On the Nature of X-ray Bright Optically Normal Galaxies

I thought it would be interesting to further delve into the XBONGs noted above in Live Science. As a result I started with Yuan's and Narayan's study.

8 January 2004

By Feng Yuan 1 and Ramesh Narayan 2

1Department of Physics, Purdue University, West Lafayette, IN 47907; fyuan@physics.purdue.edu
2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; narayan@cfa.harvard.edu

Recent X-ray surveys by Chandra and XMM-Newton have revealed a population of X-ray bright, optically normal galaxies (XBONGs) at moderate redshifts. By analogy with nearby low-luminosity active galactic nuclei, we propose that many XBONGs are powered by an inner radiatively inefficient accretion flow (RIAF)* plus an outer standard thin accretion disk. The absence of optical/UV activity in XBONGs is explained by the truncation of the thin disk near the black hole, and the relatively strong X-ray emission is explained as inverse Compton emission** from the hot RIAF. We show that the spectrum of the source P3, a prototypical XBONG, can be fit fairly well with such a model. By comparing P3 to other accreting black holes, we argue that XBONGs are intermediate in their characteristics between distant luminous active galactic nuclei and nearby low-luminosity nuclei.

The existence of this unusual class of sources was already pointed out by Elvis et al. (1981) more than 20 years ago, based on an analysis of Einstein observations. It was later confirmed by Griffiths et al. (1995) using ROSAT data. The sources have been given a variety of names, such as Optically Dull Galaxies (Elvis et al. 1981), Passive Galaxies (Griffiths et al. 1995), X-ray Bright Optically Normal Galaxies (XBONGs, Comastri et al. 2002b), and Elusive Active Galactic Nuclei (Maiolino et al. 2003). We will adopt the name XBONG in this paper. According to Maiolino et al. (2003), the fraction of XBONGs among local galaxies is comparable to or even higher than that of optically selected Seyfert nuclei.

The large X–ray–to–optical flux ratios of XBONGs, as well as their hard spectra in X–rays (at least in the brighter sources for which spectral analysis is possible), suggest that AGN activity is occurring in these objects. The lack of evident optical emission lines is, however, a puzzle. There are several possible explanations.

One explanation is that XBONGs are luminous AGNs that happen to be heavily obscured. The obscuration must be in all directions, not just in a torus, since the sources lack both broad lines and narrow lines in their spectra (Marconi et al. 1994, 2000; Genzel et al. 1998; Spoon et al. 2000; Fabian 2003). Dudley & Wynn-Williams (1997) predicted that a deep silicate absorption at 9.7μm should be detected in this case. The feature has been seen in two XBONGs (NGC 4945: Maiolino et al. 2000; NGC 4418: Spoon et al. 2001), indicating that the complete obscuration idea is correct for at least some XBONGs.

Despite this success, it seems unlikely that the obscuration model applies to all XBONGs. Severgnini et al. (2003) performed a detailed spectral analysis of three XBONGs observed with XMM-Newton and found that only one out of the three is X-ray obscured (NH ≈ 2 × 1023 cm−2), while the other two sources are relatively unobscured (NH ≈ 4 × 1021, 1 × 1021 cm−2, respectively). A similar result was obtained by Page et al. (2003), who carried out optical spectroscopy of a number of X-ray sources from the 13 hr XMM- Newton/Chandra deep survey. Of their 70 sources, 23 were found to be XBONGs, half of which were unabsorbed in X-rays.

A second explanation for XBONGs is that proposed by Moran, Filippenko, & Chornock (2002) and Severgnini et al. (2003) according to which the lack of significant emission lines in XBONGs may be due to dilution of the nuclear spectrum by starlight from the host galaxy. However, the intrinsic optical continuum emission from XBONGs is weaker than in luminous, quasar-like Active Galactic Nuclei or AGNs (e.g., Comastri et al 2002b), which suggests that the emission-line luminosities are also intrinsically low. In terms of the spectral index αox (defined by ̊Fν ∝ ν−αox ) between 2500 A and 2 keV, Severgnini et al. (2003) find that αox ≈ 1.2 for a small sample of XBONGs. This value is systematically smaller than αox ≈ 1.5 for luminous AGNs (Brandt, Laor & Wills 2000).

A third possibility is that XBONGs are BL Lac-like objects***. Observations by Chandra and XMM-Newton of at least one XBONG, the source “P3” (§2.2), indicate no significant flux or spectral variability over a time interval of seven months (Comastri et al. 2002a). This is highly unusual for a BL Lac object. The presence of a large calcium break and a lack of detectable radio emission in several XBONGs (Fiore et al. 2000) are additional arguments against the BL Lac model. Nevertheless, this model is hard to rule out and may well describe some XBONGs, though it seems unlikely to apply to a majority of XBONGs.

The last possibility, the one we focus on in this paper, is that the intrinsic weakness of XBONGs in the optical/UV waveband is because these sources lack an optically-thick accretion disk at small radii. Instead of a cool disk, we suggest that the gas at small radii is in the form of a very hot radiatively inefficient accretion flow (RIAF, also known as an advection-dominated accretion flow or ADAF).

We have investigated in this paper the nature of the population of X-ray bright, optically normal galaxies, or XBONGs. While the absence of optical activity in some XBONGs may be due to obscuration, this explanation is unlikely to apply to all XBONGs. We consider the possibility that many XBONGs possess radiatively inefficient accretion flows (RIAFs, formerly ADAFs). According to this model, the accretion flow occurs as a geometrically- thin optically-thick disk for radii larger than a transition radius Rtr, and as an optically-thin RIAF for radii below Rtr. Because of the absence of an optically-thick disk at small radii, there is very little optical or UV emission, and therefore very little broad or narrow line emission. Since RIAFs are possible only at small accretion rates (Narayan & Yi 1995; Esin et al. 1997), XBONGs are expected to populate the low-redshift, low-luminosity end of the AGN distribution. This is confirmed by observations.

XBONGs are spectrally very similar to LLAGNs in our local universe as well as some Seyfert 1 galaxies****. All of these sources have weak optical/UV emission and relatively large ̊X-ray fluxes. The spectral index αox between 2500A and 2keV is ≈ 1.2 for XBONGs, ≈ 1.1 for the Seyfert 1 galaxy NGC 5548, and ≈ 0.9 for a small sample of LLAGNs. These values are small compared to αox ≈ 1.5 for luminous AGNs. Both LLAGNs and NGC 5548 have been successfully modeled by RIAFs (Quataert et al. 1999; Chiang & Blaes 2003). The case for considering a similar model for XBONGs is thus strong. To test this proposal, we have modeled two XBONGs, Source #1 (§2.1) and P3 (§2.2). We find that the RIAF model fits the available spectral data on both sources reasonably well. We also find that any model in which the thin disk comes all the way down to the innermost stable circular orbit (ISCO) is inconsistent with the data.

It is useful to place XBONGs in the context of other accreting black hole sources. Figure 3 shows the transition radius Rtr in Schwarzschild units and the luminosity L in Eddington units for a number of objects whose spectra have been fitted with the RIAF model. Luminous black hole sources, including AGNs at high redshifts and black hole X-ray binaries in the high soft state, have standard radiatively efficient accretion disks extending down to the ISCO. When the luminosity is below a certain value, say ≈ 0.03LEdd, an advection-dominated RIAF is allowed (Esin et al. 1998), and it is postulated that the thin disk is then truncated and the innermost region of the flow is replaced by a RIAF. It is also expected that the lower the luminosity L/LEdd the larger the transition radius Rtr/Rs (e.g., Narayan & Yi 1995; Esin et al. 1997; Narayan et al. 1998; Ro ́zan ́ska & Czerny 2000; Manmoto & Kato 2000). Such a correlation is clearly seen in Figure 3. In order of decreasing luminosity L/LEdd, the sources shown are: 1) X-ray binaries in the soft state and bright AGNs; 2) X-ray binaries in the hard state and Seyfert 1 galaxies; 3) XBONGs; 4) LLAGNs; 5) X-ray binaries in the quiescent state; 6) ultra-dim AGNs.

Note that we have analyzed only two XBONGs in this paper, both of which appear to be brighter than typical LLAGN when measured in Eddington units. As a class, however, XBONGs are quite heterogeneous, and it is possible that other XBONGs are similar in luminosity to LLAGNs, or perhaps even dimmer.

One caveat to note is that our estimates of Rtr/RS for Source #1 and P3 are sensitive to the assumed optical fluxes of the two sources. The optical flux of Source #1 was obtained by Severgnini et al. (2003) by an indirect method, while P3 has no optical measurement and we have arbitrarily assumed the same flux as in Source #1. Our estimates of the transition radii are thus rather uncertain.

Finally, while we have suggested in this paper that a RIAF is present in many XBONGs, we do not mean to imply that the RIAF necessarily produces the entire emission. It is possible, for instance, that some of the radiation (e.g., radio) comes from a jet. The BL Lac-type model mentioned in §1 is an extreme example of this scenario in which most of the emission comes from a highly relativistic and beamed flow. Another significant zone of emission is the base of the jet, or the region where the jet and the disk meet (e.g., see Livio, Pringle & King 2003). The Doppler factor here is expected to be small so variability should be weak. A model of this region has been successfully worked out for Sgr A* (Yuan, Markoff, & Falcke 2002a) and NGC 4258 (Yuan et al. 2002b).

See: https://arxiv.org/pdf/astro-ph/0401117.pdf

* Radiatively inefficient accretion flow: RIAF - Black hole accretion flows could be radiatively inefficient when a mass supply rate is either much less or much larger than the Eddington rate. Because of lack of radiative cooling, such flows are hot and geometrically thick. Initially, the flows were thought to be purely hydrodynamic viscous flows. It was shown analytically and numerically that the hydrodynamic flows are convectively unstable. Convection significantly changes the flow structure, flattening the radial density profile and decreasing the accretion rate. More recently, numerical MHD models† of the flows have been developed by several groups. Models show the problems of angular momentum transport and dissipation of binding energy can be solved self-consistently. The MHD models show the flow's strong dependence on the topology of the magnetic field, and some models show efficient convection.

† - MHD or Magnetohydrodynamics is the study of electrically conducting fluids. Models in which plasma is treated as a perfectly conducting fluid (ideal MHD) are the most successful models for describing the equilibrium and large-scale stability properties of magnetized plasmas. Resistive-MHD models, in which the plasma is treated as having finite resistivity, are used to study tearing modes and magnetic reconnection†*, which may change the topology of the magnetic field and convert the energy of the magnetic field into thermal energy. Still more complex models, which may treat the ions and electrons as separate species (two-fluid MHD) or include nontrivial fluid closures, are often called extended MHD models.

†* - Magnetic reconnection is a fundamental physical process allowing magnetic fieldlines to break the “frozen-in” constraint of ideal magnetohydrodynamics (MHD), i.e. to “tear” the magnetic topology, and thereby convert stored magnetic energy into plasma energy. Reconnection powers energetic events such as solar flares, coronal mass ejections (which can disable satellites), magnetospheric substorms, and also so-called “sawtooth” crashes in fusion devices.

See: https://theory.pppl.gov/research/research.php?rid=4

** Inverse Compton scattering or non-linear inverse Compton scattering ( NICS ), also known as non-linear Compton scattering and multiphoton Compton scattering, is the scattering of multiple low-energy photons, given by an intense electromagnetic field, in a high-energy photon ( X-ray or gamma ray) during the interaction with a charged particle, in many cases an electron.

inverse Compton.jpeg
Example of inverse Compton scattering. | Download Scientific Diagramresearchgate.net
Schematic representation of inverse Compton scattering. | Download ...

Schematic representation of inverse Compton scattering. | Download ...researchgate.net

Inverse Compton Scattering: Photons Mugging Electrons - YouTube

Inverse Compton Scattering, youtube.

The measurement of the extremely high energy electron beam with the inverse Compton scattering between electrons and microwave photons requires the precise calculation of the interaction cross section of electrons and microwave photons in a resonant cavity. In the local space of the cavity, the electromagnetic field is expressed by Bessel functions. Although Bessel functions can form a complete set of orthogonal basis, it is difficult to quantify them directly as fundamental wave functions. Fortunately, with the Fourier expansion of Bessel functions, the local electromagnetic field can be considered as the superposition of a series of plane waves. Therefore, with corresponding corrections of the cross section formula of the classical Compton scattering, the cross section of the linear or nonlinear microwave Compton scattering in the local space can be described accurately. As an important application of our results in astrophysics, corresponding ground verification devices can be designed to perform experimental verifications on the prediction of the Sunyaev–Zeldovich (SZ) effect of the cosmic microwave background radiation. Our results could also provide a new way to generate wave sources with strong practical value, such as the terahertz waves, the ultra-violet (EUV) waves, or the mid-infrared beams.

compton scattering.png


See: https://link.springer.com/article/10.1140/epjd/s10053-022-00389-4

*** BL Lac, or BL Lacertae Objects, are one subclass of active galactic nuclei (AGN), the extremely energetic nuclei of active galaxies. Only some 40 BL Lac objects are known. Perhaps the most obvious property of BL Lac objects is that they look like stars. Astronomers originally thought the prototype, BL Lac, was a star . In fact, BL Lacertae is normally a variable star designation, two letters followed by a constellation name, because astronomers originally thought that BL Lac was a star whose brightness varies, a variable star. BL Lac objects do however have properties that are clearly not stellar.

Unlike most stars, BL Lac objects are very strong sources of radio and infrared emission . This emission, which is called synchrotron emission, arises from electrons traveling near the speed of light in spiral paths in strong magnetic fields. Synchrotron emission generally is polarized, so BL Lac objects have polarized emission. When light or other electromagnetic radiation is polarized, the directions of the oscillations are the same. The amount of polarization and the brightness of BL Lac objects is highly variable. This variability is very rapid and erratic. They can change very significantly in times as short as 24 hours or less. The rapid variability tells us that the energy source is very small. Nothing can travel faster than the speed of light, including whatever signal or mechanism causes the BL Lac object to change its brightness. Therefore, if a BL Lac object changes its brightness significantly in a day, its energy source must be less than one light day in radius.

The spectrum of a BL Lac object contains very few if any absorption or emission lines, which are caused by interstellar gas. Their essentially featureless spectra tell us that there is very little interstellar gas around BL Lac objects. There is evidence for a faint fuzziness in some pictures of BL Lac objects. This fuzziness is most likely the host galaxy , of which the BL Lac object is the active nucleus.

Currently, the most popular explanation of BL Lac objects is that they are the central very energetic nuclei of galaxies. The small central energy source in the nucleus is probably a supermassive black hole . However, astronomers are still very uncertain of their nature. BL Lacertae objects need further study to better understand them.

See: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/bl-lacertae-object-0

**** Seyfert 1 galaxies, any of a class of galaxies known to have active nuclei. Such galaxies were named for the American astronomer Carl K. Seyfert, who first called attention to them in 1944. The nuclear spectra of Type 1 Seyfert galaxies show broad emission lines, which are indicative of a central concentration of hot gas that is expanding outwards at speeds of up to thousands of kilometres per second.

See: https://www.britannica.com/science/Seyfert-galaxy

Multi-wavelength observations of the hard X–ray sources selected by Chandra and XMM–Newton surveys have significantly improved our knowledge of the objects responsible of the hard X–ray background. A surprising finding is the discovery of a population of optically dull, X–ray bright galaxies now known as X-ray bright, optically normal galaxies, the XBONGs.


Latest posts