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Register a new userNew Constraints on Cosmic Particle Populations at the Galactic Center using X-ray Observations of the Molecular Cloud Sagittarius B2
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Located $sim100$ pc from the dynamic center of the Milky Way, the molecular cloud Sagittarius B2 (Sgr B2) is the most massive such object in the Galactic Center region. In X-rays, Sgr B2 shows a prominent neutral Fe K$alpha$ line at 6.4 keV and continuum emission beyond 10 keV, indicating high-energy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have continued to decrease since the year 2001 as it reprocesses what are likely one or more past energetic outbursts from the supermassive black hole Sagittarius A*. The X-ray reflection model explains the observed time-variability of the Fe K$alpha$ and hard X-ray emissions, and it provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, recent interest has also focused on X-rays from Sgr B2 as a probe of low-energy (sub-GeV) cosmic particles. In contrast to the time-varying X-ray reflection, in this case we can assume that the X-ray flux contribution from interactions of low-energy cosmic particles is constant in time, such that upper limits on low-energy cosmic particle populations may be obtained using the lowest flux levels observed from the cloud. Here, we present the most recent and correspondingly dimmest NuSTAR and XMM-Newton observations of Sgr B2, from 2018. These reveal small-scale variations within lower density portions of the Sgr B2 complex, including brightening features, yet still enable the best upper limits on X-rays from low-energy cosmic particles in Sgr B2. We also present Fe K$alpha$ fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic particle interactions throughout the cloud.
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The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. Alternatively, low-energy cosmic rays propagating inside the clouds might account for the observed emission, through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons. Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure. The measured decay time is compatible with the light crossing time of the molecular cloud core . Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago.
Using {em Chandra} observations in the 2.15 deg$^{2}$ COSMOS legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3-7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT$sim$0.27 and 0.07 keV and an extragalactic power-law with photon spectral index $Gamma$=1.45$pm{0.02}$. The 1 keV normalization of the extragalactic component is 10.91$pm{0.16}$ keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$. Removing all X-ray detected sources, the remaining unresolved CXB is best-fit by a power-law with normalization 4.18$pm{0.26}$ keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$ and photon spectral index $Gamma$=1.57$pm{0.10}$. Removing faint galaxies down to i$_{AB}sim$27-28 leaves a hard spectrum with $Gammasim$1.25 and a 1 keV normalization of $sim$1.37 keV cm$^{-2}$ s$^{-1}$ sr$^{-1}$ keV$^{-1}$. This means that $sim$91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute $sim 8-9%$ of the total CXB show a marginal evidence of being harder and possibly more obscured than resolved sources. Another $sim$1% of the CXB can be attributed to still undetected star forming galaxies and absorbed AGN. According to these limits, we investigate a scenario where early black holes totally account for non source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the $zsim$6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with N$_{H}>$1.6$times$10$^{25}$ cm$^{-2}$ and form in extremely low metallicity environments $(Z_odot)sim10^{-3}$.
Diffuse X-rays from the Galactic center (GC) region were found to exhibit many K-shell lines from iron and nickel atoms in the 6--9 keV band. The strong emission lines seen in the spectrum are neutral iron K$alpha$ at 6.4~keV, He-like iron K$alpha$ at 6.7~keV, H-like iron Ly$alpha$ at 6.9~keV, and He-like iron K$beta$ at 7.8~keV. Among them, the 6.4~keV emission line is a probe of non-thermal phenomena. We have detected strong 6.4~keV emission in several giant molecular clouds, some of which were newly discovered by Suzaku. All the spectra exhibit large equivalent widths of 1-2~keV and absorption columns of $2-10times 10^{23}{rm H cm}^{-2}$. We found time variability of diffuse 6.4~keV emission in the Sgr B2 region comparing the maps and spectra obtained from 1994 to 2005 with ASCA, Chandra, XMM-Newton and Suzaku. We also report discovery of K$alpha$ lines of neutral argon, calcium, chrome, and manganese atoms in the Sgr~A region. We show that the equivalent width of the 6.4~keV emission line detected in X-ray faint region against the 6.4 keV-associated continuum (power-law component) is $sim 800 {rm eV}$. These features are naturally explained by the X-ray reflection nebula scenario rather than the low energy cosmic-ray electrons scenario. On the other hand, a 6.4~keV clump, G~0.162$-$0.217, discovered at the south end of the Radio Arc has a small equivalent width of 6.4~keV emission line of $sim200 {rm eV}$. The Radio Arc is a site of relativistic electrons. Thus, it is conceivable that the X-rays of G~0.162$-$0.217 are due to low energy cosmic-ray electrons
Here we report the detection of a 626 s periodic modulation from the X-ray source 2XMM J174016.0-290337 located in the direction of the Galactic center. We present temporal and spectral analyses of archival XMM-Newton data and photometry of archived near-infrared data in order to investigate the nature of this source. We find that the X-ray light curve shows a strong modulation at 626 +/- 2 s with a confidence level > 99.9% and a pulsed fraction of 54%. Spectral fitting demonstrates that the spectrum is consistent with an absorbed power law. No significant spectral variability was observed over the 626 s period. We have investigated the possibility that the 626 s period is orbital in nature (either that of an ultra-compact X-ray binary or an AM CVn) or related to the spin of a compact object (either an accretion powered pulsar or an intermediate polar). The X-ray properties of the source and the photometry of the candidate near-infrared counterparts are consistent with an accreting neutron star X-ray binary on the near-side of the Galactic bulge, where the 626 s period is most likely indicative of the pulsar spin period. However, we cannot rule out an ultra-compact X-ray binary or an intermediate polar with the data at hand. In the former case, if the 626 s modulation is the orbital period of an X-ray binary, it would be the shortest period system known. In the latter case, the modulation would be the spin period of a magnetic white dwarf. However, we find no evidence for absorption dips over the 626 s period, a low temperature black body spectral component, or Fe Kalpha emission lines. These features are commonly observed in intermediate polars, making 2XMM J174016.0-290337 a rather unusual member of this class if confirmed. We instead suggest that 2XMM J174016.0-290337 could be a new addition to the emerging class of symbiotic X-ray binaries.
We constrain X-ray spectral shapes for the ensemble of AGN based on the shape of the Cosmic X-ray Background (CXB). Specifically, we rule out regions of X-ray spectral parameter space that do not reproduce the CXB in the energy range 1-100 keV. The key X-ray spectral parameters are the photon index, {Gamma}; the cutoff energy, Ecutoff; and the reflection scaling factor, R. Assuming each parameter follows a Gaussian distribution, we first explore the parameter space using a Bayesian approach and a fixed X-ray luminosity function (XLF). For {sigma}_E = 36 keV and {sigma}_R = 0.14, fixed at the observed values from the Swift-BAT 70-month sample, we allow <R>, <Ecutoff > and <{Gamma}> to vary subject to reproducing the CXB. We report results for {sigma}_{Gamma} = 0.1-0.5. In an alternative approach, we define the parameter distributions, then forward model to fit the CXB by perturbing the XLF using a neural network. This approach allows us to rule out parameter combinations that cannot reproduce the CXB for any XLF. The marginalized conditional probabilities for the four free parameters are: <R> = 0.99^{+0.11}_{-0.26}, <Ecutoff> = 118^{+24}_{-23}, {sigma}_{Gamma} = 0.101^{+0.097}_{-0.001} and <{Gamma}> = 1.9^{+0.08}_{-0.09}. We provide an interactive online tool for users to explore any combination of <Ecutoff>, {sigma}_E, <{Gamma}>, {sigma}_{Gamma}, <R> and {sigma}_R including different distributions for each absorption bin, subject to the integral CXB constraint. The distributions observed in many AGN samples can be ruled out by our analysis, meaning these samples can not be representative of the full AGN population. The few samples that fall within the acceptable parameter space are hard X-ray-selected, commensurate with their having fewer selection biases.
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