Future X-ray Missions Research Articles

Multistructured Accretion Flow of Sgr A*. II. Signatures of a Cool Accretion Disk in Hydrodynamic Simulations of Stellar Winds

Hydrodynamic simulations of the stellar winds from Wolf–Rayet stars within the Galactic center can provide predictions for the X-ray spectrum of the supermassive black hole Sgr A*. Herein, we present results from updated smooth particle hydrodynamics simulations, building on the architecture of Cuadra et al. and Russell et al., and find that a “cold” (104 K) gas disk forms around Sgr A* with a simulation runtime of 3500 yr. This result is consistent with previous grid-based simulations, demonstrating that a cold disk can form regardless of numerical method. We examine the plasma scenarios arising from an environment with and without this cold disk, by generating synthetic spectra for comparison to the quiescent Fe Kα Sgr A* spectrum from Chandra HETGS, taken through the Chandra X-ray Visionary Program. We find that current and future X-ray missions are unlikely to distinguish between the kinematic signatures in the plasma in these two scenarios. Nonetheless, the stellar wind plasma model presents a good fit to the dispersed Chandra spectra within 1.″5 of Sgr A*. We compare our results to the radiatively inefficient accretion flow (RIAF) model fit to the HETGS spectrum presented in Paper I and find that the Bayesian model evidence does not strongly favor either model. With 9″ angular resolution and high spectral resolution of the X-IFU, NewAthena will offer a clearer differentiation between the RIAF plasma model and hydrodynamic simulations, but only a future X-ray mission with arcsecond resolution will significantly advance our understanding of Sgr A*’s accretion flow in X-rays.

Advancing Precision Particle Background Estimation for Future X-Ray Missions: Correlated Variability between the Alpha Magnetic Spectrometer and Chandra/XMM-Newton

Galactic cosmic-ray (GCR) particles have a significant impact on the particle-induced background of X-ray observatories, and their flux exhibits substantial temporal variability, potentially influencing background levels. In this study, we present 1 day binned high-energy reject rates derived from the Chandra-ACIS and XMM-Newton EPIC-pn instruments, serving as proxies for the GCR particle flux. We systematically analyze the ACIS and EPIC-pn reject rates and compare them with the AMS proton flux. Our analysis initially reveals robust correlations between the AMS proton flux and the ACIS/EPIC-pn reject rates when binned over 27 day intervals. However, a closer examination reveals substantial fluctuations within each 27 day bin, indicating shorter-term variability. Upon daily binning, we observe finer temporal structures in the data sets, demonstrating the presence of recurrent variations with periods of ∼25 days and 23 days in the ACIS and EPIC-pn reject rates, respectively, spanning the years 2014–2018. Notably, during the 2016–2017 period, we additionally detect periodicities of ∼13.5 days and 9 days in the ACIS and EPIC-pn reject rates, respectively. Intriguingly, we observe a time lag of ∼6 days between the AMS proton flux and the ACIS/EPIC-pn reject rates during the second half of 2016. This time lag is not visible before 2016 and after 2017. The underlying physical mechanisms responsible for this time lag remain a subject of ongoing investigation.

Search for Synchrotron Emission from Secondary Electrons of Proton–Proton Interactions in Galactic PeVatron Candidate HESS J1641–463

HESS J1641−463 is an unidentified gamma-ray source with a hard TeV gamma-ray spectrum, and thus it has been proposed to be a possible candidate for a cosmic-ray (CR) accelerator up to PeV energies (a PeVatron candidate). The source spatially coincides with the radio supernova remnant G338.5+0.1 but has not yet been fully explored in the X-ray band. We analyzed newly taken NuSTAR data, pointing at HESS J1641−463, with 82 ks effective exposure time. There is no apparent X-ray counterpart of HESS J1641−463, while nearby stellar cluster, Mercer 81, and stray-light X-rays are detected. Combined with the archival Chandra data, partially covering the source, we derived an upper limit of ∼6 × 10−13 erg cm−2 s−1 in 2–10 keV (∼3 × 10−13 erg cm−2 s−1 in 10–20 keV). If the gamma-ray emission is originated from the decay of π 0 mesons produced in interactions between CR protons and ambient materials, secondary electrons in the proton–proton interactions can potentially emit synchrotron photons in the X-ray band, which can be tested by our X-ray observations. Although the obtained X-ray upper limits cannot place a constraint on the primary proton spectrum, it will be possible with a future hard X-ray mission.

The history of the Milky Way: The evolution of star formation, cosmic rays, metallicity, and stellar dynamics over cosmic time

Abstract We study the long-term evolution of the Milky Way (MW) over cosmic time by modeling the star formation, cosmic rays, metallicity, stellar dynamics, outflows, and inflows of the galactic system to obtain various insights into the galactic evolution. The mass accretion is modeled by the results of cosmological N-body simulations for the cold dark matter. We find that the star formation rate is about half the mass accretion rate of the disk, given the consistency between observed Galactic diffuse X-ray emissions (GDXEs) and possible conditions driving the Galactic wind.Our model simultaneously reproduces the quantities of star formation rate, cosmic rays, metals, and the rotation curve of the current MW. The most important predictions of the model are that there is an unidentified accretion flow with a possible number density of ∼10−2 cm−3 and that part of the GDXEs originates from a hot, diffuse plasma which is formed by consuming about $10\%$ of supernova explosion energy. The latter is the science case for future X-ray missions: XRISM, Athena, and so on. We also discuss further implications of our results for the planet formation and observations of external galaxies in terms of multi-messenger astronomy.

An Aluminum-coated sCMOS Sensor for X-Ray Astronomy

In recent years, tremendous progress has been made on scientific Complementary Metal Oxide Semiconductor (sCMOS) sensors, making them a promising device for future space X-ray missions. We have customized a large-format sCMOS sensor, G1516BI, dedicated for X-ray applications. In this work, a 200 nm thick aluminum layer is successfully sputtered on the surface of this sensor. This Al-coated sensor, named EP4K, shows consistent performance with the uncoated version. The readout noise of the EP4K sensor is around 2.5 e− and the dark current is less than 0.01 e− pixel−1 s−1 at −30°C. The maximum frame rate is 20 Hz in the current design. The ratio of single pixel events of the sensor is 45.0%. The energy resolution can reach 153.2 eV at 4.51 keV and 174.2 eV at 5.90 keV at −30°C. The optical transmittance of the aluminum layer is approximately 10−8 to 10−10 for optical lights from 365 to 880 nm, corresponding to an effective aluminum thickness of around 140 to 160 nm. The good X-ray performance and low optical transmittance of this Al-coated sCMOS sensor make it a good choice for space X-ray missions. The Lobster Eye Imager for Astronomy, which has been working in orbit for about one year, is equipped with four EP4K sensors. Furthermore, 48 EP4K sensors are used on the Wide-field X-ray Telescope on the Einstein Probe satellite, which will be launched at the end of 2023.

O, Ne, Mg, and Fe Abundances in Hot X-Ray-emitting Halos of Galaxy Clusters, Groups, and Giant Early-type Galaxies with XMM-Newton RGS Spectroscopy

Chemical elements in the hot medium permeating early-type galaxies, groups, and clusters make such objects an excellent laboratory for studying metal enrichment and cycling processes on the largest scales of the universe. Here, we report the analysis by the XMM-Newton Reflection Grating Spectrometer of 14 early-type galaxies, including the well-known brightest cluster galaxies of Perseus, for instance. The spatial distribution of the O/Fe, Ne/Fe, and Mg/Fe ratios is generally flat in the central 60″ regions of each object, irrespective of whether or not a central Fe abundance drop has been reported. Common profiles between noble gas and normal metal suggest that the dust depletion process does not work predominantly in these systems. Therefore, observed abundance drops are possibly attributed to other origins, such as systematics in the atomic codes. Giant systems with a high ratio of gas mass to luminosity tend to hold a hot gas (∼2 keV) yielding the solar N/Fe, O/Fe, Ne/Fe, Mg/Fe, and Ni/Fe ratios. Contrarily, light systems in a sub-keV temperature regime, including isolated or group-centered galaxies, generally exhibit supersolar N/Fe, Ni/Fe, Ne/O, and Mg/O ratios. We find that the latest supernova nucleosynthesis models fail to reproduce such a supersolar abundance pattern. Possible systematic uncertainties contributing to these high abundance ratios of cool objects are also discussed in tandem with the crucial role of future X-ray missions.

Doppler Broadening and Line-of-sight Effects in Core-collapse Supernovae and Young Remnants

The dynamics and spectral characteristics of supernova ejecta reveal details of the supernova energetics, explosive nucleosynthesis, and evolution of the progenitor. However, in practice, this important diagnostic information is only derived from CCD-resolution X-ray spectra of shock-heated material. If the spectra were to be observed at higher resolution, then important clues to the explosion energetics would be obvious through measurements of bulk Doppler motions and turbulence in the ejecta. Likewise, the unshocked ejecta in supernovae and young remnants are responsible for obscuring the emission from ejecta on the back side of the remnant. In light of these important effects, we present line-of-sight spectral maps of core-collapse supernova remnant models. We explore the bulk Doppler broadening of spectral lines, including line-of-sight effects. We also explore the time-dependent absorption from both shocked and unshocked ejecta. Finally, we discuss how future X-ray missions such as XRISM and Athena will be able to resolve these effects in nearby and extragalactic supernovae and their remnants.

Clumpy Wind Studies and the Nondetection of a Cyclotron Line in OAO 1657–415

Winds of massive stars are suspected to be inhomogeneous (or clumpy), which biases the measures of their mass-loss rates. In high-mass X-ray binaries (HMXBs), the compact object can be used as an orbiting X-ray point source to probe the wind and constrain its clumpiness. We perform a spectrotiming analysis of the HMXB OAO 1657–415 with nonsimultaneous NuSTAR and NICER observations. We compute the hardness ratio from the energy-resolved light curves, and, using an adaptive rebinning technique, we thus select appropriate time segments to search for rapid spectral variations on timescales of a few hundred to thousands of seconds. The column density and intensity of the iron Kα line were strongly correlated, and the recorded spectral variations were consistent with accretion from a clumpy wind. We also illustrate a novel framework to measure clump sizes and masses in HMXBs more accurately based on the absorption measurements and orbital parameters of the source. We then discuss the limitations posed by current X-ray spacecraft in such measurements and present prospects with future X-ray missions. We find that the source pulse profiles show a moderate dependence on energy. We identify a previously undetected dip in the pulse profile visible throughout the NuSTAR observation near spin phase 0.15 possibly caused by intrinsic changes in accretion geometry close to the neutron star. We do not find any evidence for the debated cyclotron line at ∼36 keV in the time-averaged or phase-resolved spectra with NuSTAR.

The 100-m X-ray test facility at IHEP

The 100-m X-ray Test Facility of the Institute of High Energy Physics (IHEP) was initially proposed in 2012 for the test and calibration of the X-ray detectors of the Hard X-ray Modulation Telescope (HXMT) with the capability to support future X-ray missions. The large instrument chamber connected with a long vacuum tube can accommodate the X-ray mirror, focal plane detector and other instruments. The X-ray sources are installed at the other end of the vacuum tube with a distance of 105 m, which can provide an almost parallel X-ray beam covering 0.2sim60 keV energy band. The X-ray mirror modules of the Einstein Probe (EP) and the enhanced X-ray Timing and Polarimetry mission (eXTP) and payload of the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) have been tested and calibrated with this facility. It has been also used to characterize the focal plane camera and aluminum filter used on the Einstein Probe. In this paper, we will introduce the overall configuration and capability of the facility, and give a brief introduction of some calibration results performed with this facility.

Comparing reflection and absorption models for the soft X-ray variability in the NLS1 AGN UGC 11763

ABSTRACT We present a spectral analysis of two XMM–Newton observations of the narrow-line Seyfert 1 galaxy UGC 11763. UGC 11763 shows very different soft X-ray spectral shapes in the two observations separated by 12 yr. Three spectral models are considered to explain the multi-epoch X-ray variability of UGC 11763, one based on the relativistic disc reflection model, one based on multiple partially covering absorbers combined with the warm corona model, and a hybrid model. In the first model, the X-ray variability of UGC 11763 is caused by the emission from a compact coronal region with a variable size. The resulting disc reflection component changes accordingly. A warm absorption model with a modest column density is required in this model too. In the partially covering absorption scenario, the X-ray variability of UGC 11763 is caused by the variable covering factors of two absorbers located within a region of r < ≈100rg. Moreover, the temperature and strength of the warm corona have to change significantly too to explain the variable underlying soft X-ray emission. Lastly, we investigate the possibility of variable intrinsic power-law emission from the hot corona combined with variable absorption in UGC 11763 without changing the geometry of the corona in the third model. This hybrid model provides a slightly better fit than the partially covering absorption model with improvements in fitting the iron emission band. Current CCD-resolution data cannot distinguish these spectral models for UGC 11763. Future high-resolution X-ray missions, e.g. Athena and XRISM, will test them by resolving different spectral components.

The X-ray disc/wind degeneracy in AGN

ABSTRACT Relativistic Fe K emission lines from accretion discs and from disc winds encode key information about black holes, and their accretion and feedback mechanisms. We show that these two processes can in principle produce indistinguishable line profiles, such that they cannot be disentangled spectrally. We argue that it is likely that in many cases both processes contribute to the net line profile, and their relative contributions cannot be constrained purely by Fe K spectroscopy. In almost all studies of Fe K emission to date, a single process (either disc reflection or wind Compton scattering) is assumed to dominate the total line profile. We demonstrate that fitting a single process emission model (pure reflection or pure wind) to a hybrid line profile results in large systematic biases in the estimates of key parameters, such as mass outflow rate and spin. We discuss various strategies to mitigate this effect, such as including high-energy data covering the Compton hump, and the implications for future X-ray missions.

A Detection of Red Noise in PSR J1824–2452A and Projections for PSR B1937+21 Using NICER X-Ray Timing Data

Abstract We have used X-ray data from the Neutron Star Interior Composition Explorer (NICER) to search for long-timescale temporal correlations (“red noise”) in the pulse times of arrival (TOAs) from the millisecond pulsars PSR J1824−2452A and PSR B1937+21. These data more closely track intrinsic noise because X-rays are unaffected by the radio-frequency-dependent propagation effects of the interstellar medium. Our search yields strong evidence (natural log Bayes factor of 9.634 ± 0.016) for red noise in PSR J1824−2452A, but the search is inconclusive for PSR B1937+21. In the interest of future X-ray missions, we devise and implement a method to simulate longer and higher-precision X-ray data sets to determine the timing baseline necessary to detect red noise. We find that the red noise in PSR B1937+21 can be reliably detected in a 5 yr mission with a TOA error of 2 μs and an observing cadence of 20 observations per month compared to the 5 μs TOA error and 11 observations per month that NICER currently achieves in PSR B1937+21. We investigate detecting red noise in PSR B1937+21 with other combinations of observing cadences and TOA errors. We also find that time-correlated red noise commensurate with an injected stochastic gravitational-wave background having an amplitude of A GWB = 2 × 10−15 and spectral index of timing residuals of γ GWB = 13/3 can be detected in a pulsar with similar TOA precision to PSR B1937+21. This is with no additional red noise in a 10 yr mission that observes the pulsar 15 times per month and has an average TOA error of 1 μs.

A multiwavelength-motivated X-ray model for the Circinus Galaxy

ABSTRACT Reprocessed X-ray emission in active galactic nuclei can provide fundamental information about the circumnuclear environments of supermassive black holes. Recent mid-infrared studies have shown evidence of an extended dusty structure perpendicular to the torus plane. In this work, we build a self-consistent X-ray model for the Circinus Galaxy including the different physical components observed at different wavelengths and needed to reproduce both the morphological and spectral properties of this object in the mid-infrared. The model consists of four components: the accretion disc, the broad-line region (BLR), a flared disc in the equatorial plane, and a hollow cone in the polar direction. Our final model reproduces well the 3–70 keV Chandra and NuSTAR spectra of Circinus, including the complex Fe Kα zone and the spectral curvature, although several additional Gaussian lines, associated with either ionized iron or broadened Fe Kα/Kβ lines, are needed. We find that the flared disc is Compton-thick ($N_{\rm H,d}= \rm 1.01^{+0.03}_{-0.24}\times 10^{25}\, cm^{-2}$) and geometrically thick ($\mathrm{ CF}=0.55^{+0.01}_{-0.05}$), and that the hollow cone has a Compton-thin column density ($N_{\rm H,c}= \rm 2.18^{+0.47}_{-0.43}\times 10^{23}\, cm^{-2}$), which is consistent with the values inferred by mid-infrared studies. Including also the BLR, the effective line-of-sight column density is $N_{\rm H}= \rm 1.47^{+0.03}_{-0.24}\times 10^{25}\, cm^{-2}$. This approach to X-ray modelling, i.e. including all the different reprocessing structures, will be very important to fully exploit data from future X-ray missions.

On the dynamical and morphological state of the CHEX-MATE clusters

The CHEX-MATE sample was built to provide an overview of the statistical properties of the underlying cluster population and to set the stage for future X-ray missions. In this work, we perform a morphological analysis of the 118 clusters included in the sample with the aim to provide a classification of their dynamical state which will be useful for future studies of the collaboration.

Scattering efficiencies measurements of soft protons at grazing incidence from an Athena Silicon Pore Optics sample

Soft protons are a potential threat for X-ray missions using grazing incidence optics, as once focused onto the detectors they can contribute to increase the background and possibly induce radiation damage as well. The assessment of these undesired effects is especially relevant for the future ESA X-ray mission Athena, due to its large collecting area. To prevent degradation of the instrumental performance, which ultimately could compromise some of the scientific goals of the mission, the adoption of ad-hoc magnetic diverters is envisaged. Dedicated laboratory measurements are fundamental to understand the mechanisms of proton forward scattering, validate the application of the existing physical models to the Athena case and support the design of the diverters. In this paper we report on scattering efficiency measurements of soft protons impinging at grazing incidence onto a Silicon Pore Optics sample, conducted in the framework of the EXACRAD project. Measurements were taken at two different energies, sim 470 keV and sim 170 keV, and at four different scattering angles between 0.6∘ and 1.2∘. The results are generally consistent with previous measurements conducted on eROSITA mirror samples, and as expected the peak of the scattering efficiency is found around the angle of specular reflection.

Impact of the Returning Radiation on the Analysis of the Reflection Spectra of Black Holes

Abstract A fraction of the electromagnetic radiation emitted from the surface of a geometrically thin and optically thick accretion disk of a black hole returns to the disk because of the strong light bending in the vicinity of the compact object (returning radiation). While such radiation clearly affects the observed spectrum of the source, it is often neglected in theoretical models. In the present paper, we study the impact of the returning radiation on relativistic reflection spectra. Assuming neutral material in the disk, we estimate the systematic uncertainties on the measurement of the properties of the system when we fit the data with a theoretical model that neglects the returning radiation. Our NICER simulations show that the inclination angle of the disk and the black hole spin parameter tend to be overestimated for low viewing angles, while no clear bias is observed for high viewing angles. The iron abundance of the disk is never overestimated. In the most extreme cases (in particular, for maximally rotating black holes), the returning radiation flattens the radial emissivity beyond a few gravitational radii. In such cases, it also produces residuals that cannot be compensated for by adjusting the parameters of models that neglect the returning radiation. This may be an important issue for the interpretation of data from future X-ray missions (e.g., Athena). When we simulate some observations with NuSTAR and fit data above 10 keV, we find that some conclusions that are valid for the NICER simulations are no longer true (e.g., we can obtain a high iron abundance).

Intrusion of MeV–TeV Cosmic Rays into Molecular Clouds Studied by Ionization, the Neutral Iron Line, and Gamma Rays

Abstract Low-energy (∼ MeV) cosmic rays (CRs) ionize molecular clouds and create the neutral iron line (Fe i Kα) at 6.4 keV. On the other hand, high-energy (≳GeV) CRs interact with the dense cloud gas and produce gamma rays. Based on a one-dimensional model, we study the spatial correlation among ionization rates of gas, 6.4 keV line fluxes, and gamma-ray emissions from a molecular cloud illuminated by CRs accelerated at an adjacent supernova remnant. We find that the spatial distributions of these three observables depend on how CRs intrude the cloud and on the internal structure of the cloud. If the intrusion is represented by slow diffusion, the 6.4 keV line should be detected around the cloud edge where ionization rates are high. On the other hand, if CRs freely stream in the cloud, the 6.4 keV line should be observed where gamma rays are emitted. In the former, the cooling time of the CRs responsible for the 6.4 keV line is shorter than their cloud crossing time, and it is opposite in the latter. Although we compare the results with observations, we cannot conclude whether the diffusion or the free-streaming is dominantly realized. Our predictions can be checked in more detail with future X-ray missions such as XRISM and Athena and by observations of ionization rates that cover wider fields.

Analysis of the Unconcentrated Background of the EPIC pn Camera on Board XMM-Newton

Abstract Our understanding of the background of the EPIC/pn camera on board XMM-Newton is incomplete. This affects the study of extended sources and can influence the predictions of the expected background of future X-ray missions, such as the Advanced Telescope for High Energy Astrophysics (ATHENA). Here we provide new results based on the analysis of the largest data set ever used. We focus on the unconcentrated component of the EPIC/pn background, supposedly related to cosmic rays interacting with detector and telescope structures. We show that the so-called out field-of-view region of the pn detector is actually exposed to the sky. After carefully cleaning from the sky contamination, the unconcentrated background measured in the out field-of-view region does not show significant spatial variations, and its time behavior is anticorrelated with the solar cycle. We find a very tight linear correlation between unconcentrated backgrounds detected in the EPIC/pn and EPIC/MOS2 cameras. This relationship permits the correct evaluation of the pn unconcentrated background of each exposure on the basis of MOS2 data, avoiding the use of the contaminated out field-of-view region of the pn, as done in standard techniques. We find a tight linear correlation between the pn unconcentrated background and the proton flux in the 630–970 MeV energy band, as measured by the EPHIN instrument on board SOHO. Through this relationship, we quantify the contribution of cosmic-ray interaction to the pn unconcentrated background. This reveals a second source that contributes to the pn unconcentrated background for a significant fraction (30%–70%). This agent does not depend on the solar cycle or vary with time and is roughly isotropic. After having ruled out several candidates, we find that the hard X-ray photons of the cosmic X-ray background satisfy all known properties of the constant component. Our findings provide an important observational confirmation of simulation results on ATHENA and suggest that a high-energy particle monitor could contribute decisively to the reproducibility of the background for both experiments on ATHENA.

OUP accepted manuscript

Blazars are some of the brightest UV and X-ray sources in the sky and are valuable probes of the elusive warm-hot intergalactic medium (WHIM; $T{\simeq} 10^5-10^7$ K). However, many of the brightest blazars$-$called BL Lac objects such as 1ES1553+113$-$have quasi-featureless spectra and poorly constrained redshifts. Here, we significantly improve the precision of indirect redshift constraints for blazars based on the edge of the $\rm{H\,I}$ Ly$\alpha$ forest observed in their UV spectra. We develop a robust technique to constrain the redshift of a $z<0.5$ AGN or blazar with a $1\sigma$ uncertainty of ${\approx}0.01$ using only the position of its highest-redshift Ly$\alpha$ absorber with $\log N_{\rm{H\,I}}/{\rm cm^{-2}} > 12.6$. We use a large sample of 192 AGN/QSOs at $0.01\lesssim z\lesssim0.45$ that have high-quality COS FUV spectra to characterize the intrinsic scatter in the gap between the AGN redshift and the edge of their Ly$\alpha$ forest. We present new COS NUV data for 1ES1553+113 and confirm its redshift of $z=0.433$ using our technique. We apply our Ly$\alpha$-forest-based redshift estimation technique to nine additional blazars with archival ${\it HST}$ UV spectra, most of which are key targets for future X-ray missions. Our inferred redshift constraints improve estimates for two BL Lacs (1ES1118+424 and S50716+714) and are consistent with previous estimates for the rest. Our results emphasize the need to obtain further UV spectra of bright blazars, of which many have uncertain redshifts, in order to maximize the scientific value of future X-ray WHIM observations that will improve our understanding of galaxy evolution.

Prediction and Understanding of Soft-proton Contamination in XMM-Newton: A Machine Learning Approach

Abstract One of the major and unfortunately unforeseen sources of background for the current generation of X-ray telescopes are few tens to hundreds of keV (soft) protons concentrated by the mirrors. One such telescope is the European Space Agency’s (ESA) X-ray Multi-Mirror Mission (XMM-Newton). Its observing time lost due to background contamination is about 40%. This loss of observing time affects all the major broad science goals of this observatory, ranging from cosmology to astrophysics of neutron stars and black holes. The soft-proton background could dramatically impact future large X-ray missions such as the ESA planned Athena mission (http://www.the-athena-x-ray-observatory.eu/). Physical processes that trigger this background are still poorly understood. We use a machine learning (ML) approach to delineate related important parameters and to develop a model to predict the background contamination using 12 yr of XMM-Newton observations. As predictors we use the location of the satellite and solar and geomagnetic activity parameters. We revealed that the contamination is most strongly related to the distance in the southern direction, Z (XMM-Newton observations were in the southern hemisphere), the solar wind radial velocity, and the location on the magnetospheric magnetic field lines. We derived simple empirical models for the first two individual predictors and an ML model that utilizes an ensemble of the predictors (Extra-Trees Regressor) and gives better performance. Based on our analysis, future missions should minimize observations during times associated with high solar wind speed and avoid closed magnetic field lines, especially at the dusk flank region in the southern hemisphere.