZh/30/430 : INTEGRAL X-ray source in Galactic center (Revnivtsev+, 2004)
J/ApJS/165/173 : Chandra sources in Galaxy central 150pc (Muno+, 2006)
J/MNRAS/371/38 : X-ray observations of the Galaxy center (Wang+, 2006)
J/ApJ/706/223 : X-ray point sources near the Galactic Center (Hong+, 2009)
J/ApJS/181/110 : Galactic center X-ray sources (Muno+, 2009)
J/ApJ/836/99 : NuSTAR serendipitous survey: the 40-month catalog
(Lansbury+, 2017)
Byte-by-byte Description of file: table2.dat table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 I1 --- Group [1/2] Group number (G1)
3- 4 I2 --- ID [1/77] NuSTAR Galactic Center Point
(NGP) source identifier
6- 14 F9.5 deg RAdeg Right Ascension in decimal degrees
(J2000) (1)
16- 24 F9.5 deg DEdeg Declination in decimal degrees (J2000)
(1)
26- 29 I4 --- ChandraM [321/8943] Chandra counterpart
identifier by Muno et al. 2009
(J/ApJS/181/110)
31- 45 A15 --- Chandra Chandra counterpart name
(HHMMSS.s+DDMMSS)
47- 52 F6.1 10-6ph/cm2/s FX(2-8) Chandra 2-8 keV flux of the counterpart
(in cgs)
54- 57 F4.1 arcsec Offset Angular offset between the NuSTAR and
Chandra positions
59- 61 I3 ks Exp Combined exposure of the two NuSTAR
focal plane modules (FPMs) at
the Chandra source position
63- 64 A2 --- Det [S H SH] An indicator of the soft
(S<10 keV) and/or hard (H>10 keV) band
detection
66- 72 F7.1 --- Trial Trial map value at the Chandra position
(2)
74- 75 I2 keV b_Band [3/10] Energy band of the trial map
with the local peak value, lower value
77- 78 I2 keV B_Band [10/79] Energy band of the trial map
with the local peak value, upper value
80- 81 I2 % Size [15/30] Source cell size of the trial
map with the local peak value
83- 84 I2 --- NDetec [0/15] Number of trial maps above their
respective thresholds at the Chandra
position
86-123 A38 --- Com A known name, nearby Chandra source,
and/or notable diffuse feature nearby
--------------------------------------------------------------------------------
Note (1): The local peak location of the trial map within 30" of the Chandra
position. They are weighted average values among the trial maps with
detections. For sources with bright neighbors, we limit the search
radius to 10" or 15" depending on the proximity. The peak position is
determined by a 2D Gaussian fit on the trial map.
Note (2): The sources are ordered by this value.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat table5.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 I1 --- Group [1/2] Group number (G1)
3- 4 I2 --- ID [1/77] NuSTAR Galactic Center Point
(NGP) source identifier
6- 9 I4 --- ChandraM [321/8943] Chandra source identifier
by Muno et al. 2009 (J/ApJS/181/110)
11- 15 I5 ct Count Net counts in the 3-40 keV band
17- 19 I3 ct e_Count Uncertainty in Count (1)
21- 27 F7.4 --- BEHR The mode of the posterior
distribution of Bayesian Enhanced
X-ray Hardness Ratio (BEHR) (2)
29- 34 F6.4 --- e_BEHR Uncertainty in BEHR (1)
35 A1 --- neBEHR [*] Note on e_BEHR (3)
37- 42 F6.3 keV E50 Median energy of the NuSTAR spectrum
in 3-40 keV
44- 48 F5.3 keV e_E50 Uncertainty in E50 (1)
49 A1 --- neE50 [*] Note on e_E50 (3)
51- 56 F6.4 --- QR A relative ratio of 25% and 75%
quartiles:
3(E25-3 keV)/(E75-3 keV) (4)
58- 63 F6.4 --- e_QR Uncertainty in QR (1)
64 A1 --- neQR [*] Note on e_QR (3)
66- 68 F3.1 10+22/cm2 NH An estimate of NH along the line of
sight (Nishiyama et al.
2008ApJ...680.1174N 2008ApJ...680.1174N)
70- 73 F4.2 --- Index An estimate of photon index using the
median energy for an absorbed
power-law model with
NH=6x1022/cm2 (Section 4.3)
75- 78 F4.2 --- e_Index Uncertainty in Index (1)
79 A1 --- neIndex [+] Note on e_Index (5)
81- 86 F6.1 10-6ph/cm2/s FX(2-8) The observed (i.e., absorbed) Chandra
2-8 keV flux from Muno et al. 2009
(J/ApJS/181/110) (6)
88- 94 F7.1 10-6ph/s/cm2 FX(3-10) The observed NuSTAR 3-10 keV flux
96-101 F6.1 10-6ph/s/cm2 e_FX(3-10) Uncertainty in FX(3-10) (1)
102 A1 --- neFX(3-10) [*] Note on e_FX(3-10) (3)
104-109 F6.1 10-6ph/s/cm2 FX(10-40) The observed NuSTAR 10-40 keV flux
111-115 F5.1 10-6ph/s/cm2 e_FX(10-40) Uncertainty in FX(10-40) (1)
116 A1 --- neFX(10-40) [*] Note on e_FX(10-40) (3)
118-124 F7.1 10+25W LX(3-10) The observed NuSTAR 3-10 keV
luminosity at 8 kpc (in 1032erg/s)
126-131 F6.1 10+25W e_LX(3-10) Uncertainty in LX(3-10) (1)
132 A1 --- neLX(3-10) [*] Note on e_LX(3-10) (3)
134-140 F7.1 10+25W LX(10-40) The observed NuSTAR 10-40 keV
luminosity at 8 kpc (in 1032erg/s)
142-147 F6.1 10+25W e_LX(10-40) Uncertainty in LX(10-40) (1)
148 A1 --- neLX(10-40) [*] Note on e_LX(10-40) (3)
150 A1 --- Size1 Source and background aperture radii
flag, for the first aperture set (7)
151 A1 --- n_Size1 [a] Note on Size1 (8)
153 A1 --- Size2 Source and background aperture radii
flag, for the second aperture set
(7)
154 A1 --- n_Size2 [a] Note on Size2 (8)
156-162 A7 --- Flag The NuSTAR flags (9)
--------------------------------------------------------------------------------
Note (1): The errors quoted in Tables 4 and 5 are the largest of the three
estimates: a statistical error and two different estimates of
systematic errors. The statistical error is estimated from the
uncertainty of the observed net counts after background subtraction.
Note (2): Park et al. 2006ApJ...652..610P 2006ApJ...652..610P (see Section 4.3): (H-S)/(H+S) where
H and S are net counts in 3-10 and 10-40 keV, respectively.
Note (3): Note as follows:
* = A systematic error is given by the difference in the photometry results
between two aperture sets.
Note (4): Equivalent to the y-axis value in the NuSTAR quantile diagram
(Hong et al. 2004ApJ...614..508H 2004ApJ...614..508H, see Section 4.3).
Note (5): Note as follows:
+ = In calculating the photon indices and the X-ray luminosities, another
systematic error is estimated based on the selection of spectral model
parameters (see Section 4.4).
Note (6): X-ray photon fluxes and luminosities quoted in this paper are all
absorbed quantities using the assumed or estimated NH values unless
otherwise noted.
Note (7): We performed aperture photometry using two aperture sets for each
source: the first set to provide the basic photometry results, and
the second set to estimate the systematic errors originating from the
aperture selection (Section 4.1). The two baseline choices are (e)
and (f).
Source and background aperture radii as follows:
a = 20"/35"-42";
b = 20"/30"-46";
c = 20"/45"-75",
d = 30"/45"-45";
e = 30"/50"-80";
f = 40"/60"-90";
g = 8"/130"-145";
h = 70"/145"-145";
i = 70"/210"-230";
j = 100"/210"-230".
Note (8): Note as follows:
a = Indicates the aperture has additional exclusion zones (see Section 4.1).
Note (9): Flag as follows:
f = Sources showing the iron lines (Section 4.5);
k = Sources with short-term variability according to a Kolmogorov-Smirnov (KS)
test of individual observations (Section 4.6);
r = Sources with long-term variablility according to the maximum-to-minimum
flux ratio of multiple observations (Section 4.6);
c = The Chandra source flags (Muno et al. 2009, J/ApJS/181/110): sources
confused with another nearby source;
g = Sources that fell near the edge of a detector in one or more observations;
b = Sources for which the source and background spectra have a >10% chance of
being drawn from the same distribution according to the KS tests;
s = Sources variable on short timescales, as indicated by probabilities of
<1% that the event arrival times for at least one observation were
consistent with a uniform distribution according to the KS test;
l = Sources that were variable on long timescales, as indicated by a
probability of <1% that the fluxes for all observations were consistent
with a uniform distribution according to the KS test;
t = Transients identified in Degenaar et al. (2012A&A...545A..49D 2012A&A...545A..49D);
p = Sources that suffered from photon piled-up.
--------------------------------------------------------------------------------
Global notes:
Note (G1): Group as follows:
1 = Clear Association. The sources in group 1 have a relatively clear Chandra
counterpart which is usually the only bright (FX>3x10-6 ph/cm2/s1)
Chandra source around the NuSTAR detection;
2 = Unclear Association. The sources in group 2 have solid NuSTAR detections
(except for NGP 65, which is a bit marginal), but their association with
the Chandra sources is not as clear either because multiple Chandra
sources of similar fluxes are found within the uncertainty of the NuSTAR
positions (e.g., NGPs 55 and 56) or because a diffuse origin of the hard
X-ray emission cannot be ruled out (e.g., NGPs 53 and 59,
see Section 6.1).
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History:
From electronic version of the journal
References:
Mori et al., Paper I., 2015ApJ...814...94M 2015ApJ...814...94M
(End) Tiphaine Pouvreau [CDS] 20-Feb-2018