Discovery of Variable Iron Fluorescence from Reflection Nebulae in the Galactic Center

“Finally, the absolute astrometric frame was registered using X-ray sources with counterparts in the 2MASS catalog (Skrutskie et al. 2006), making it accurate to 0.”

“To generate these, we first calculated the one-sided, upper 1σ Poisson uncertainty on the counts in each pixel of each image, following Gehrels (1986).”

“…with electrons would require a large, and arguably unreasonable, amount of energy in keV electrons (Revnivtsev et al. 2004b), and there is currently no X-ray source in the Galactic center bright enough to illuminate these features (Sunyaev et al. 1993; Koyama et al. 1996; Revnivtsev et al. 2004a).”

“However, it is uncommon for the outburst of an X-ray binary to persist at this luminosity for 2–3 years (Chen et al. 1997)8, so we prefer the hypothesis that the source of the outburst was Sgr A∗.”

“The pervasive nature of these variations is not surprising, becuase then entire field contains molecular gas (Tsuboi et al. 1999) and exhibits diffuse iron Kα emission (Muno et al. 2004).”

“The pervasive nature of these variations is not surprising, becuase then entire field contains molecular gas (Tsuboi et al. 1999) and exhibits diffuse iron Kα emission (Muno et al. 2004).”

“…with electrons would require a large, and arguably unreasonable, amount of energy in keV electrons (Revnivtsev et al. 2004b), and there is currently no X-ray source in the Galactic center bright enough to illuminate these features (Sunyaev et al. 1993; Koyama et al. 1996; Revnivtsev et al. 2004a).”

“Although this is 100 times brighter than the outbursts in 2002 and 2004 (Porquet et al. 2005; Muno et al. 2005), this outburst would have occurred 20 years ago, before the advent of both Chandra and the wide-field X-ray monitors on BeppoSAX and the Rossi X-ray Timing Explorer, so it too could have…”

“…with electrons would require a large, and arguably unreasonable, amount of energy in keV electrons (Revnivtsev et al. 2004b), and there is currently no X-ray source in the Galactic center bright enough to illuminate these features (Sunyaev et al. 1993; Koyama et al. 1996; Revnivtsev et al. 2004a).”

“The most likely source of the reflected X-rays is an outburst of Sgr A∗ with a luminosity of at least 1038 erg s−1, which would have occurred about 50 years ago and lasted 2–3 years.”

“The Fe Kβ emission is ≈10-15% of the strength of the Kα line, as is expected for fluorescent emission (e.g., Murakami et al. 2000).”

“The fluorescence is thought to be produced when low-ionization iron in the molecular clouds is bombarded either by ∼30 keV electrons (Valinia et al. 2000; Yusef-Zadeh, Law, & Wardle 2002; Bykov 2002) or by ∼8 keV X-rays (Sunyaev & Churazov 1998; Revnivtsev et al. 2004b).”

“One manifestation of these interactions are iron fluorescence nebulae that are visible in the X-ray band, and that are associated with the molecular cloud complexes Sgr B1 and B2 (Murakami et al. 2000, 2001b), Sgr C (Murakami et al. 2001a), and M 0.11-0.11 (Park et al. 2004).”

“…with electrons would require a large, and arguably unreasonable, amount of energy in keV electrons (Revnivtsev et al. 2004b), and there is currently no X-ray source in the Galactic center bright enough to illuminate these features (Sunyaev et al. 1993; Koyama et al. 1996; Revnivtsev et al. 2004a).”

“…are located near α=266.492◦, δ=–28.943◦ (J2000), or 6′ from Sgr A∗, and are covered by all of our observations.5 At this offset, the point-spread function (PSF) at 6 keV has a full-width at half-maximum (FWHM) of 6′′, which corresponds to a physical scale of 0.2 pc (D=8 kpc; McNamara et al. 2000).”

“One manifestation of these interactions are iron fluorescence nebulae that are visible in the X-ray band, and that are associated with the molecular cloud complexes Sgr B1 and B2 (Murakami et al. 2000, 2001b), Sgr C (Murakami et al. 2001a), and M 0.11-0.11 (Park et al. 2004).”

“Such rapid changes over the course of a few years would not be produced by steady, slow-moving sources such as ejecta from supernovae (Bykov 2002).”

“The changes in the intensities and morphologies of hard X-ray nebulosities on parsec scales have been discovered (Muno et al. 2007).”

“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).”

“Nevertheless, the superb imaging capabilities of Chandra allowed the detection of changes in the shape of the emitting regions (Muno et al. 2007).”

“…of studying the propagation of echoes of the past GC activity within the CMZ (Sunyaev et al. 1993; Koyama et al. 1996; 2008; Revnivtsev et al. 2004; Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010; 2013; Terrier et al. 2010; Nobukawa et al. 2011; Capelli et al. 2011; 2012; Clavel et al.…”

“…physics of the flares (as well as to continue detailed study of diffuse emission and transients in the Galactic center; e.g., Baganoff et al. 2003; Muno et al. 2004, 2007, 2009; Park et al. 2004; Ponti et al. 2010; Capelli et al. 2012), Chandra undertook an unprecedented X-ray Visionary Project…”

“…emanating from the molecular clouds at the Galactic center (Sunyaev et al. 1993; Koyama et al. 1996; Murakami et al. 2000; Revnivtsev et al. 2004; Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010, 2013; Terrier et al. 2010; Capelli et al. 2011, 2012; Nobukawa et al. 2011; Ryu et al. 2012).”

“First, Muno et al. (2007) have detected time varying diffuse emission in the 4–8 keV range a few arcmin away from Sgr A comparing Chandra images taken before and after 2004.”

“Capelli et al. (2012) present evidence for the fact that the luminosity of SgrA* decreased during the past 150 years (in good agreement with Muno et al. 2007) with upwards and downwards trends lasting from a few to perhaps 10 years.”

“Nethertheless, none of the peaks reached the ∼ 1039 erg s−1needed to explain the putative flare in Sgr A* X-ray luminosity a few hundred years ago (Revnivtsev et al., 2004, see also Muno et al. (2007)).”

“A similar time variability has also been reported for the other cloud near Sgr A by Muno et al. (2007) and Koyama et al. (2009), which are the time variabilities of a few years span.”

“…big flare may have been that in some 100 yr ago at Sgr A , which is well established by the time variability of the 6.4 keV line from molecular clouds (Muno et al. 2007; Koyama et al. 2008; Inui et al. 2009; Koyama et al. 2009; Ponti et al. 2010; Nobukawa et al. 2011; Capelli et al. 2011, 2012).”

“…by evidence for an eventful history of outbursts within 106 years by the currently quiescent black hole at the Galactic Center, from X-ray echoes (Muno et al. 2007), the ionization structure of the Magellanic Stream (Bland-Hawthorn et al. 2013), and possibly in the “Fermi bubbles” (Zubovas et…”

“First, Muno et al. (2007) found rapid changes in the intensities and morphologies of two relatively compact, hard X-ray continuum nebulosities, a result that could be explained by a moving reflection nebula created by an X-ray front propagating at lightspeed, but which is not consistent with the…”

“Detections of time variability of the 6.4 keV line from Sgr B2 (Inui et al. 2009), as well as from molecular clouds within 15′ to the east of Sgr A∗ (Muno et al. 2007; Ponti et al. 2010), are best explained by the assumption that the Fe Kα line emission from these regions is a fluorescence…”

“Muno et al. (2007) and Koyama et al. (2009) reported variability of the 6.4 keV line from the Sgr A cloud in a short time-scale of 3–5 years at a ∼5σ sigma level.”

“The first such detection was reported by Muno et al. (2007), who showed a ∼ 2− 4 σ continuum variability from filamentary (∼ 0.3× 2 pc) regions that are dominated by Fe Kα emission and coincident with molecular structures near Sgr A (Muno et al. 2007).”

“These features are discussed extensively in Park et al. (2004) and Muno et al. (2007), and are produced because hard X-rays (>7 keV) from a transient source are being scattered by molecular gas, and causing iron to fluoresce.”

“1996; Muno et al. 2007; Su et al. 2010; Terrier et al. 2010; Ponti et al. 2010, 2012).”

“11−0.11, and their Fe Kα emission was attributed to reflection of an outburst of Sgr A* or an Xray binary with a few year duration (Muno et al. 2007).”

“This is consistent with previous findings (e.g., Muno et al. 2007; Ponti et al. 2010; Capelli et al. 2012; Clavel et al. 2013).”

“…cosmic rays (LECR; e.g. Yusef-Zadeh et al. 2002; Dogiel et al. 2009; Tatischeff et al. 2012), the great variability observed in the Sgr A complex (Muno et al. 2007; Ponti et al. 2010; Capelli et al. 2011; Clavel et al. 2013, 2014) and in Sgr B2 (Koyama et al. 2008; Inui et al. 2009; Terrier et…”

“In actual fact, detections of time variability of the iron fluorescent line from Sgr B2 (Koyama et al. 2008; Inui et al. 2009) and molecular clouds around Sgr A (Muno et al. 2007; Ponti et al. 2010) in a timescale of a few years have provided strong support for the XRN model.”

“Moreover, Muno et al. (2007) have reported the evolution in intensity and morphology of the 4–8 keV continuum emission in two filamentary regions located close to Sgr A*.”

“Muno et al.(2007) reported the Chandra discovery of a 2–3 years variability of the 6.40 keV line within 20 pc of Sgr A∗.”

“Interestingly, the clouds exhibit Fe Kα line variability in different ways, some rising, others are decreasing and an emission peak is detected in the “Bridge” structure with a time scale as short as two years (Muno et al. 2007; Ponti et al. 2010; Capelli et al. 2012; Clavel et al. 2013).”

“In addition, Muno et al. (2007) found flickering continuum X-ray emissions from small clouds near the GC.”

“The most direct evidence to favor the X-ray origin is the time variability of the clumps, as was reported by Muno et al. (2007) for the sub-structures of source 1.”

“Variation of Fe K α line intensity across the clouds as well as during repeated observations indicate that Sgr A ∗ is variable on a ∼ (1 − 10) yr time-scale (Muno et al. 2007; Chuard et al. 2018).”

“However, other reflecting nebulae are present around the central black hole, which are also varying with time (Muno et al. 2007(39)).”

“However, strong support for the XRN scenario comes from the variability in the intensity and morphology of the 4–8 keV X-ray emission that Muno et al. (2007) have reported in the Sgr A complex.”

“This region was not studied in detailed in the recent Chandra and XMM-Newton compilations of 6.4- keV cloud properties (Muno et al. 2007; Ponti et al. 2010), although earlier Chandra results for the AC have been reported by Yusef-Zadeh et al. (2002b), Law & Yusef-Zadeh (2004), and Wang et al.…”

“Each net count and exposure time map was then reprojected with the Chandra tool reproject_image_grid to a common, specified grid.”

“4 value varies strongly with position, especially in the central region, and sometimes with time(e.g., Muno et al. 2007; Yuasa et al. 2012).”

“Variable X-ray emission has been recently observed withChandrain a few molecular clouds in the Galactic Center region by Muno et al. (2007).”

“Since the early 90s, many authors have argued that Sgr A* was more active in the past, specifically about hundreds of years ago (Sunyaev et al. 1993; Koyama et al. 1996; Ponti et al. 2010), and even as little as decades ago (Muno et al. 2007).”

“Muno et al. (2007) listed up 2 Fe I-Kα sources named Feature 1 and Feature 2, the same sources as No1 and No2, respectively.”

“One possibility is a time variability, as was found in Sgr B2 and Radio Arc regions (Muno et al. 2007; Koyama et al. 2008; Inui et al. 2009).”

“…in its vicinity (first proposed by Sunyaev et al. (1993) and followed by Koyama et al. (1996); Murakami et al. (2000); Revnivtsev et al. (2004); Muno et al. (2007); Inui et al. (2009); Ponti et al. (2010); Terrier et al. (2010); Capelli et al. (2012); Nobukawa et al. (2011); Gando Ryu et al.…”

“Some of them were also found to be time variable in both flux and morphology (Muno et al. 2007; Koyama et al. 2008; Inui et al. 2009).”

“Theoretically, the accretion of a single clump of a large enough mass could be responsible for the more active past of the SMBH inferred from observations of X-ray echoes (e.g. Sunyaev et al. 1993; Sunyaev & Churazov 1998; Muno et al. 2007; Ponti et al. 2010).”

“…flux at 6.4 keV was indeed observed in the GMC Sgr B2 by Koyama et al 1996, and has been extensively studied ever since (Murakami et al. 2000a; Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010; Terrier et al. 2010; Capelli et al. 2012; Nobukawa et al. 2011; Gando Ryu et al. 2012; Clavel…”

“These observations have shown that the X-ray emission from some of the molecular clouds varies significantly with time (Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010; Terrier et al. 2010; Clavel et al. 2013, 2014; Zhang et al. 2015).”

“…seem to imply that just a few hundred years ago Sgr A* was orders of magnitude brighter than it is currently (Sunyaev et al. 1993, 1998; Koyama et al. 1996; Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010, 2011; Terrier et al. 2010; Nobukawa et al. 2011; Capelli et al. 2012; Ryu et al. 2013).”

“Observationally, it was first reported in Muno et al. (2007) and then found in all regions bright in the reflected emission (e.g. Ponti et al. 2010; Terrier et al. 2010; Capelli et al. 2012; Clavel et al. 2013; Ryu et al. 2013; Mori et al. 2015; Zhang et al. 2015).”

“…of time variability of the X-ray emission from Sgr B2 (see e.g. Nobukawa et al. 2011) and from several clouds within 15′ to the east of Sgr A∗ (Muno et al. 2007; Ponti et al. 2010), strongly suggest that the Fe Kα line emission from these regions is a fluorescence radiation excited by a…”

“In addition, the time evolution of the γ -ray spectrum may be relevant to the recent Fe Kα line maps of the Galactic Centre environment which indicate that Sgr A* was more luminous ∼100 yr ago (e.g. Muno et al. 2007; Inui et al. 2009; Ponti et al. 2010; Terrier et al. 2010).”

“Propagation of X-ray echoes is also detected from other MCs in the CMZ (e.g. Muno et al., 2007; Ponti et al., 2010; Clavel et al., 2013; Ryu et al., 2013; Chuard et al., 2018; Terrier et al., 2018).”

“…supermassive black hole (SMBH) Sgr A∗ over past few hundred years (Sunyaev, Markevitch, & Pavlinsky 1993; Koyama et al. 1996; Revnivtsev et al. 2004; Muno et al. 2007; Ponti et al. 2010; Terrier et al. 2010; Capelli et al. 2012; Ryu et al. 2013; Zhang et al. 2015; Krivonos et al. 2017; Chuard et…”

“In agreement with these simple considerations, it is observed that larger clouds vary on longer time-scales than smaller ones and, in particular, that their halving time is comparable to the light crossing time of their nucleus (Muno et al. 2007; Terrier et al. 2010; Ponti et al. 2010).”

“The hypothesis of a previous Sgr A* outburst is appealing because it implies that the X-ray variability of the CMZ is a fossil memory of how our Galactic nucleus acted a few hundred years ago (Muno et al. 2007).”

“The strong variability of this emission, detected in both Sgr B2 (Inui et al. 2009, Terrier et al. 2010, Nobukawa et al. 2011) and the Sgr A region (Muno et al. 2007, Ponti et al. 2010, Capelli et al. 2012, Clavel et al. 2013), proves that a significant fraction of the diffuse emission correlated…”

“4 keV line emission in several distant clouds (Muno et al. 2007; Inui et al. 2009; Terrier et al. 2010) is contradicting the idea that CRs are producing the 6.”