We present measurements of the H-like potassium (K XIX) X-ray spectrum and its Helike (K XVIII) satellite lines, which are situated in the wavelength region between 3.34 and 3.39 Å, which has been of interest for the detection of dark matter. The measurements were taken with a high-resolution X-ray spectrometer from targets irradiated by a long-pulse (2 ns) beam from the Orion laser facility. We obtain experimental wavelength values of dielectronic recombination satellite lines and show that the ratio of the Lyα lines and their dielectronic satellite lines can be used to estimate the electron temperature, which in our case was about 1.5±0.3 keV

Beiersdorfer, P

Brown, GV

Burridge, D

Hatfield, P

Hill, MP

Hoarty, DJ

Hobbs, LMR

Lockard, TE

McKelvey, A

Morton, J

Nilsen, J

Rose, SJ

Shepherd, R

Soukhanovskii, VA

Weller, ME

Wilson, L

The Astrophysical Journal

English

We present measurements of the H-like potassium (K xix) X-ray spectrum and its He-like (K xviii) satellite lines, which are situated in the wavelength region between 3.34 and 3.39 Å, which has been of interest for the detection of dark matter. The measurements were taken with a high-resolution X-ray spectrometer from targets irradiated by a long-pulse (2 ns) beam from the Orion laser facility. We obtain experimental wavelength values of dielectronic recombination satellite lines and show that the ratio of the Lyα lines and their dielectronic satellite lines can be used to estimate the electron temperature, which in our case was about 1.5 ± 0.3 keV

Oxford University Research Archive (ORA)

Observation of He-like satellite lines of the H-like potassium K XIX emission

Rose, Steven

Oxford University Research Archive

ORA - Oxford University Research Archive

Spiral - Imperial College Digital Repository

 1 Observation of He-like Satellite Lines of the H-like Potassium K XIX Emission  M.E. Weller1, P. Beiersdorfer1, T.E. Lockard1, G.V. Brown1, A. McKelvey1,2, J. Nilsen1, R. Shepherd1, V.A. Soukhanovskii1, M.P. Hill3, L.M.R. Hobbs3, D. Burridge3, D.J. Hoarty3, J. Morton3, L. Wilson3, S.J. Rose4, P. Hatfield5 1Lawrence Livermore National Laboratory, Livermore, California 94550, USA 2University of Michigan, Ann Arbor, Michigan 48109, USA 3Directorate of Research and Applied Science, AWE plc, Reading RG7 4PR, UK 4Plasma Physics Group, Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK 5Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK  Abstract   We present measurements of the H-like potassium (K XIX) X-ray spectrum and its He-like (K XVIII) satellite lines, which are situated in the wavelength region between 3.34 and 3.39 Å, which has been of interest for the detection of dark matter.  The measurements were taken with a high-resolution X-ray spectrometer from targets irradiated by a long-pulse (2 ns) beam from the Orion laser facility.  We obtain experimental wavelength values of dielectronic recombination satellite lines and show that the ratio of the Lyα lines and their dielectronic satellite lines can be used to estimate the electron temperature, which in our case was about 1.5±0.3 keV.  Key words: techniques: spectroscopic – atomic data – plasmas – X-rays: galaxies: clusters – cosmology: dark matter – methods: laboratory: atomic 1. Introduction There has been a recent debate about the origin of a 3.5 keV X-ray emission feature measured in the Galactic Center and the Perseus galaxy cluster.  The line was first detected by Bulbul et al. (2014), and it was hypothesized that the line was either caused by the decay of sterile neutrino dark matter or by an atomic transition, the intensity of which was underestimated.  To offer potential solutions to the phenomenon, Jeltema et al. (2015) and Carlson et al. (2015) concluded that a likely candidate for the line is a pair of K XVIII lines at 3.48 and 3.52 keV and the feature is not caused by the decay of sterile neutrino dark matter.  Recent observations of the Perseus galaxy cluster with the Hitomi satellite (Hitomi Collaboration 2017) have ruled out anomalously bright K XVIII lines as the cause of the 3.5 keV emission reported by Bulbul et al. (2014).  However, the Hitomi observations appeared to identify (weak) flux from both K XVIII and K XIX.  This debate and the potential presence of potassium emission lines have sparked interest in producing and verifying the associated atomic data and in studying potassium emission from laboratory sources, especially as few experiments have focused on the X-ray emission of this element and as a deeper understanding of X-ray lines generated in this region is needed (Gu et al. 2015, Shah et al. 2016).   To our knowledge, only K XVIII X-ray lines have been previously studied in the laboratory, i.e. the He-like potassium lines 1snp 1P1 → 1s2 1S0 were measured in laser-produced plasmas (Feldman et al. 1974) with n=2 and subsequently in a tokamak plasma from the Princeton Large Torus (PLT; Beiersdorfer et al. 1989) with n=2,3.  No laboratory X-ray spectra have been reported for K XIX.  Here we present a measurement of the 2p 2P3/2 → 1s 2S1/2 Lyα1 and 2p 2P1/2 → 1s 2S1/2 Lyα2 lines and the associated He-like satellite lines.  The He-like satellite lines are of particular interest, because they are produced by dielectronic recombination.  Such  3 lines have been measured previously only for neighboring elements, e.g. argon (Bartiromo et al. 1985, Rice et al. 2011), calcium (Rice et al. 2014), chromium (Bartiromo et al. 1989), and iron (Decaux et al. 1991). The paper is organized as follows.  In Section II we present our theoretical predictions for the He-like satellite lines associated with the K XIX spectrum that were generated using the Flexible Atomic Code (FAC, Gu 2008).  We also include a theoretical estimate of the relative dielectronic satellite intensity as a function of electron temperature, which was generated with the Spectroscopic Collisional-Radiative Atomic Model (SCRAM; Hansen 2007, 2011).  In Section III we present our experimental results and give wavelength estimates for two resolved dielectronic recombination lines.  Section IV concludes.  2. Theoretical Predictions of He-like Satellite Dielectronic Recombination Lines A non-local thermodynamic equilibrium (non-LTE) kinetic model of potassium was developed to help in the analysis.  The atomic rates were generated using FAC (version 1.1.1), which calculates energy levels, collisional excitation, collisional ionization, radiative decay, Auger decay, photoionization, and radiative recombination.  The description of the energy levels for H- and He-like ions calculated using FAC are as follows: The singly excited states are included up to n = 6 for H-like ions, where there are a total of 30 levels, and n = 5 for He-ions, where there are a total of 90 levels.  The doubly excited states include up to n = 3 for He-like ions.   Table 1 gives a list of the FAC calculated wavelengths of the H-like and He-like potassium lines compared with previous Vainshtein and Safronova (1978), Johnson and Soff (1985), and Garcia and Mack (1965) calculations.  The two H-like lines are the doublet Lyα1,2 2p  4 2P3/2,1/2 → 1s 2S1/2 resonance lines at approximately 3.3466 Å and 3.3521 Å, respectively.  These FAC-calculated wavelengths are approximately 0.001 Å greater than those calculated by Vainshtein and Safronova (1978) and in agreement (to 0.0001 Å) with those calculated by Johnson and Soff (1985) and Garcia and Mack (1965).  The rest of the He-like lines are on average 0.0005 Å different, with the clear majority of FAC wavelengths being slightly greater than those of the Vainshtein and Safronova (1978) calculations.  The two prominent dielectronic recombination satellite lines are the 2s2p 1P1 → 1s2s 1S0 line and the 2p2 1D2 → 1s2p 1P1 line, which have been historically labelled as T and J, respectively.  In particular, the ratio of T/Lyα1 has been used to determine the electron temperature in previous works (see Bartiromo et al. 1985).   Because it may be difficult in observational or experimental measurements to resolve satellite T or even Lyα1 from Lyα2 we show in Fig. 1(a) our calculations for the temperature dependence of IDR/ILyα, which is the ratio of the flux from all dielectronic satellite lines in the range between 3.387 and 3.364 Å and the combined flux of both Lyα lines.  These calculations were carried out with the SCRAM code.  In Fig. 1(a), we have plotted the electron temperature on the y-axis to stress that the temperature can be inferred from the measured IDR/ILyα, ratio.  The dependence is given for several different electron densities, ranging from 1010 – 1021 cm-3, and essentially no such dependence is seen.  The fact that the ratio of the sum of all dielectronic satellite lines to the sum of the two Lyα lines is sensitive to the electron temperature has also been pointed out by Gu et al. (2012) in the case of the H-like Fe XXVI Lyα emission.       5 3.  Experimental Results The present experiments were carried out on the Orion laser facility (Hopps et al. 2015) with targets containing varying amounts of potassium.  The data were collected with a recently developed high-resolution crystal spectrometer (Beiersdorfer et al. 2016) designed specifically for the determination of plasma characteristics from the line shape of X-ray lines produced by the Orion laser.  The instrument is dubbed the Orion high-resolution X-ray (OHREX) spectrometer.  In the present work, OHREX was fielded with a quartz ( ) crystal (lattice spacing, 2d = 8.512 Å) that allowed for a wavelength coverage of 6.4 – 6.8 Å and a resolving power of approximately E/ΔE ≈ 9000.  The present measurements were carried out in second order Bragg reflection, i.e. 3.2 – 3.4 Å.  Since OHREX employed an image plate for X-ray detection, all measurements were integrated over the entire shot. The data of interest were collected with OHREX from six long-pulse Orion laser experiments, which irradiated thin foils (0.8 – 0.9 μm thick) typically with two 1.5 ns, 200 J pulses of 3ω light.  The thin foils were of different compositions and are as follows: parylene dichloride (PyD; Orion shot # 6519), KF (Orion shot # 6523), KF/NaCl/KF (Orion shot # 6525), KF/KCl/KF (Orion shot # 6527), NaF/KCl/NaF (Orion shot # 6534), and KCl (Orion shot # 6539).  The targets containing potassium were overcoated on one side with 0.025 μm Al and on both sides with 0.3 μm plastic.  Aluminum line emission can be seen in first order Bragg reflection unless a 8 μm thick Ti filter is used to screen out the first order emission.   Two of our spectra are shown in Fig. 2, one of the KCl target and the other of the PyD target.  The chlorine spectrum includes the Lyγ 4p 2P3/2,1/2 → 1s 2S1/2 lines, the Lyδ 5p 2P3/2,1/2 → 1s 2S1/2 lines, and the Lyε 6p 2P3/2,1/2 → 1s 2S1/2 lines, which were used to establish the dispersion in the wavelength calibration process.  The wavelength values used for these chlorine lines come  6 from Garcia and Mack (1965) and are 3.3508 Å, 3.2724 Å, and 3.2312 Å, respectively.  In addition, the K Lyα1,2 2p 2P3/2,1/2 → 1s 2S1/2 lines at 3.3467 Å and 3.3521 Å, respectively, were used in the wavelength calibration process.  Even with the dispersion known, and OHREX calibrated by Hell et al. (2016) for different crystals, each spectrum had to be uniquely calibrated, i.e. anchored to the Lyα1,2 lines, due to uncertainties in positioning the image plate in its holder and on the scanner.  The resolution of OHREX can differentiate between the two K Lyα1,2 lines, with the Lyα2 line blended with the Cl Lyγ line.  In fact, the width of the lines shown in Fig. 2 does not reflect the instrumental resolution; instead it is an expression of the opacity effects, plasma motion, and the ion temperature.  It is also interesting to note that chlorine radiated approximately at the same relative intensities, whether it was in a mix with potassium or not. The spectrum from the KF target (shot # 6523) is shown in Fig. 3 along with the synthetic spectrum from our non-LTE potassium model from SCRAM.  The SCRAM model includes opacity effects related to the finite thickness of the plasma, which is required to achieve a reasonable fit of relative intensities of the two Ly lines.  Although the fits we achieved between our modeling calculations and the measured spectrum is far from perfect, our modeling suggests an electron density on the order of 8x1020 cm-3 – although this density is only poorly constrained by the opacity affecting the relative intensity of the Ly lines – and an electron temperature of approximately 1.6 keV.  The standard error for our modeling has been previously reported as an order of magnitude for electron density and around 10% for electron temperature (Weller et al. 2014).  In the present case, we double the uncertainty for the temperature because of the uncertainty in the intensity of the Ly lines due to opacity effects.  7 The centroids of both T and J lines were estimated by Gaussian fitting for all five spectra and their wavelengths determined from our dispersion curve.  The results are shown in Table 2.  All experimental wavelength measurements have an error of approximately ± 0.0005 Å.  The average measured value for T is 3.3668 Å and for J is 3.3792 Å, compared to the theoretical values of 3.3664 Å and 3.3812 Å, respectively, from FAC.  The experimental value for J is found to be 0.0004 Å greater than that of FAC calculations, while that for T is 0.002 Å less.  4. Discussion and Conclusion The K XIX 2p 2P3/2 → 1s 2S1/2 Lyα1 and Lyα2 lines and the associated He-like dielectronic recombination satellite lines have been measured at electron temperatures near 1.6 keV.  We showed that the ratio between the dielectronic satellite lines and the Ly lines can be used to infer the electron temperature.  In addition, the experimental wavelengths for the T and J dielectronic recombination lines were shown to be within 0.002 Å of calculations.   It would be useful to compare the measured ratio between the dielectronic satellite lines and the Lyα lines to spectral codes such as AtomDB (Foster 2013) and SPEX (Kaastra 1996). Although the K XIX Lyα lines are included in AtomDB, the He-like dielectronic lines to the K XIX Lyα spectrum are not. The reason for the omission may be that the K line positions in AtomDB derive from the NIST Atomic Spectra Database, which also only lists the Lyα lines but none of the satellite lines. Similarly, none of these dielectronic satellite lines appear to be included in SPEX. Our present laboratory identification of these lines, thus, expands the list of “critically” evaluated atomic data for future use in these astrophysical plasma emission codes. Bulbul et al. (2014) relied on AtomDB for calculating the flux of the K XIX lines. Because AtomDB does not include the associated He-like dielectronic satellite lines, it appears  8 that their computed flux underestimates the contribution of the potassium line emission in their analysis of the 3.5 keV X-ray emission feature. The enhancement of the K XIX emission feature associated with the dielectronic satellite lines is given as a function of temperature in Fig. 1(b). Although the enhancement at the temperature of maximum emission for the K XIX Lyα lines between 3 and 4 keV is only a few percent, it reaches up to 50% at 1 keV.  Bulbul et al. (2019) have recently considered enhanced flux of the Ar XVII He feature due to the dielectronic satellites, which may also affect the 3.5 keV feature. Their conclusion was that the added flux was not sufficient to account for the bulk of the 3.5 keV feature. This is also true for the added flux of the K XIX feature we studied here. However, each time additional flux is included in the plasma emission models the significance of the 3.5 keV feature diminishes, and thus all such incremental enhancements must be accounted for. The diagnostic importance of the Lyα spectra will certainly increase with future X-ray missions.  The recent Hitomi spectrum of the Perseus cluster fell short of the expected observation length, and future missions will undoubtedly revisit this and other clusters with much higher spectral fidelity.  Consequently, the statistical significance, and, thus, the diagnostic utility of the K XIX spectrum and that of other H-like X-ray spectra will become very relevant for determining the electron temperatures associated with such ions.    9 Acknowledgements This work was performed under the auspices of the U.S. DOE by LLNL under Contract No. DE-AC52-07NA27344.  We like to thank Dr. Stephanie Hansen for making the SCRAM code available as well as many helpful comments.  References Bartiromo, R., et al., 1985, Nucl. Instrum. Methods, 225, 378 Bartiromo, R., et al., 1989, Phys. Rev. A, 40, 7387 Beiersdorfer, P., et al., 2016, RSI, 87, 063501 Beiersdorfer, P., et al., 1989, Phys. Rev. A, 40, 150 Bulbul, E., et al., 2014, ApJ, 783, 13 Bulbul, E., et al., 2019, ApJ, 820, 21 Carlson, E., et al., 2015, JCAP, 02, 009 Decaux, V., et al., 1991, Phys. Rev. A, 43, 228 Feldman, U., et al., 1974, ApJ, 192, 213 Foster, A. R., Ji, L., Yamaguchi, H., Smith, R. K., and Brickhouse, N. S., 2013, AIPCP, 1545, 252  Garcia, M. J., and Mack, J. E., 1965, J. Opt. Soc. Amer., 55, 654 Gu, L., et al., 2015, A&A, 584, L11 Gu, M. F., 2008, Can. J. Phys., 86, 675 Gu, M. F., 2012, Can. J. Phys., 90, 351 Hell, N., Beiersdorfer, P., Magee, E. W., and Brown G. V., 2016, RSI, 87, 11D604 Hansen, S. B., Bauche, J., Bauche-Arnoult, C., and Gu, M. F., 2007, HEDP 3, 109  Hansen, S. B., Can. J. Phys., 2011, 89, 633  10 Hitomi Collaboration, 2017, ApJL, 837, 215 Hopps, N., et al., 2015, Plasma Phys. Control. Fusion, 57, 064002 Jeltema, T., and Profumo, S., 2015, MNRAS, 450, 2143 Johnson, W. R., and Soff, G., 1985, Atom. Data Nucl. Data T., 33, 405 Kaastra, J. S., Mewe, R., and Nieuwenhuijzen, H., 1996, 11th C. on UV and X-ray Spec. of Astro. and Lab. Plas., 411  Rice, J. E., et al, 2011, J. Phys. B: At. Mol. Opt. Phys., 44, 165702 Rice, J. E., et al, 2014, J. Phys. B: At. Mol. Opt. Phys., 47, 075701 Shah, C., et al, 2016, ApJ, 833, 52 Vainshtein, L. A., and Safronova, U. I., 1978, Atom. Data Nucl. Data T., 21, 49 Weller, M. E., et al., 2014, PoP, 21, 031206               11 Ion Transition FAC λ (Å) Vainshtein/Safronova λ (Å) Johnson/Soff λ (Å) Garcia/Mack λ (Å) He 2p2 1S0 → 1s2p 3P1 3.3391 3.3392 - - H 2p 2P3/2 → 1s 2S1/2 3.3466 3.3456 3.3467 3.3467 He 2s2p 1P1 → 1s2s 3S1 3.3461 3.3469 - - H 2p 2P1/2 → 1s 2S1/2 3.3521 3.3510 3.3521 3.3521 He 2p2 1S0 → 1s2p 1P1 3.3552 3.3549 - - He 2p2 1D2 → 1s2p 3P1 3.3648 3.3646 - - He - T 2s2p 1P1 → 1s2s 1S0 3.3664 3.3667 - - He 2p2 1D2 → 1s2p 3P2 3.3680 3.3678 - - He 2s2p 3P2 → 1s2s 3S1 3.3718 3.3714 - - He 2p2 3P2 → 1s2p 3P1 3.3741 3.3733 - - He 2s2p 3P1 → 1s2s 3S1 3.3756 3.3751 - - He 2p2 3P1 → 1s2p 3P0 3.3760 3.3752 - - He 2p2 3P1 → 1s2p 3P1 3.3767 3.3759 - - He 2p2 3P2 → 1s2p 3P2 3.3773 3.3765 - - He 2s2p 3P0 → 1s2s 3S1 3.3771 3.3766 - - He 2p2 3P0 → 1s2p 3P1 3.3787 3.3779 - - He 2p2 3P1 → 1s2p 3P2 3.3799 3.3791 - - He - J 2p2 1D2 → 1s2p 1P1 3.3812 3.3806 - - He 2p2 3P2 → 1s2p 1P1 3.3905 3.3894 - - He 2p2 3P1 → 1s2p 1P1 3.3932 3.3919 - - He 2p2 3P0 → 1s2p 1P1 3.3951 3.3940 - - He 2s2p 3P1 → 1s2s 1S0 3.3954 3.3953 - - He 2s2 1S0 → 1s2p 3P1 3.3996 3.3996 - - He 2s2 1S0 → 1s2p 1P1 3.4163 3.4159 - -  Table 1: Theoretical wavelengths for potassium He-like dielectronic lines and H-like resonance lines from the Flexible Atomic Code compared to those from the calculations of Vainshtein and Safronova (1978), Johnson and Soff (1985), and Garcia and Mack (1965).         12 Ion Transition Orion 6523 Orion 6525  Orion 6527 Orion 6534 Orion 6539 Average He - T 2s2p 1P1 → 1s2s 1S0 3.3670 3.3669 3.3669 3.3665 3.3668 3.3668 He - J 2p2 1D2 → 1s2p 1P1 3.3794 3.3793 3.3791 3.3791 3.3791 3.3792  Table 2: Experimental wavelengths (Å) for the two strongest potassium He-like dielectronic lines.  Error for all measurements is approximately ± 0.0005 Å.    13  Figure 1: (a) Electron temperature vs the ratio of IDR/ILyα for different electron densities and (b) percent enhancement of the K XIX emission feature vs Te due to the dielectronic satellite lines for the low-density limit (ne ≤ 1010 cm-3).  14  Figure 2: Time-integrated spectra from two OHREX measurements performed on the Orion laser employing a chlorinated plastic target and a KCl target.  The wavelength scale shown is for second order Bragg reflection.  Aluminum features appear in first order.  15  Figure 3: Measured and calculated spectra of H- and He-like emission of potassium.  The OHREX measurement (black trace) was performed on the Orion laser employing a KF target (Orion shot #6523). The synthetic spectrum (smooth, red trace) was calculated with SCRAM using FAC data using Te = 1600 eV and ne = 8x1020 cm-3. 

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Observation of He-like satellite lines of the H-like potassium K XIX emission

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