DISCOVERY OF A SUPERLUMINAL Fe K ECHO AT THE GALACTIC CENTER: THE GLORIOUS PAST OF Sgr A* PRESERVED BY MOLECULAR CLOUDS

By G. Ponti, R. Terrier, A. Goldwurm, G. Belanger, G. M. L. Apc, Paris, U. Southampton, CNRS-CEASaclay, ESA-ESAC Madrid. 2010

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We present the result of a study of the X-ray emission from the galactic center (GC) molecular clouds (MCs) within 15 arcmin from Sgr A*. We use XMM-Newton data (about 1.2 Ms of observation time) spanning about eight years. The MC spectra show all the features characteristic of reflection: (1) intense Fe Kα, with equivalent width of about 0.7–1 keV, and the associated Kβ line; (2) flat power-law continuum, and (3) a significant Fe K edge (τ ∼ 0.1–0.3). The diffuse low ionization Fe K emission follows the MC distribution, nevertheless not all MC are Fe K emitters. The long baseline monitoring allows the characterization of the temporal evolution of the MC emission. A complex pattern of variations is shown by the different MCs, with some having constant Fe K emission, some increasing, and some decreasing. In particular, we observe an apparent superluminal motion of a light front illuminating a molecular nebula. This might be due to a source outside the MC (such as Sgr A* or a bright and long outburst of a X-ray binary), though it cannot be due to low energy cosmic rays or a source located inside the cloud. We also observe a decrease of the X-ray emission from G0.11-0.11, behavior similar to that of Sgr B2. The line intensities, clouds dimensions, columns densities, and positions with respect to Sgr A* are consistent with being produced by the same Sgr A* flare. The required high luminosity (about 1.5 × 1039 erg s−1) can hardly be produced by a binary system, while it is in agreement with a flare of Sgr A* fading about 100 years ago. The low intensity of the Fe K emission coming from the 50 and the 20 km s−1 MC places an upper limit of 1036 erg s−1 to the mean luminosity of Sgr A* in the last 60–90 years. The Fe K emission and variations from these MC might have been produced by a single flare of Sgr A*.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/5ad41a7bcff81262ad6192eeaba710cfcf960616

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Citations in the corpus

A CORRELATION BETWEEN STAR FORMATION RATE AND AVERAGE BLACK HOLE ACCRETION IN STAR-FORMING GALAXIES (2013, background): “There is also observational evidence in the X-ray Fe Kα echoes from the quiescent SMBH in the center of the Milky Way, suggesting it might have been a low-luminosity AGN a few hundred years ago (e.g. Revnivtsev et al. 2004; Ponti et al. 2010; Nobukawa et al. 2011).”
Cool outflows in galaxies and their implications (2020, background): “10−5), based on the detection of strong fluorescent Fe Kα line emission off of molecular clouds near the GC (Sunyaev et al. 1993; Koyama et al. 1996; Ponti et al. 2010).”
BLACK HOLE VARIABILITY AND THE STAR FORMATION–ACTIVE GALACTIC NUCLEUS CONNECTION: DO ALL STAR-FORMING GALAXIES HOST AN ACTIVE GALACTIC NUCLEUS? (2013, background): “…of X-ray emission off molecular clouds indicate that the Galactic Center varied in brightness by up to∼ 103 times over the past ∼ 500 years (e.g., Ponti et al. 2010; Capelli et al. 2012; Gando Ryu et al. 2012), while the observed “Fermi bubbles” observed in γ-ray emission may be relics of an…”
What drives the growth of black holes (2011, background): “These data suggest that Sgr A* may have been up-to ≈ 3 orders of magnitude brighter ≈ 100 years ago (e.g., Muno et al., 2007; Ponti et al., 2010).”
GIANT GAMMA-RAY BUBBLES FROM FERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND? (2010, unknown): “The luminosity is ∼1.5×1039 erg s−1 in 2 − 200 keV with a power law spectrum dN/dE ∝ E−γ with γ=1.8±0.2 (Ponti et al. 2010; Revnivtsev et al. 2004; Nobukawa et al. 2008).”
Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ∼105 yr (2015, background): “X-ray light echoes in the Galactic Center show moderate-luminosity ﬂares in the last few centuries (Revnivtsev et al. 2004; Ponti et al. 2010), though these were likely too short and not sufﬁciently luminous to generate narrow lines across the Milky Way.”
Observational Signatures of Particle Acceleration in Supernova Remnants (2012, background): “Besides, the observation of an apparent superluminal propagation of the 6.4 keV emission in a molecular structure (Ponti et al., 2010) is also a clear sign that the clouds are in fact X-Ray Reflection Nebulae.”