We model thermal X-ray emission from the accreting supermassive black hole Sagittarius A Ã at the Galactic center. For the region inside 1B5 of the center, we use a generalized radiatively inefficient accretion flow ( RIAF) model, and for the region out to 10 00 we use published models of the ambient hot gas. We calculate the equivalent widths of hydrogen-like and helium-like emission lines of various elements, notably iron. We predict that a strong helium-like iron line with an equivalent width $1 keV should be emitted by both the external medium and the RIAF. The equivalent width in the external medium is sensitive to the metallicity Z of the gas as well as the mean temperature. For reasonable choices of these parameters, the calculated results agree with Chandra's detection of an iron line with an equivalent width of 1.3 keV within 10 00 . The emission from within 1B5 is not sensitive to the external temperature, but is sensitive to the density and, especially, temperature profile inside the Bondi radius. For the range of profiles we consider, we calculate the equivalent width of the iron line to be $0.6-1.5(Z/Z ) keV, where Z is the solar metallicity. We present a new Chandra spectrum of the quiescent emission within 1B5 of Sgr A Ã . The measured equivalent width of the iron line is 0.7 keV. Although this measurement has a large uncertainty, it is consistent with our predictions, provided the metallicity of the gas is approximately solar.
In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow-Owen dividing line for the FR I/FR II dichotomy in the optical absolute magnitude of the host galaxy-radio luminosity (M R -L Rad ) plane can be translated to the line in the black hole mass-jet power (M bh -Q jet ) plane by using two empirical relations: Q jet -L Rad and M bh -M R . We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow-Owen FR I/II dividing line in the M bh -Q jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L bol /L Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L bol /L Edd ∼ 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.
No abstract
We study the relation of optical/UV and X-ray emission in the low luminosity active galactic nuclei (LLAGNs), using a sample of 49 sources including 28 local Seyfert galaxies and 21 low-ionization nuclear emission-line regions with optical/UV spectral luminosity at wavelength λ = 2500 Å, 23.0 log L ν(2500 Å) (erg s −1 Hz −1 ) 27.7, and X-ray spectral luminosity at 2 keV, 20.5 log L ν(2 keV) 25.3. Strong correlations are found between the X-ray luminosity and the optical/UV-to-X-ray index, α ox , with the optical/UV luminosity, with slopes very similar to the findings for the luminous AGNs in previous works. The correlation between α ox and L ν(2 keV) is very weak, as is that found for the luminous AGNs in the majority of previous similar works. We also study the relation between α ox and the Eddington ratio L bol /L Edd for our sample and find a significant anti-correlation for the sources with L bol /L Edd 10 −3 , which is opposite to the correlation between the two variables for the luminous AGNs. Using the advection-dominated accretion flow (ADAF) model, we roughly reproduce this anti-correlationship for the two variables for the LLAGNs. This result strongly supports the ADAF as a candidate for the accretion mode in LLAGNs.
We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect 47 X-ray point sources, most of which are likely low mass X-ray binaries (LMXBs), within 4 effective radii (R e ). The brightest X-ray source coincides with the optical, UV and radio center of the galaxy, and shows variability on > 1 hr timescales, indicating the possible existence of a low-luminosity AGN. The 46 offcenter sources and the unresolved point sources contribute about 29% and 20% to the total luminosity of the galaxy, respectively. We find that, after correcting for the incompleteness at the low luminosity end, the observed cumulative X-ray luminosity function (XLF) of the off-center sources is best fit by a broken powerlaw model with a break at L b = 4.4 +2.0 −1.4 ×10 38 ergs s −1 . We identified 210 globular clusters (GCs) candidates in a HST WFPC2 optical image of the galaxy's central region. Of the 25 off-center LMXBs that fall within the WFPC2 field of view, 10 sources are coincident with a GC. Thus the fraction of the GCs hosting bright LMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%, respectively. In the V and I bands, the GCs hosting bright LMXBs are typically 1-2 magnitudes brighter than the GCs with no detected LMXBs. There are about 1.9 ± 0.4 times as many LMXBs in the red, metal-rich GCs as there are in the blue, metal-poor ones. We find no obvious difference between the luminosity distributions of LMXBs in GCs and in field, but the cumulative spectrum of the LMXBs in GCs tends to be softer than that of the LMXBs in field. We detected
We present a study of the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing the high quality Chandra and ROSAT X-ray spectroscopic data. In order to probe the stellar mass distribution we calculated the B-band mass-to-light ratio profile by comparing the observed line-strength indices and multi-color photometric data with different stellar synthesis model predictions. Based on the recent survey results we also have modeled the contribution from other group members to the stellar mass. We find that the gas is single-phase with a temperature of ≃ 0.7 keV within 1R e (1R e = 9.0 kpc), which is typical for elliptical galaxies. Outside 1R e the gas temperature increases quickly outwards to > 1 keV, indicating its group origin. With the high spatial resolution of Chandra we reveal that the X-ray surface brightness profile shows a central excess in the innermost region, and on both the total mass and dark matter profiles there is a flattened feature at about < ∼ 1R e , which coincides with the gas temperature transition from the galactic level to the group level. We speculate that this may be a mark of the boundary between the galaxy and group halos, as has been seen in some other cluster/group-dominating galaxies. The total mass and dark matter distributions within 0.85R e are cuspy and can be approximated by power-law profiles with indices of ≃ 2, which are marginally consistent with the generalized NFW profiles with ζ = 2. The mass in outer regions can be well fitted by a single NFW profile, and the derived concentration parameter c (18.6 ± 1.5) is larger than the 68% upper limit for a halo at z = 0 with the given M vir . We find that the NGC 1407 group has a baryondominated core, while the mass in the > 1R e is dominated by dark matter. At the virial radius r 200 = 572 ± 118 kpc, the inferred mass and mass-to-light ratio are M 200 = 2.20±0.42×10 13 M ⊙ and M vir /L B = 311±60 M ⊙ /L B,⊙ , respectively, showing that the NGC 1407 group is an extremely dark system even comparable to many clusters of galaxies. Since the obtained total mass is lower than those given in the earlier galaxy kinematic works, we speculate that NGC 1400 is not a virialized member in the group's gravitational potential well.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.