J/A+A/649/A104 Long-term X-ray spectral evolution of ULXsE (Gurpide+, 2021)
Long-term X-ray spectral evolution of ultraluminous X-ray sources:
implications on the accretion flow geometry and the nature of the accretor.
Gurpide A., Godet O., Koliopanos F., Webb N., Olive J.-F.
<Astron. Astrophys. 649, A104 (2021)>
=2021A&A...649A.104G 2021A&A...649A.104G (SIMBAD/NED BibCode)
ADC_Keywords: Binaries, X-ray
Keywords: stars: neutron - stars: black holes - X-rays: binaries -
accretion, accretion disks
Abstract:
The discovery of pulsations in several ultraluminous X-ray sources
(ULXs) has demonstrated that a fraction of them are powered by
super-Eddington accretion onto neutron stars (NSs). This has raised
questions regarding the NS to black hole (BH) ratio within the ULX
population and the physical mechanism that allows ULXs to reach
luminosities well in excess of their Eddington luminosity. Is this
latter the presence of strong magnetic fields or rather the presence
of strong outflows that collimate the emission towards the observer?
In order to distinguish between these scenarios, namely,
supercritically accreting BHs, weakly magnetised NSs, or strongly
magnetised NSs, we study the long-term X-ray spectral evolution of a
sample of 17 ULXs with good long-term coverage, 6 of which are known
to host NSs. At the same time, this study serves as a baseline to
identify potential new NS-ULX candidates.
We combine archival data from Chandra, XMM-Newton, and NuSTAR
observatories in order to sample a wide range of spectral states for
each source. We track the evolution of each source in a
hardness-luminosity diagram (HLD) in order to identify spectral
changes, and show that these can be used to constrain the accretion
flow geometry, and in some cases the nature of the accretor.
We find NS-ULXs to be among the hardest sources in our sample with
highly variable high-energy emission. On this basis, we identify M81
X-6 as a strong NS-ULX candidate, whose variability is shown to be
akin to that of NGC 1313 X-2. For most softer sources with an unknown
accretor, we identify the presence of three markedly different
spectral states, which we interpret by invoking changes in the
mass-accretion rate and obscuration by the supercritical wind/funnel
structure. Finally, we report on a lack of variability at high
energies (≳10keV) in NGC 1313 X-1 and Holmberg IX X-1, which we
argue may offer a means to differentiate BH-ULXs from NS-ULXs.
We support a scenario in which the hardest sources in our sample might
be powered by strongly magnetised NSs, meaning that the high-energy
emission is dominated by the hard direct emission from the accretion
column. Instead, softer sources may be explained by weakly magnetised
NSs or BHs, in which the presence of outflows naturally explains their
softer spectra through Compton down-scattering, their spectral
transitions, and the dilution of the pulsed-emission should some of
these sources contain NSs.
Description:
Tables containing all the datasets analysed of each source
(table1.dat), the results of the F-test noting in which observations
the simpl model was finally used (table7.dat) and the results of the
spectral fitting (parameters in spparam.dat and absorption corrected
fluxes in table3.dat). All Tables follow the nomenclature given in
Table table1.dat.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 109 238 Sample of sources selected for this study
with their respective distance
table2.dat 159 177 Best-fit spectral fit parameters )
table3.dat 126 176 *Estimated unabsorbed luminosities for each epoch
using the distances from Table 1 and models
from Table 2
table7.dat 64 139 Results of the F-test between (diskbb+ diskbb)
(model 1) and (diskbb+ simpl*diskbb) (model 2)
models
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Note on table3.dat: For jointly fit data, the error estimation takes into
account the errors on the joint parameters. Uncertainties are quoted at 90%
confidence level.
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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- 15 A15 --- Name Name
16- 18 A3 --- n_Name [P NS] Note on Name (1)
20- 24 F5.2 Mpc Dist Distance
25 A1 --- r_Dist Distance reference (2)
27- 34 E8.2 cm-2 NH Galactic absorption (3)
36- 41 A6 --- Epoch Epoch number (G1)
46- 55 A10 --- Tel Telescope of the observation
57- 68 A12 --- Inst Instruments considered, indicates that a certain
instrument was not used, Inst1/Inst2/Inst3 (4)
70- 79 A10 "date" Date Date of the observation
81- 91 I11 --- ObsID Observation id
93- 97 F5.1 ks Exp1 ?=- Good exposure time for Inst1
98 A1 --- --- [/]
99-103 F5.1 ks Exp2 ?=- Good exposure time for Inst2
104 A1 --- --- [/]
105-109 F5.1 ks Exp3 ?=- Good exposure time for Inst3
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Note (1): Note as follows:
P = indicates a source for which pulsations have been identified
NS = No pulsations have been detected in this source but it was identified as
a NS through the detection of a cyclotron line Brightman et al.
(2018NatAs...2..312B 2018NatAs...2..312B)
Note (2): Distances references as follows:
a = Tully et al. (2016AJ....152...50T 2016AJ....152...50T, Cat. J/AJ/152/50)
b = Lelli et al. (2016ApJ...816L..14L 2016ApJ...816L..14L, Cat. J/ApJ/816/L14)
c = Karachentsev et al. (2017AJ....153....6K 2017AJ....153....6K, Cat. J/AJ/153/6)
d = Tully et al. (2008ApJ...676..184T 2008ApJ...676..184T, Cat. J/ApJ/676/184)
e = Karachentsev et al. (2002A&A...383..125K 2002A&A...383..125K)
f = Cappellari et al. (2011MNRAS.413..813C 2011MNRAS.413..813C, Cat. J/MNRAS/413/813)
Note (3): From Kalberla et al. (2005A&A...440..775K 2005A&A...440..775K, Cat. VIII/76):
HI4PI Collaboration, 2016A&A...594A.116H 2016A&A...594A.116H, Cat. J/A+A/594/A116).
Note (4): Observations for which an instrument was not available because it was
not active or because the source was not in the field of view are marked
with "-".
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 15 A15 --- Name Name
17- 22 A6 --- Epoch Epoch number (G1) (1)
24- 27 F4.1 10+20cm-2 nH Extragalactic neutral hydrogen absorption
column
29- 32 F4.1 10+20cm-2 E_nH ? Error on nH (upper value) (2)
34- 37 F4.1 10+20cm-2 e_nH ? Error on nH (lower value) (2)
38 A1 --- n_nH [b] Note on nH (3)
40- 44 F5.3 keV kTsoft Temperature of the soft diskbb
46- 50 F5.3 keV E_kTsoft ? Temperature of the soft diskbb (2)
51 A1 --- nEkTsoft [a] Note on kTsoft (3)
52- 56 F5.3 keV e_kTsoft Temperature of the soft diskbb (2)
58- 63 F6.2 --- Normsoft Normalization of the soft diskbb
66- 70 F5.2 --- E_Normsoft Normalization of the soft diskbb (2)
72- 76 F5.2 --- e_Normsoft Normalization of the soft diskbb (2)
78- 81 F4.2 keV kThard Temperature of the hard diskbb
83- 87 F5.3 keV E_kThard ? Temperature of the hard diskbb (2)
88 A1 --- nEkThard [a] Note on kThard (3)
89- 92 F4.2 keV e_kThard Temperature of the hard diskbb (2)
94-100 F7.3 10-2 Normhard Normalization of the hard diskbb
102-109 F8.3 10-2 E_Normhard Normalization of the hard diskbb (2)
110-116 F7.3 10-2 e_Normhard Normalization of the hard diskbb (2)
118-121 F4.2 --- Gamma ?=- Power law index of the simpl model
123-126 F4.2 --- E_Gamma ? Power law index of the simpl model (2)
128-131 F4.2 --- e_Gamma ? Power law index of the simpl model (2)
132 A1 --- n_Gamma [b] Note on Gamma (3)
133-140 F8.4 % fscatt ?=- Fraction of scattered photons of the
simpl model
143-144 I2 % E_fscatt ? Fraction of scattered photons of the
simpl model (2)
145 A1 --- nEfscatt [a] Note on fE_scatt (3)
146-147 I2 % e_fscatt ? Fraction of scattered photons of the
simpl model (2)
148 A1 --- nefscatt [a] Note on e_fscatt (3)
150-154 F5.3 --- chir Reduced chi square
155 A1 --- --- [/]
156-159 I4 --- dof Degrees of freedom
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Note (1): The first epochs in each source correspond to the high-quality
datasets that were jointly fitted and from which we determine Γ and nH.
The rest of the data were fitted individually by freezing nH and/or Γ
to the best-fit value found in the joint fit of the high-quality data.
Note (2): Errors are quoted at a 90% confidence level.
Note (3): Note as follows:
a = Parameter frozen at the value determined in the joint fits
b = Unbound parameter
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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- 15 A15 --- Name Name
19- 24 A6 --- Epoch Epoch number (G1)
27- 31 F5.2 10+32W Lsoft Unabsorbed luminosity in the 0.3-1.5keV band
33- 36 F4.2 10+32W E_Lsoft Error on Lsoft (upper value)
38- 41 F4.2 10+32W e_Lsoft Error on Lsoft (lower value)
44- 48 F5.2 10+32W Lhard Unabsorbed luminosity in the 1.5-10keV band
50- 53 F4.2 10+32W E_Lhard Error on Lhard (upper value)
56- 59 F4.2 10+32W e_Lhard Error on Lhard (lower value)
61- 65 F5.2 10+32W Ltot Unabsorbed luminosity in the 0.3-10keV band
67- 70 F4.2 10+32W E_Ltot Error on Ltot (upper value)
72- 75 F4.2 10+32W e_Ltot Error on Ltot (lower value)
77- 81 F5.2 10+32W Lsoftdbb Luminosity of the soft diskbb component
83- 85 F3.1 10+32W E_Lsoftdbb Error on Lsoftdbb (upper value)
87- 90 F4.2 10+32W e_Lsoftdbb Error on Lsoftdbb (lower value)
92- 96 F5.2 10+32W Lharddbb Luminosity of the hard diskbb component
98-102 F5.2 10+32W E_Lharddbb Error on Lharddbb (upper value)
104-108 F5.2 10+32W e_Lharddbb Error on Lharddbb (upper value)
110-115 F6.2 10+32W Lsimpl ? Unabsorbed luminosity of the
simpl*diskbb_hard model component
(Flux luminosity for entire component)
117-121 F5.2 10+32W E_Lsimpl ? Error on Lsimpl (upper value)
123-126 F4.2 10+32W e_Lsimpl ? Error on Lsimpl (lower value)
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Byte-by-byte Description of file: table7.dat
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Bytes Format Units Label Explanations
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1- 15 A15 --- Name Name
17- 22 A6 --- Epoch Epoch number (G1)
25- 28 F4.2 --- chir1 Reduced chi square of model 1 (diskbb+diskbb)
29 A1 --- --- [/]
30- 33 I4 --- dof1 Degrees of freedom of model 1 (diskbb+diskbb)
35- 38 F4.2 --- chir2 Reduced chi square of model 2
(diskbb+simpl*diskbb)
39 A1 --- --- [/]
40- 43 I4 --- dof2 Degrees of freedom of model 2
(diskbb+simpl*diskbb)
45- 50 F6.2 --- Deltachi ?=- Delta chi improvement between
model 1 and model 2
52- 57 F6.2 % Prej ?=- Rejection probability of model 1 as given
by the F-test
60- 62 A3 --- simpl? [yes/no ] indicating if the simpl model was
used
64 A1 --- n_simpl? [cd] Note on simpl? (1)
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Note (1): Note as follows:
c = model included as we could constrain it with another observation where
the source was similar in flux and hardness ratio
d = model not included as we could not constrain its parameters with other
observations where the source was similar in flux and hardness ratio
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Global notes:
Note (G1): where the number of Xs, Cs and Ns indicate the number of XMM-Newton,
Chandra and Nustar observations considered together.
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Acknowledgements:
Andres Gurpide, agurpidelash(at)irap.omp.eu
(End) Patricia Vannier [CDS] 05-Mar-2021