X-Ray Spectroscopic Evidence for Intermediate-Mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-Ray Sources

By J. Miller, G. Fabbiano, M. Miller, A. M. C. F. Astrophysics, U. Maryland, U. I. D. O. Astronomy. 2002

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We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultraluminous" X-ray (ULX) sources. We measure isotropic luminosities of LX = 2.0 × 1040 ergs s-1 and LX = 6.6 × 1039 ergs s-1 for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 σ level of confidence for X-1; the improvement for X-2 is significant at the 3 σ level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of kT ≃ 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar mass (10 M☉) black holes in their brightest states (kT ≃ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: MBH ≃ 103 M☉. Simple Eddington scaling arguments suggest a minimum mass of MBH ~ 102 M☉. NGC 1313 X-1 and X-2 are found in optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

XMM–Newton observations of the brightest ultraluminous X-ray sources (2006, methodology): “…qualityXMM-Newton/EPIC spectroscopy of ULXs, that it has been demonstrated that the addition of a soft thermal disc component to a power-law continuum spectrum provides a strong statistical improvementto the best fitting models to ULX data (e.g., Miller et al. 2003; Miller, Fabian, & Miller 2004a).”
The Ultraluminous X-Ray Source Population from the Chandra Archive of Galaxies (2004, background): “A few ULX candidates with soft accretion disk components have been reported (e.g., Miller et al. 2003; Roberts & Colbert 2003).”
X-ray pre-ionization powered by accretion on the first black holes – I. A model for the WMAP polarization measurement (2003, background): “The observed spectra of ULX sources tentatively show a subdominant soft component fitted by multicolour discs blackbody with kT disc ∼ 0.15 keV, which has been interpreted as a tentative evidence of intermediate mass BHs (Miller et al. 2003).”
Young Massive Star Clusters (2010, background): “The black holes that may be responsible for the ULXs NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc) may have masses in the range 100–1000M⊙ (Miller et al. 2003), and in these cases YMCs have been identified as optical counterparts.”
Early Reionization by Miniquasars (2003, background): “While the soft components are well fit by cool multicolor disk blackbodies with kTmax ≃ 0.15 keV, which may indicate IMBHs (e.g. Miller et al. 2003), the nonthermal power-law component has spectral slope Lν ∝ ν−α, with α ≈ 1.”
The nature of SS433 and the ultraluminous X-ray sources (2006, methodology): “Strong soft components of this type are indeed seen in ULXs with sufficiently low absorption columns (e.g. Miller et al. 2003).”
Observational evidence for intermediate-mass black holes (2017, background): “…of IMBHs in ULXs came from the fit of the X-ray spectrum by a cool disk (soft excess of 0.1-0.3 keV) plus a power-law tail: if coming from a standard BH, the inverse proportionality between disk temperature and BH mass (Shakura & Sunyaev 1973) implied masses ∼ 104 M (e.g., Miller et al. 2003).”
Intermediate - mass black holes (2003, background): “For example, Miller et al. (2003a) analyzed XMM data of the brightest ULXs in NGC 1313, and found that a two-component fit is necessary (see Figure 7), with an inferred inner disk temperature kTin = 0.15 keV.”
Ultraluminous X-ray sources in the Chandra and XMM-Newton era (2011, background): “Characteristic ages& 10 Myr for the surrounding stellar population were also found for example in the case of NGC 1313 X-1 and X-2 (Yang et al. 2011; Grisé et al. 2008), IC 342 X-1 (Feng & Kaaret 2008), NGC 4559 X-1 (Soria et al. 2005), and NGC 1073 IXO 5 (Kaaret 2005).”
Populations of X-Ray Sources in Galaxies (2006, background): “A very soft component was first reported by Kaaret et al. (2003) for the ULX NGC5408 X-1, and soon after by Miller et al. (2003) for the ULX NGC1313 X-2, with temperatures of ∼110 and 150 eV, respectively.”
BlackCAT: A catalogue of stellar-mass black holes in X-ray transients (2015, background): “The source of these luminosities is still uncertain, and it has been proposed that they may be produced by intermediate mass BHs (∼103 M , see e.g. Miller et al. 2003) or stellar-mass BHs (e.g., Poutanen et al. 2007; Kawashima et al. 2012).”
The Ultraluminous State (2009, background): “2 Here and throughout the paper we distinguish IMBHs as BHs with masses> 100M⊙, as suggested by the X-ray spectral modelling of Miller et al. (2003) and subsequent work.”
Formation of Massive Black Holes in Dense Star Clusters. I. Mass Segregation and Core Collapse (2003, background): “…and whose high X-ray luminosities in many cases suggest a compact object mass of at least ∼ 10 2 M ⊙ (Ebisuzaki et al. 2001; Kaaret et al. 2001; Miller et al. 2003), although beamed emission by an accreting stellarmass BH provides an alternative explanation (King et al. 2001; Zezas &…”
Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates (2007, background): “The origin of IMBHs is unknown, but if they are produced by Pop III stars their number may fall short in explaining the observed ULXs population (e.g., Kuranov et al. 2007; Pelupessy et al. 2007).”
X-ray Source Populations in Galaxies (2003, methodology): “The discovery of low-temperature components in the XMM-Newton spectra of ULXs, such as in the NGC 1313 ULXs, is consistent with emission from an IMBH accretion disk (Miller et al. 2003a).”
Supercritically accreting stellar mass black holes as ultraluminous X-ray sources (2006, background): “A strong argument in favour of IMBHs is the presence of a soft, 0.1–0.2 keV component in their spectra (Kaaret et al. 2003; Miller et al. 2003, 2004).”