Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets in which to resolve the innermost region of an SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multiwavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ∼704 ± 102
The mass-loss rates from single massive stars are high enough to form radio photospheres at large distances from the stellar surface, where the wind is optically thick to (thermal) free–free opacity. Here we calculate the far-infrared, millimeter, and radio band spectral energy distributions (SEDs) that can result from the combination of free–free processes and synchrotron emission, to explore the conditions for nonthermal SEDs. Simplifying assumptions are adopted in terms of scaling relations for the magnetic field strength and the spatial distribution of relativistic electrons. The wind is assumed to be spherically symmetric, and we consider the effect of Razin suppression on the synchrotron emission. Under these conditions, long-wavelength SEDs with synchrotron emission can be either more steep or more shallow than the canonical asymptotic power-law SED from a nonmagnetic wind. When nonthermal emission is present, the resultant SED shape is generally not a power law; however, the variation in the slope can change slowly with wavelength. Consequently, over a limited range of wavelengths, the SED can masquerade as approximately a power law. While most observed nonthermal long-wavelength spectra are associated with binarity, synchrotron emission can have only a mild influence on single-star SEDs, requiring finer levels of wavelength sampling for the detection of the effect.
more » « less- Award ID(s):
- 2009412
- NSF-PAR ID:
- 10486138
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 932
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 12
- Size(s):
- ["Article No. 12"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract μ as (axial ratio ∼μ as (axial ratio ∼1.28 ± 0.2) at 1.349 cm and 6.95 mm, respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power law, with an index ∼1.2 ± 0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes. -
ABSTRACT The great power offered by photoionization models of active galactic nuclei emission line regions has long been mitigated by the fact that very little is known about the spectral energy distribution (SED) between the Lyman limit, where intervening absorption becomes a problem, and 0.3 keV, where soft X-ray observations become possible. The emission lines themselves can, to some degree, be used to probe the SED, but only in the broadest terms. This paper employs a new generation of theoretical SEDs that are internally self-consistent, energy conserving, and tested against observations, to infer properties of the emission-line regions. The SEDs are given as a function of the Eddington ratio, allowing emission-line correlations to be investigated on a fundamental basis. We apply the simplest possible tests, based on the foundations of photoionization theory, to investigate the implications for the geometry of the emission-line region. The SEDs become more far-ultraviolet bright as the Eddington ratio increases, so the equivalent widths of recombination lines should also become larger, an effect that we quantify. The observed lack of correlation between Eddington ratio and equivalent width shows that the cloud covering factor must decrease as Eddington ratio increases. This would be consistent with recent models proposing that the broad-line region is a failed dusty wind off the accretion disc.more » « less
-
Context. Accreting black holes (BHs) may be surrounded by a highly magnetized plasma threaded by an organized poloidal magnetic field. Nonthermal flares and power-law spectral components at high energy could originate from a hot, collisionless, and nearly force-free corona. The jets we often observe from these systems are believed to be rotation-powered and magnetically driven. Aims. We study axisymmetric BH magnetospheres, where a fraction of the magnetic field lines anchored in a surrounding disk are connected to the event horizon of a rotating BH. For different BH spins, we identify the conditions and sites of magnetic reconnection within 30 gravitational radii. Methods. With the fully general relativistic particle-in-cell code GRZeltron , we solve the time-dependent dynamics of the electron–positron pair plasma and of the electromagnetic fields around the BH. The aligned disk is represented by a steady and perfectly conducting plasma in Keplerian rotation, threaded by a dipolar magnetic field. Results. For prograde disks around Kerr BHs, the topology of the magnetosphere is hybrid. Twisted open magnetic field lines crossing the horizon power a Blandford-Znajek jet, while open field lines with their footpoint beyond a critical distance on the disk could launch a magneto-centrifugal wind. In the innermost regions, coupling magnetic field lines ensure the transfer of significant amounts of angular momentum and energy between the BH and the disk. From the Y point at the intersection of these three regions, a current sheet forms where vivid particle acceleration via magnetic reconnection takes place. We compute the synchrotron images of the current sheet emission. Conclusions. Our estimates for jet power and BH–disk exchanges match those derived from purely force-free models. Particles are accelerated at the Y point, which acts as a heat source for the so-called corona. It provides a physically motivated ring-shaped source of hard X-rays above the disk for reflection models. Episodic plasmoid ejection might explain millisecond flares observed in Cygnus X-1 in the high-soft state, but are too fast to account for daily nonthermal flares from Sgr A * . Particles flowing from the Y point down to the disk could produce a hot spot at the footpoint of the outermost closed magnetic field line.more » « less
-
more » « less
Abstract Rapid X-ray phase-dependent flux enhancement in the archetype classical Cepheid star
δ Cep was observed by XMM-Newton and Chandra. We jointly analyze thermal and nonthermal components of the time-resolved X-ray spectra prior to, during, and after the enhancement. A comparison of the timescales of shock particle acceleration and energy losses is consistent with the scenario of a pulsation-driven shock wave traveling into the stellar corona and accelerating electrons to ∼GeV energies, and with Inverse Compton (IC) emission from the UV stellar background leading to the observed X-ray enhancement. The index of the nonthermal IC photon spectrum, assumed to be a simple power law in the [1–8] keV energy range, radially integrated within the shell [3–10] stellar radii, is consistent with an enhanced X-ray spectrum powered by shock-accelerated electrons. An unlikely ∼100-fold amplification via turbulent dynamo of the magnetic field at the shock propagating through density inhomogeneities in the stellar corona is required for the synchrotron emission to dominate over the IC; the lack of time correlation between radio synchrotron and stellar pulsation contributes to make synchrotron as an unlikely emission mechanism for the flux enhancement. Although current observations cannot rule out a high-flux two-temperature thermal spectrum with a negligible nonthermal component, this event might confirm for the first time the association of Cepheids pulsation with shock-accelerated GeV electrons. -
Abstract While most simulations of the epoch of reionization have focused on single-stellar populations in star-forming dwarf galaxies, products of binary evolution are expected to significantly contribute to emissions of hydrogen-ionizing photons. Among these products are stripped stars (or helium stars), which have their envelopes stripped from interactions with binary companions, leaving an exposed helium core. Previous work has suggested these stripped stars can dominate the Lyman Continuum (LyC) photon output of high-redshift, low-luminosity galaxies post-starburst. Other sources of hard radiation in the early universe include zero-metallicity Population iii stars, which may have similar spectral energy distribution (SED) properties to galaxies with radiation dominated by stripped-star emissions. Here, we use four metrics (the power-law exponent over wavelength intervals 240–500 Å, 600–900 Å, and 1200–2000 Å, and the ratio of total luminosity in FUV wavelengths to LyC wavelengths) to compare the SEDs of simulated galaxies with only single-stellar evolution, galaxies containing stripped stars, and galaxies containing Population iii stars, with four different initial mass functions (IMFs). We find that stripped stars significantly alter SEDs in the LyC range of galaxies at the epoch of reionization. SEDs in galaxies with stripped stars have lower power-law indices in the LyC range and lower FUV to LyC luminosity ratios. These differences in SEDs are present at all considered luminosities ( M UV > − 15 , AB system), and are most pronounced for lower-luminosity galaxies. Intrinsic SEDs as well as those with interstellar medium absorption of galaxies with stripped stars and Population iii stars are found to be distinct for all tested Population iii IMFs.more » « less
