A Supermassive Object in the Nucleus of NGC 4258?

Greenhill, L. J.; Dong, Jiwen; Moran, J. M.; Reid, M. J.; Lo, K. Y.; Claussen, M. J.; Henkel, C.; Wilson, Thomas L.; Becker, R. H.; Wouterloot, J. G. A.

doi:10.1007/978-94-010-9374-3_98

Cited by 60 publications

(86 citation statements)

References 8 publications

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“…High resolution 22 GHz VLBI observations of the water maser emission reveal a thin circumnuclear disk in Keplerian orbit (Greenhill et al 1995a;Miyoshi et al 1995;Moran et al 1995;Herrnstein et al 1996). The maser emission extends from 0.13 pc to 0.26 pc, and the perfect Keplerian rotation curve requires a central binding mass, M, of (3.5 ± 0.1) × 10 7 M within 0.13 pc.…”

Section: Introductionmentioning

confidence: 96%

NGC 4258: A jet-dominated low-luminosity AGN?

Yuan

Markoff

Falcke

et al. 2002

A&A

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Abstract. Low-luminosity AGNs (LLAGNs) are a very important class of sources since they occupy a significant fraction of local galaxies. Their spectra differ significantly from the canonical luminous AGNs, most notably by the absence of the "big blue bump". In the present paper, taking a typical LLAGN-NGC 4258-as an example, we investigate the origin of their spectral emission. The observational data of NGC 4258 is extremely abundant, including water maser emission, putting very strict constraints to its theoretical models. The infrared (IR) spectrum is well described by a steep power-law form f ν ∝ ν −1.4 , and may extend to the optical/UV band. Up until now there is no model which can explain such a steep spectrum, and we here propose a coupled jet plus accretion disk model for NGC 4258. The accretion disk is composed of an inner ADAF (or radiatively inefficient accretion flow) and an outer standard thin disk. A shock occurs when the accretion flow is ejected out of the ADAF to form the jet near the black hole, accelerating the electrons into a power-law energy distribution. The synchrotron and self-Comptonized emission from these electrons greatly dominates over the underlying accretion disk and can well explain the spectrum ranging from IR to X-ray bands. The further propagation of the shocked gas in the jet can explain the flat radio spectrum of NGC 4258. Several predictions of our model are presented for testing against future observations, and we briefly discuss the application of the model to other LLAGNs.

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Section: Introductionmentioning

confidence: 96%

NGC 4258: A jet-dominated low-luminosity AGN?

Yuan

Markoff

Falcke

et al. 2002

A&A

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“…The systemic velocity features lie along the central line of sight, while the highvelocity features lie along the midline of the disk perpendicular to our line of sight. Independent confirmation that this is the correct geometry is provided by observations of centripetal accelerations of the maser features; the systemic features have a measured acceleration of a ~ 9 km s -1 yr -1 , while the high-velocity features have accelerations a < 1 km s _ 1 y r -1 (Greenhill et al 1995b). The best determination is for the bright high-velocity feature at 1306 km s _ 1 which gives a = 0.0 ± 0.1 km s _ 1 yr -1 , constraining this feature to lie within about two degrees of the perpendicular midline.…”

Section: The Warped Maser Disk Of Ngc 4258mentioning

confidence: 85%

Probing Accretion Disks in AGN with Water Masers

Maloney

1997

International Astronomical Union Colloquium

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Abstract.Extremely luminous extragalactic water masers -the socalled "megamasers", with isotropic luminosities of tens to hundreds of solar luminosities -appear to be uniquely associated with active galactic nuclei. The recent survey of Braatz et al. indicates that 20% of Seyfert 2 galaxies have detectable water maser emission. Although originally suggested to arise in shocks, it now seems likely that the masers arise from the irradiation of high-pressure molecular gas by X-rays from the AGN. Quantitative modelling shows that the observed megamaser luminosities can plausibly be produced in this fashion. Both observational limits on the size scales and the high gas pressures required indicate that the water maser emission arises on very small scales, either in a circumnuclear "torus" or the accretion disk itself. In the best-studied case, NCG 4258, the masers are produced in a geometrically thin, warped accretion disk. The maser models can be used to derive quantitative information about the physical conditions in the disk, namely, the mass accretion rate, and therefore the radiative efficiency. I discuss the implications of water maser observations and models for the study of accretion disks and circumnuclear tori in AGN.

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“…This implies that the most intense maser emission at low velocities (velocities near the systemic velocity) comes mainly from the outer edge of ring (for q < 1) or from the inner edge (for q > 1), either the near or far side of the ring, as is indicated in Figure 4. Measurements of positive acceleration of the maser emission around the systemic velocity show that this emission certainly comes from the near side of the ring at the proximity of its inner edge (e.g., Greenhill et al 1995). If we suppose that the gas inside the masing ring is thermalized probably due to its higher density, then an important fraction of the maser emission from the backside of the ring would be absorbed and the detected emission would come from the front side of the ring at the outer or inner edge depending on the value of q.…”

Section: Discussionmentioning

confidence: 99%

“…The systemic maser features drift at a mean rate of $9 km s À1 yr À1 ( Haschick et al 1994;Greenhill et al 1995;Nakai et al 1995;Bragg et al 2000), while the high-velocity maser features drift by P1 km s À1 yr À1 . In a recent spectroscopic study, Bragg et al (2000) detected accelerations for the high-velocity features in the range of À0.77 to 0.38 km s À1 yr À1 .…”

Section: Introductionmentioning

confidence: 98%

Position‐Velocity Diagrams for the Maser Emission Coming from a Keplerian Ring

Uscanga

Cantó

Raga

2007

ApJ

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We have studied the maser emission from a thin, planar, gaseous ring in Keplerian rotation around a central mass observed edge-on. The absorption coefficient within the ring is assumed to follow a power-law dependence on the distance from the central mass as ¼ 0 r Àq . We have calculated position-velocity diagrams for the most intense maser features, for different values of the exponent q. We have found that, depending on the value of q, these diagrams can be qualitatively different. The most intense maser emission at a given velocity can come mainly from either regions close to the inner or outer edges of the amplifying ring or the line perpendicular to the line of sight and passing through the central mass (as is commonly assumed). Particularly, when q > 1 the position-velocity diagram is qualitatively similar to the one observed for the water maser emission in the nucleus of the galaxy NGC 4258. In the context of this simple model, we conclude that in this object the absorption coefficient depends on the radius of the amplifying ring as a decreasing function, in order to have significant emission coming from the inner edge of the ring.

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A Supermassive Object in the Nucleus of NGC 4258?

Cited by 60 publications

References 8 publications

NGC 4258: A jet-dominated low-luminosity AGN?

NGC 4258: A jet-dominated low-luminosity AGN?

Probing Accretion Disks in AGN with Water Masers

Position‐Velocity Diagrams for the Maser Emission Coming from a Keplerian Ring

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