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NuSTAR observations of Mrk 766: distinguishing reflection from absorption
摘要： We present two new NuSTAR observations of the narrow-line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The NuSTAR spectra show a strong hard (>15 keV) X-ray excess, while simultaneous soft X-ray coverage of one of the observations provided by XMM–Newton constrains the ionized absorption in the source. The pure reflection model requires a black hole of high spin (a > 0.92) viewed at a moderate inclination (i = 46+1?4°). The pure partial covering model requires extreme parameters: the cut-off of the primary continuum is very low (22+7?5 keV) in one observation and the intrinsic X-ray emission must provide a large fraction (75 per cent) of the bolometric luminosity. Allowing a hybrid model with both partial covering and reflection provides more reasonable absorption parameters and relaxes the constraints on reflection parameters. The fractional variability reduces around the iron K band and at high energies including the Compton hump, suggesting that the reflected emission is less variable than the continuum.
关键词： accretion, accretion discs,galaxies: individual: Mrk 766,galaxies: Seyfert,black hole physics
更新于2025-09-23 15:22:29
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Optical and <i>J, K</i> -photometry of the quiescent black hole X-ray nova A0620?00 in the passive and active states
摘要： In 2015–2016, the X-ray nova A0620?00 in quiescence was observed in the passive state, according to the terminology of Cantrell et al., and in less than 230 d it transited into the active state. The system mean luminosity increased by 0.20 mag in the optical and by 0.25 and 0.30 mag in J and K infrared bands, respectively, while the orbital light curves changed drastically, and the ?ickering amplitude more than doubled. The mean light-curve analysis performed in the context of two models argued that the interaction region where the gas stream reaches the disc is responsible for these effects. The growth of this region’s luminosity in the active state implies the increase of the mass transfer rate via the L1 point, likely due to non-stationary processes in the donor star atmosphere. The non-stellar spectrum Fλ in the observed range λλ6400–22 000 ? obeys a power law of λα where α = ?(2.13 ± 0.1) in the passive state and α = ?(1.85 ± 0.1) in the active, while the absolute mean square ?ickering amplitude behaves like (cid:4)F?(λ) ～ λ?2.36 in both states of activity.
关键词： X-rays: individual: 1A 0620?00,X-rays: binaries,accretion discs,techniques: photometric,starspots,radiation mechanisms: non-thermal,accretion
更新于2025-09-23 15:22:29
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The power of infrared AGN selection in mergers: a theoretical study
摘要： The role of galaxy mergers in fuelling active galactic nuclei (AGNs) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGNs are often obscured in late-stage mergers. Mid-infrared (IR) colour selection of dust-enshrouded AGN with, e.g. the Wide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly to AGN that dominate emission from the host. To understand how selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of AGN throughout galaxy mergers. Although mid-IR colours closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGNs are missed with common mid-IR selection criteria, even in late-stage, gas-rich major mergers. At z (cid:2) 0.5, where merger signatures and dual nuclei can most easily be detected, we find that a more lenient W1 ? W2 > 0.5 cut greatly improves completeness without significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-colour cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR colour selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (<3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope.
关键词： galaxies: interactions,infrared: galaxies,accretion,accretion discs,galaxies: active,black hole physics
更新于2025-09-23 15:21:21
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A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability
摘要： We develop a new spectral model for the broad-band spectral energy distribution (SED) of active galactic nuclei (AGN). This includes an outer standard disc, an inner warm Comptonizing region to produce the soft X-ray excess and a hot corona. We tie these together energetically by assuming Novikov–Thorne emissivity, and use this to de?ne a size scale for the hard X-ray corona as equal to the radius where the remaining accretion energy down to the black hole can power the observed X-ray emission. We test this on three AGN with well-de?ned SEDs as well as on larger samples to show that the average hard X-ray luminosity is always approximately a few per cent of the Eddington luminosity across a large range of Eddington ratio. As a consequence, the radial size scale required for gravity to power the X-ray corona has to decrease with increasing Eddington fraction. For the ?rst time, we hardwire this into the spectral models, and set the hard X-ray spectral index self-consistently from the ratio of the hard X-ray luminosity to intercepted seed photon luminosity from the disc. This matches the observed correlation of steeper spectral index with increasing Eddington ratio, as well as reproducing the observed tight UV/X relation of quasars. We also include the reprocessed emission produced by the hot inner ?ow illuminating the warm Comptonization and standard disc regions and show that this predicts a decreasing amount of optical variability with increasing Eddington ratio as observed, though additional processes may also be required to explain the observed optical variability.
关键词： galaxies: Seyfert,accretion,black hole physics,accretion discs
更新于2025-09-23 15:21:01
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The donor of Aquila X-1 revealed by high-angular resolution near-infrared spectroscopy
摘要： The low-mass X-ray binary Aquila X-1 is one of the most active neutron star X-ray transients. Despite its relatively bright quiescent optical counterpart, the detection of its companion has been hampered by the presence of a nearby interloper star. Using the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) on the Very Large Telescope-8.2m telescope, we unambiguously single out Aquila X-1 from the interloper. Phase-resolved near-infrared spectroscopy reveals absorption features from a K4 ± 2 companion star moving at a projected velocity of K2 = 136 ± 4 km s?1. We here present the ?rst dynamical solution and associated fundamental parameters of Aquila X-1, imposing new constraints on the orbital inclination (36? < i < 47?) and the distance (d = 6 ± 2 kpc) to this prototypical neutron star transient.
关键词： accretion,stars: neutron,accretion discs,X-rays: binaries
更新于2025-09-16 10:30:52
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The study of laser cooling of TeH- anion in theoretical approach
摘要： For systems with extremely low accretion rate, such as Galactic Center Sgr A* and M87 galaxy, the ion collisional mean free path can be considerably larger than its Larmor radius. In this case, the gas pressure is anisotropic to magnetic ?eld lines. In this paper, we pay attention to how the properties of out?ow change with the strength of anisotropic pressure and the magnetic ?eld. We use an anisotropic viscosity to model the anisotropic pressure. We solve the two-dimensional magnetohydrodynamic (MHD) equations in spherical coordinates and assume that the accretion ?ow is radially self-similar. We ?nd that the work done by anisotropic pressure can heat the accretion ?ow. The gas temperature is heightened when anisotropic stress is included. The out?ow velocity increases with the enhancement of strength of the anisotropic force. The Bernoulli parameter does not change much when anisotropic pressure is involved. However, we ?nd that the energy ?ux of out?ow can be increased by a factor of 20 in the presence of anisotropic stress. We ?nd strong wind (the mass out?ow is about 70% of the mass in?ow rate) is formed when a relatively strong magnetic ?eld is present. Out?ows from an active galactic nucleus can interact with gas in its host galaxies. Our result predicts that out?ow feedback effects can be enhanced signi?cantly when anisotropic pressure and a relatively powerful magnetic ?eld is considered.
关键词： accretion discs,hydrodynamics,accretion,black hole physics
更新于2025-09-11 14:15:04
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Spherical photon orbits in the field of Kerr naked singularities
摘要： For the Kerr naked singularity (KNS) space-times, we study properties of spherical photon orbits (SPOs) con?ned to constant Boyer-Lindquist radius r . Some new features of the SPOs are found, having no counterparts in the Kerr black hole (KBH) spacetimes, especially stable orbits that could be pure prograde/retrograde, or with turning point in the azimuthal direction. At r > 1 (r < 1) the covariant photon energy E > 0 (E < 0), at r = 1 there is E = 0. All unstable orbits must have E > 0. It is shown that the polar SPOs can exist only in the spacetimes with dimensionless spin a < 1.7996. Existence of closed SPOs with vanishing total change of the azimuth is demonstrated. Classi?cation of the KNS and KBH spacetimes in dependence on their dimensionless spin a is proposed, considering the properties of the SPOs. For selected types of the KNS spacetimes, typical SPOs are constructed, including the closed paths. It is shown that the stable SPOs intersect the equatorial plane in a region of stable circular orbits of test particles, depending on the spin a. Relevance of this intersection for the Keplerian accretion discs is outlined and observational effects are estimated.
关键词： Keplerian accretion discs,Kerr black hole,dimensionless spin,stable orbits,covariant photon energy,observational effects,closed SPOs,spherical photon orbits,azimuthal direction,Kerr naked singularity
更新于2025-09-11 14:15:04
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Study of the reflection spectra of SAX J1748.9?2021
摘要： the spectral analysis of accretion powered millisecond X-ray pulsar SAX J1748.9?2021 from the NuSTAR observation made during its 2015 outburst. The spectra displayed a broad emission line at ～6.5 keV with line width of ～0.5 keV and excess above ～20 keV due to Compton hump. The persistent emission of SAX J1748.9?2021 is described by a combination of soft thermal component with kT = 0.62+0.03 ?0.05 keV and thermally Comptonized component with kTe = 2.50+0.06 ?0.03 keV re?ected from the disc with re?ection fraction of 0.30+0.08 ?0.04. We have modelled the re?ection spectrum with self-consistent model relxillCP and have found the inclination of the accretion disc to be 32.?3+4.8 ?4.7 and log ξ is equal to 3.05+0.40 ?0.34. We have obtained an upper limit of 57 km for inner disc radius; and the estimated magnetic ?eld strength at the poles is less than 3.4 × 109 G. This kind of detailed investigation of re?ection spectrum of SAX J1748.9?2021, especially above 10 keV, has been achieved only because of high statistics NuSTAR data of the source.
关键词： X-rays: binaries,accretion,accretion discs,X-rays: individual: SAX J1748.9?2021,stars: neutron
更新于2025-09-10 09:29:36
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Hard X-ray view on intermediate polars in the <i>Gaia</i> era
摘要： The hardness of the X-ray spectra of intermediate polars (IPs) is determined mainly by the white dwarf (WD) compactness (mass–radius ratio, M/R) and, thus, hard X-ray spectra can be used to constrain the WD mass. An accurate mass estimate requires the finite size of the WD magnetosphere Rm to be taken into the account. We suggested to derive it either directly from the observed break frequency in power spectrum of X-ray or optical light curves of a polar, or assuming the corotation. Here we apply this method to all IPs observed by NuSTAR (10 objects) and Swift/BAT (35 objects). For the dwarf nova GK Per, we also observe a change of the break frequency with flux, which allows us to constrain the dependence of the magnetosphere radius on the mass accretion rate. For our analysis, we calculated an additional grid of two-parameter (M and Rm/R) model spectra assuming a fixed, tall height of the accretion column Hsh/R = 0.25, which is appropriate to determine WD masses in low mass accretion IPs like EX Hya. Using the Gaia Data Release 2, we obtain for the first time reliable estimates of the mass accretion rate and the magnetic field strength at the WD surface for a large fraction of objects in our sample. We find that most IPs accrete at a rate of ～10?9 M(cid:3) yr?1 and have magnetic fields in the range 1–10 MG. The resulting WD mass average of our sample is 0.79 ± 0.16 M(cid:3), which is consistent with earlier estimates.
关键词： methods: numerical,accretion,cataclysmic variables,X-rays: binaries,accretion discs,X-rays: individual: (EX Hya, GK Per),novae
更新于2025-09-10 09:29:36
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Analysis of colour and polarimetric variability of RW?Aur?A in 2010–2018
摘要： The results of UBVRIJHKLM photometry and VRI polarimetry of a young star RW Aur A observed during unprecedented long and deep (up to (cid:2)V ≈ 5 mag) dimming events in 2010–11 and 2014–18 are presented. The polarization degree p of RW Aur A at this period has reached 30 per cent in the I band. As in the case of UX Ori-type stars (UXORs), the so-called ‘bluing effect’ in the colour–magnitude V versus V ? Rc, V ? Ic diagrams of the star and a strong anticorrelation between p and brightness were observed. However, the duration and the amplitude of the eclipses as well as the value and orientation of the polarization vector in our case differ signi?cantly from those of UXORs. We conclude that the dimmings of RW Aur A occurred due to eclipses of the star and inner regions of its disc by the axisymmetric dust structure located above the disc and created by the disc wind. Taking into account both scattering and absorption of stellar light by the circumstellar dust, we explain some features of the light curve and the polarization degree–magnitude dependence. We found that near the period of minimal brightness the mass-loss rate of the dusty wind was >10?9 M(cid:3) yr?1.
关键词： binaries: general,stars: winds, out?ows,stars: individual: RW Aur,accretion, accretion discs,stars: variables: T Tauri, Herbig Ae/Be
更新于2025-09-09 09:28:46