Astrophysical Plasma Emission Code Research Articles

Investigating the Soft X-Ray Spectra of Solar Flare Onsets

In this study, we present the analysis of six solar flare events that occurred in 2022, using new data from the third-generation Miniature X-Ray Solar Spectrometer, also known as the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS). The primary focus of this study is on the flare’s onset phase, which is characterized by elevated soft X-ray emissions even before the flare’s impulsive phase. We analyze the temporal evolution of plasma temperature, emission measure, and elemental abundance factors during the flare onset phase by fitting the DAXSS spectra with the Astrophysical Plasma Emission Code model. The model fitting results indicate that the flaring plasma is already at a high temperature (10–15 MK) during the onset period. The temperature rises during the onset phase, followed by a decrease and subsequent increase during the impulsive phase. Elemental abundance factors show a trend of falling below preflare values during the onset phase, with some recovery before the impulsive phase. During the impulsive phase, the abundance factors decrease from elevated coronal values to about photospheric values. We also analyze images from the 193 Å channel of the Atmospheric Imaging Assembly (AIA), highlighting the formation or brightening of coronal loop structures during the onset phase. Two distinct onset loop configurations are observed, which are referred to as one-loop and two-loop onsets. Both DAXSS and AIA observations indicate that the flare onset phase exhibits hot coronal plasma properties similar to those of the impulsive phase, suggesting that the onset phase may act as a preconditioning effect for some flares.

Cosmological Simulation of Galaxy Groups and Clusters. II. Studying Different Modes of Feedback through X-Ray Observations

The impact of feedback from active galactic nuclei (AGNs) on the cosmological evolution of the large-scale structure is a long-studied problem. However, it is still not well understood how the feedback energy couples to the ambient medium to influence the properties of AGN host galaxies and dark matter halos. In this work we investigate different modes of AGN feedback and their effect on the surrounding medium by probing the diffuse X-ray emission from the hot gas inside galaxy groups and clusters. For this purpose, we use the cosmological hydrodynamic simulation SIMBA to theoretically calculate the X-ray emission from simulated galaxy clusters/groups with the help of the Astrophysical Plasma Emission Code. We also perform synthetic observations of these systems with the Chandra X-ray telescope using the ray-tracing simulator Model of AXAF Response to X-rays. Our results show that in addition to the radiative wind mode of feedback from the AGNs, jet and X-ray modes of feedback play significant roles in suppressing the X-ray emission from the diffuse gas in the vicinity of the black hole. Our mock observational maps suggest that the signatures of AGN feedback from high-redshift objects may not be detected with the instrumental resolution of current X-ray telescopes like Chandra, but provide promising prospects for detection of these features with potential X-ray missions such as Lynx.

X-ray emission from cosmic web filaments in SRG/eROSITA data

Using the publicly available eROSITA Final Equatorial Depth Survey (eFEDS) data, we detected the stacked X-ray emissions at the position of 463 filaments at a significance of 3.8σbased on the combination of all energy bands. In parallel, we found that the probability of the measurement under the null hypothesis is ∼0.0017. The filaments were identified with galaxies in the Sloan Digital Sky Survey survey, ranging from 30 Mpc to 100 Mpc in length at 0.2 < z < 0.6. The stacking of the filaments was performed with the eFEDS X-ray count-rate maps in the energy range between 0.4 and 2.3 keV after masking the resolved galaxy groups and clusters and the identified X-ray point sources from the ROSAT,Chandra,XMM-Newton, and eROSITA observations. In addition, diffuse X-ray foreground and background emissions or any residual contribution were removed by subtracting the signal in the region between 10 and 20 Mpc from the filament spines. For the stacked signal, we performed an X-ray spectral analysis, which indicated that the signal is associated with a thermal emission. According to a model with the astrophysical plasma emission code for the plasma emission and with aβ-model gas distribution withβ = 2/3, the detected X-ray signal can be interpreted as emission from hot gas in the filaments with an average gas temperature of 1.0−0.2+0.3keV and a gas overdensity of 21 ± 5 at the center of the filaments.

Simulated X‐ray emission in galaxy clusters with feedback from active galactic nuclei

To investigate the effect of feedback from active galactic nuclei (AGN) on their surrounding medium, we study the diffuse X‐ray emission from galaxy groups and clusters by coupling the Astrophysical Plasma Emission Code (APEC) with the cosmological hydrodynamic simulation involving AGN feedback. We construct a statistical sample of synthetic Chandra X‐ray photon maps to observationally characterize the effect of AGN on the ambient medium. We show that AGN are effective in displacing the hot X‐ray emitting gas from the centers of groups and clusters, and that these signatures remain evident in observations of the X‐ray surface brightness profiles.

First detection of stacked X-ray emission from cosmic web filaments

We report the first statistical detection of X-ray emission from cosmic web filaments in ROSAT data. We selected 15 165 filaments at 0.2 < z < 0.6 ranging from 30 Mpc to 100 Mpc in length, identified in the Sloan Digital Sky Survey survey. We stacked the X-ray count-rate maps from ROSAT around the filaments, excluding resolved galaxy groups and clusters above the mass of ∼3 × 1013 M⊙as well as the detected X-ray point sources from the ROSAT,Chandra, andXMM-Newtonobservations. The stacked signal results in the detection of the X-ray emission from the cosmic filaments at a significance of 4.2σin the energy band of 0.56−1.21 keV. The signal is interpreted, assuming the Astrophysical Plasma Emission Code model, as an emission from the hot gas in the filament-core regions with an average gas temperature of 0.9−0.6+1.0keV and a gas overdensity ofδ ∼ 30 at the center of the filaments. Furthermore, we show that stacking the SRG/eROSITA data for ∼2000 filaments only would lead to a ≳5σdetection of their X-ray signal, even with an average gas temperature as low as ∼0.3 keV.

The advection-dominated accretion flow+thin accretion disk model for two low-luminosity active galactic nuclei: M81 and NGC 4579

It was found that advection-dominated accretion flow (ADAF)+thin disk model calculations can reproduce the observed spectral energy distributions (SEDs) of two low-luminosity active galactic nuclei (AGNs), provided they are accreting at ∼ 0.01 – 0.03 Eddington rates and the thin disks are truncated to ADAFs at ∼ 100RS (RS is the Schwarzschild radius) for M81 and NGC 4579 (Quataert et al. 1999). However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF+thin disk model calculations can reproduce the observed SEDs of these two low-luminosity AGNs, if the black hole is accreting at 2.5 ′ 10−4 Eddington rates with the thin disk truncated at Rtr = 120RS for M81 (ṁ = 3.3 ′ 10−3 and Rtr = 80RS are required for NGC 4579). The transition zones with temperature from the thin disk with ∼ 104 – 105 to ∼ 109 – 1010 K in the ADAF will inevitably emit thermal X-ray lines, which provides a useful diagnosis of their physical properties. The observed widths of the thermal X-ray iron lines at ≃ 6.8 keV are consistent with Doppler broadening by Keplerian motion of the gases in the transition zones at ∼ 100RS. We use the structure of the transition zone between the ADAF and the thin disk derived by assuming the turbulent diffusive heat mechanism to calculate their thermal X-ray line emission with the standard software package Astrophysical Plasma Emission Code (APEC). Comparing them with the equivalent widths of the observed thermal X-ray iron lines in these two sources, we find that the turbulent diffusive heat mechanism seems to be unable to reproduce the observed thermal X-ray line emission. The test of the evaporation model for the accretion mode transition with the observed thermal X-ray line emission is briefly discussed.

Coronal Density Diagnostics with Six:ChandraLETGS Observations of Procyon, α Centauri A and B, Capella, and ε Eridani

Electron density diagnostics based on the line intensity ratio of Si X are applied to the X-ray spectra of Procyon, α Cen A and B, Capella, and Eri measured with the Low Energy Transmission Grating Spectrometer combined with the High Resolution Camera on board the Chandra X-Ray Observatory. The ratio R1 of the intensities of the Si X lines at 50.524 and 50.691 Å is adopted. A certain temperature effect in R1 appears near the low-density limit region, which is due to the contamination of the Si X line at 50.703 Å. Using the emission measure distribution model derived by Audard and coworkers for Capella and emissivities calculated with the Astrophysical Plasma Emission Code model by Smith and coworkers, we successfully estimate the contributions of the Fe XVI lines at 50.367 and 50.576 Å (73% and 62%, respectively). A comparison between the observed ratios and theoretical predictions constrains the (logarithmic) electron densities for Procyon to be 8.61 cm-3, while for α Cen A and B, Capella, and Eri they are 8.81, 8.60, 9.30-0.48, and 9.11 cm-3, respectively. The comparison of our results with those constrained by the triplet of He-like carbon shows good agreement. For normal stars, our results display a narrow uncertainty, while for active stars, a relatively larger uncertainty due to contamination from Fe XVI lines is found. Another possible reason may be the uncertainty of the continuum level, since the emission lines of Si X become weak for active stars. For Eri, an electron density in the C V-forming region was first estimated through Si X emission.

An Assessment of the Fe xviii and Fe xix Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, extreme-UV, and far-UV line emission, formed at the peak of Capella's (α Aurigae's) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than a factor of 2 are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common use. In particular, the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton-impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 107 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

Spectral Modeling with APEC

AbstractThe Astrophysical Plasma Emission Code (APEC) collaboration now provides public models for X-ray spectra of collisional equilibrium plasmas. These models facilitate the diagnosis of temperature, density, elemental abundance, charge state, and optical depth. We report benchmarking studies of the APEC models from the Emission Line Project, a project to test these models using high quality stellar coronal spectra. We discuss the implications of the benchmarked atomic data for non-equilibrium collisional models as well. Finally, we discuss the extension of APEC to other applications, such as opacity models for AGN.

Modeling the NeixTriplet Spectral Region of Capella with theChandraandXMM‐NewtonGratings

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (ne < 2 × 1010 cm-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions

New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of high-resolution X-ray spectra in which hydrogen- and helium-like ions are usually strong features. We present results from a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic data in the companion Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains the requisite atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. We compare the APEC results to other plasma codes for hydrogen- and helium-like diagnostics, and test the sensitivity of our results to the number of levels included in the models. We find that dielectronic recombination with hydrogen-like ions into high (n=6-10) principal quantum numbers affects some helium-like line ratios from low-lying (n=2) transitions.