Galactic Black Hole Research Articles

Dissimilar donuts in the sky? Effects of a pressure singularity on the circular photon orbits and shadow of a cosmological black hole

The black hole observations obtained so far indicate one thing: similar “donuts” exist in the sky. But what if some of the observed black hole shadows that will be obtained in the future are different from the others? In this work the aim is to show that a difference in the shadow of some observed black holes in the future might explain the H 0-tension problem. In this letter we investigate the possible effects of a pressure cosmological singularity on the circular photon orbits and the shadow of galactic supermassive black holes at cosmological redshifts. Since the pressure singularity is a global event in the Universe, the effects of the pressure singularity will be imposed on supermassive black holes at a specific redshift. As we show, the pressure singularity affects the circular photon orbits around cosmological black holes described by the McVittie metric, and specifically, for some time before the time instance that the singularity occurs, the photon orbits do not exist. We discuss the possible effects of the absence of circular photon orbits on the shadow of these black holes. Our idea indicates that if a pressure singularity occurred in the near past, then this could have a direct imprint on the shadow of supermassive galactic black holes at the redshift corresponding to the time instance that the singularity occurred in the past. Thus, if a sample of shadows is observed in the future for redshifts , and for a specific redshift differences are found in the shadows, this could be an indication that a pressure singularity occurred, and this global event might resolve the H 0-tension as discussed in previous work. However, the observation of several shadows at redshifts is a rather far future task.

Probing bumblebee gravity with black hole X-ray data

Bumblebee gravity is one of the simplest gravity theories with spontaneous Lorentz symmetry breaking. Since we know a rotating black hole solution in bumblebee gravity, we can potentially test this model with the available astrophysical observations of black holes. In this work, we construct a reflection model in bumblebee gravity and we use our model to analyze the reflection features of a NuSTAR spectrum of the Galactic black hole EXO 1846-031 in order to constrain the Lorentz-violating parameter ell . We find that the analysis of the reflection features in the spectrum of EXO 1846-031 cannot constrain the parameter ell because of a very strong degeneracy between the estimates of ell and of the black hole spin parameter a_*. Such a degeneracy may be broken by combining other observations.

Accretion flow properties of MAXI J1910-057/Swift J1910.2–0546 during its 2012–13 outburst

Galactic black hole candidate MAXI J1910-057/Swift J1910.2–0546 was simultaneously discovered by MAXI/GSC and Swift/BAT satellites during its first outburst in 2012. We study the detailed spectral and temporal properties of the source in a broad energy range using archival data from Swift/XRT, MAXI/GSC, and Swift/BAT satellites/instruments. Low frequency quasi periodic oscillations are observed during the outburst. The combined 1–50 keV spectra are analyzed using the transonic flow solution based Two Component Advective Flow (TCAF) model. Based on the variations of soft and hard X-ray fluxes, their hardness ratios and the variations of the spectral model fitted parameters, we find that the source has evolved through six spectral states. We interpret this spectral state evolution to be a result of the release of the leftover matter from the pile-up radius due to a sudden rise of viscosity causing a rebrightening. We show a possible configuration of the evolution of accretion flow during the outburst. From the spectral analysis with TCAF model, we estimate the probable mass of the black hole to lie in the range 6.31 M⊙ to 13.65 M⊙, and the source distance is estimated to be 1.9-8.3 kpc from transition luminosity considerations.

High resolution X-ray spectroscopy of V4641 Sgr during its 2020 outburst

ABSTRACT We observed the Galactic black hole X-ray binary V4641 Sgr with the high resolution transmission gratings on Chandra during the source’s 2020 outburst. Over two epochs of Chandra gratings observations, we see numerous highly ionized metal lines, superimposed on a hot, disc-dominated X-ray continuum. The measured inner disc temperatures and luminosities imply an unfeasibly small inner disc radius, such that we suggest that the central engine of V4641 Sgr is obscured, and we are viewing scattered X-rays. We find that the emission lines in the Chandra spectra cannot be constrained by a single photoionized model, instead finding that two separate photoionized model components are required, one to reproduce the iron lines and a second for the other metals. We compare the observed X-ray spectra of V4641 Sgr to optical studies during previous outbursts of the source, suggesting that the lines originate in an accretion disc wind, potentially with a spherical geometry.

Regularized 3D spectroscopy with CubeFit: Method and application to the Galactic Center circumnuclear disk

Context. The Galactic Center black hole and the nuclear star cluster are surrounded by a clumpy ring of gas and dust, the circumnuclear disk (CND), that rotates about them at a standoff distance of ≃1.5 pc. The mass and density of individual clumps in the CND are disputed. Aims. We seek to use H2 to characterize the clump size distribution and to investigate the morphology and dynamics of the interface between the ionized interior layer of the CND and the molecular reservoir lying farther out (corresponding to the inner rim of the CND, illuminated in ultraviolet light by the central star cluster). Methods. We have observed two fields of approximately 20″ × 20″ in the CND at near-infrared wavelengths with the OSIRIS spectro-imager at the Keck Observatory. These two fields, located at the approaching and receding nodes of the CND, best display this interface. Our data cover two H2 lines as well as the Brγ line (tracing H II). We have developed the tool CubeFit, an original method for extracting maps of continuous physical parameters (such as the velocity field and velocity dispersion) from integral-field spectroscopy data, using regularization to largely preserve spatial resolution in regions of low signal-to-noise ratio. Results. This original method enables us to isolate compact, bright features in the interstellar medium of the CND. Several clumps in the southwestern field assume the appearance of filaments, many of which are parallel to one another. We conclude that these clumps cannot be self-gravitating.

Tidal effects of a dark matter halo around a galactic black hole* *Supported by the National Natural Science Foundation of China (11875026, 11875025, 12035005, 2020YFC2201400)

We investigate tidal forces and geodesic deviation motion in the spacetime of a black hole in a galaxy with a dark matter halo. Our results show that tidal forces and geodesic deviation motion depend on the mass of the dark matter halo and the typical lengthscale of the galaxy. The effect of the typical lengthscale of the galaxy on the tidal force is opposite to that of dark matter mass. With increasing dark matter mass, the radial tidal force increases in the region far from the black hole but decreases in the region near the black hole. Furthermore, the absolute value of angular tidal force monotonously increases with the dark matter halo mass. The angular tidal force also depends on the particle energy, and the effects of dark matter become more distinct for the test particle at higher energies, which differs from the behavior observed in typical static black hole spacetimes. We also present the change in the geodesic deviation vector with dark matter halo mass and the typical lengthscale of a galaxy under two types of initial conditions.

A 2-hr binary period for the black hole transient MAXI J0637-430

ABSTRACT We revisit various sets of published results from X-ray and optical studies of the Galactic black hole (BH) candidate MAXI J0637-430, which went into outburst in 2019. Combining the previously reported values of peak outburst luminosity, best-fitting radii of inner and outer accretion disc, viewing angle, exponential decay time-scale, and peak-to-peak separation of the He II λ4686 disc emission line, we improve the constraints on the system parameters. We estimate a heliocentric distance d ≈ (8.7 ± 2.3) kpc, a projected Galactocentric distance R ≈ (13.2 ± 1.8) kpc and a height |z| ≈ (3.1 ± 0.8) kpc from the Galactic plane. It is the currently known Milky Way BH candidate located farthest from the Galactic Centre. We infer a BH mass M1 ≈ (5.1 ± 1.6)M⊙, a spin parameter a* ≲ 0.25, a donor star mass M2 ≈ (0.25 ± 0.07)M⊙, a peak Eddington ratio λ ≈ 0.17 ± 0.11 and a binary period $P_{\rm orb} \approx 2.2^{+0.8}_{-0.6}$ hr. This is the shortest period measured or estimated so far for any Galactic BH X-ray binary. If the donor star is a main-sequence dwarf, such a period corresponds to the evolutionary stage where orbital shrinking is driven by gravitational radiation and the star has regained contact with its Roche lobe (low end of the period gap). The three Galactic BHs with the shortest period (≲3 hr) are also those with the highest vertical distance from the Galactic plane (≳2 kpc). This is probably because binaries with higher binding energies can survive faster natal kicks.

Polarized Optical Emission of the Blazar PKS 1222+216: Discovery of a 420 day Quasiperiodic Signal

We report our search for quasiperiodic signals in long-term optical and γ-ray data for the blazar PKS 1222+216, where the data are from the Steward Observatory blazar monitoring program and the all-sky survey with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope, respectively. A quasiperiodic signal, with a period of ≃420 days and a significance of >5σ, is found in the measurements of the optical linear polarization degree for the source, while no similar signals are found in the optical and γ-ray light curves covering approximately the same time period of ∼10 yr. We study the quasiperiodic variations by applying a helical jet model and find that the model can provide a good explanation. This work shows that polarimetry can be a powerful tool for revealing the physical properties, in particular the configuration of the magnetic fields of jets from galactic supermassive black holes.

The peculiar spectral evolution of the new X-ray transient MAXI J0637–430

ABSTRACT We studied the transient Galactic black hole candidate MAXI J0637−430 with data from Insight-HXMT, Swift, and XMM–Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637–430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disc, we suggest a most likely distance ≲7 kpc. Compared with other black hole transients, MAXI J0637–430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at ≈0.7 keV and ≲0.1 times Eddington, respectively); a short decline time-scale; a low soft-to-hard transition luminosity (≲0.01 times Eddington). We argue that such properties are consistent with a small binary separation, short binary period (P ∼ 2 h), and low-mass donor star (M2 ∼ 0.2 M⊙). Moreover, spectral modelling shows that a single disc blackbody component is not a good fit to the thermal emission. Soft spectral residuals, and deviations from the standard $L_{\rm disc} \propto T_{\rm in}^4$ relation, suggest the need for a second thermal component. We propose and discuss various scenarios for such component, in addition to those presented in previous studies of this source. For example, a gap in the accretion disc between a hotter inner ring near the innermost stable orbit, and a cooler outer disc. Another possibility is that the second thermal component is the thermal plasma emission from an ionized outflow.

Multiwavelength observations of MAXI J1820+070 during its outburst decay and subsequent mini-outburst

ABSTRACT We present results from quasi-simultaneous multiwavelength observations of the Galactic black hole X-ray transient MAXI J1820+070 during the decay of the 2018 outburst and its entire subsequent mini-outburst in March 2019. We fit the X-ray spectra with phenomenological and Comptonizaton models and discuss the X-ray spectral evolution in comparison with the multiwavelength behaviour of the system. The system showed a rebrightening in UV/Optical/NIR bands 7 d after the soft-to-hard transition during the main outburst decay while it was fading in X-rays and radio. In contrast, the mini-outburst occurred 165 d after the hard state transition of the initial outburst decay and was detected in all wavelengths. For both events, the measured time-scales are consistent with those observed in other black hole systems. Contemporaneous hard X-ray/soft γ-ray observations indicate a non-thermal electron energy distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a thermal distribution can fit the data during the hard mini-outburst activity. The broad-band spectral energy distributions until the rebrightening are consistent with the irradiated outer accretion disc model. However, both the SEDs produced for the peak of rebrightening and close to the peak of mini-outburst provided good fits only with an additional power law component in the UV/Optical/NIR frequency ranges, which is often interpreted with a jet origin.

Multiwavelength observations of the black hole X-ray binary MAXI J1820+070 in the rebrightening phase

Abstract We report the results of quasi-simultaneous multiwavelength (near-infrared, optical, UV, and X-ray) observations of the Galactic X-ray black hole binary MAXI J1820+070 performed in 2019 May 10–13, ∼60 d after the onset of the first rebrightening phase. It showed a much larger optical-to-X-ray luminosity ratio (∼8) than in the initial outburst epoch. The primary components of the spectral energy distribution (SED) can be best interpreted by a radiatively inefficient accretion flow (RIAF) spectrum showing a luminosity peak in the optical band. By comparison with theoretical calculations, we estimate the mass accretion rate to be $\dot{M}/(8 L_{\rm Edd}/c^2) \sim 10^{-3}$, where c is the light speed and LEdd is the Eddington luminosity. In addition to the RIAF emission, a blue power-law component is detected in the optical–UV SED, which is most likely synchrotron radiation from the jet. The optical spectrum taken at the Seimei telescope shows a weak and narrow Hα emission line, the emitting region of which is constrained to be ≳2 × 104 times the gravitational radius. We suggest that the entire disk structure cannot be described by a single RIAF solution but cooler material responsible for the Hα emission must exist at the outermost region.

Do radio active galactic nuclei reflect X-ray binary spectral states?

Context.Over recent years there has been mounting evidence that accreting supermassive black holes in active galactic nuclei (AGNs) and stellar mass black holes have similar observational signatures: thermal emission from the accretion disk, X-ray coronas, and relativistic jets. Further, there have been investigations into whether or not AGNs have spectral states similar to those of X-ray binaries (XRBs) and what parallels can be drawn between the two using a hardness-intensity diagram (HID).Aims.To address whether AGN jets might be related to accretion states as in XRBs, we explore whether populations of radio AGNs classified according to their (a) radio jet morphology, Fanaroff-Riley classes I and II (FR I and II), (b) excitation class, high- and low-excitation radio galaxies (HERG and LERG), and (c) radio jet linear extent, compact to giant, occupy different and distinct regions of the AGN HID (total luminosity vs. hardness).Methods.We do this by cross-correlating 15 catalogs of radio galaxies with the desired characteristics from the literature withXMM-NewtonandSwiftX-ray and ultraviolet (UV) source catalogs. We calculate the luminosity and hardness from the X-ray and UV photometry, place the sources on the AGN HID, and search for separation of populations and analogies with the XRB spectral state HID.Results.We find that (a) FR Is and IIs, (b) HERGs and LERGs, and (c) FR I-LERGs and FR II-HERGs occupy distinct areas of the HID at a statistically significant level (p-value < 0.05), and we find no clear evidence for population distinction between the different radio jet linear extents. The separation between FR I-LERG and FR II-HERG populations is the strongest in this work.Conclusions.Our results indicate that radio-loud AGNs occupy distinct areas of the HID depending on the morphology and excitation class, showing strong similarities to XRBs.

A Multiwavelength Study of GRS 1716-249 in Outburst: Constraints on Its System Parameters

We present a detailed study of the evolution of the Galactic black hole transient GRS 1716−249 during its 2016–2017 outburst at optical (Las Cumbres Observatory), mid-infrared (Very Large Telescope), near-infrared (Rapid Eye Mount telescope), and ultraviolet (the Neil Gehrels Swift Observatory Ultraviolet/Optical Telescope) wavelengths, along with archival radio and X-ray data. We show that the optical/near-infrared and UV emission of the source mainly originates from a multi-temperature accretion disk, while the mid-infrared and radio emission are dominated by synchrotron emission from a compact jet. The optical/UV flux density is correlated with the X-ray emission when the source is in the hard state, consistent with an X-ray irradiated accretion disk with an additional contribution from the viscous disk during the outburst fade. We find evidence for a weak, but highly variable jet component at mid-infrared wavelengths. We also report the long-term optical light curve of the source and find that the quiescent -band magnitude is 21.39 ± 0.15 mag. Furthermore, we discuss how previous estimates of the system parameters of the source are based on various incorrect assumptions, and so are likely to be inaccurate. By comparing our GRS 1716−249 data set to those of other outbursting black hole X-ray binaries, we find that while GRS 1716−249 shows similar X-ray behavior, it is noticeably optically fainter, if the literature distance of 2.4 kpc is adopted. Using several lines of reasoning, we argue that the source distance is further than previously assumed in the literature, likely within 4–17 kpc, with a most likely range of ∼4–8 kpc.

On the origin of core radio emissions from black hole sources in the realm of relativistic shocked accretion flow

ABSTRACT We study the relativistic, inviscid, advective accretion flow around the black holes and investigate a key feature of the accretion flow, namely the shock waves. We observe that the shock-induced accretion solutions are prevalent and such solutions are commonly obtained for a wide range of the flow parameters, such as energy (${\cal E}$) and angular momentum (λ), around the black holes of spin value 0 ≤ ak < 1. When the shock is dissipative in nature, a part of the accretion energy is released through the upper and lower surfaces of the disc at the location of the shock transition. We find that the maximum accretion energies that can be extracted at the dissipative shock ($\Delta {\cal E}^{\rm max}$) are $\sim 1{{\ \rm per\ cent}}$ and $\sim 4.4{{\ \rm per\ cent}}$ for Schwarzschild black holes (ak → 0) and Kerr black holes (ak → 1), respectively. Using $\Delta {\cal E}^{\rm max}$, we compute the loss of kinetic power (equivalently shock luminosity, Lshock) that is enabled to comply with the energy budget for generating jets/outflows from the jet base (i.e. post-shock flow). We compare Lshock with the observed core radio luminosity (LR) of black hole sources for a wide mass range spanning 10 orders of magnitude with sub-Eddington accretion rate and perceive that the present formalism seems to be potentially viable to account LR of 16 Galactic black hole X-ray binaries (BH-XRBs) and 2176 active galactic nuclei. We further aim to address the core radio luminosity of intermediate-mass black hole (IMBH) sources and indicate that the present model formalism perhaps adequate to explain core radio emission of IMBH sources in the sub-Eddington accretion limit.

First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole

We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a “variability noise budget” in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of ∼50 μas, consistent with the expected “shadow” of a 4 × 106 M ⊙ black hole in the Galactic center located at a distance of 8 kpc.

Simple Analytic Formula Relating the Mass and Spin of Accreting Compact Objects to Their Rapid X-Ray Variability

Abstract Following the previous research on epicyclic oscillations of accretion disks around black holes (BHs) and neutron stars (NSs), a new model of high-frequency quasiperiodic oscillations (QPOs) has been proposed, so-called cusp torus (CT) model, which deals with oscillations of fluid in marginally overflowing accretion tori (i.e., tori terminated by cusps). According to preliminary investigations, the model provides better fits of the NS QPO data compared to the relativistic precession (RP) model. It also implies a significantly higher upper limit on the Galactic microquasar BH spins. A short analytic formula has been noticed to well reproduce the model’s predictions on the QPO frequencies in Schwarzschild spacetimes. Here we derive an extended version of this formula that applies to rotating compact objects. We start with the consideration of Kerr spacetimes and derive a formula that is not restricted to a particular specific angular momentum distribution of the inner accretion flow, such as a Keplerian or constant one. Finally, we consider Hartle–Thorne spacetimes and include corrections implied by the NS oblateness. For a particular choice of a single parameter, our relation provides frequencies predicted by the CT model. For another value, it provides frequencies predicted by the RP model. We conclude that the formula is well applicable to rotating oblate NSs and both models. We briefly illustrate the application of our simple formula on several NS sources and confirm the expectation that the CT model is compatible with realistic values of the NS mass and provides better fits of data than the RP model.

The dark mass signature in the orbit of S2

Context.The Schwarzschild precession of star S2, which orbits the massive black hole at the centre of the Milky Way, has recently been detected with the result of ∼12 arcmin per orbit. The same study also improved the 1σupper bound on a possibly present dark continuous extended mass distribution (e.g. faint stars, stellar remnants, stellar mass black holes, or dark matter) within the orbit of S2 to ∼4000 M⊙. The secular (i.e. net) effect of an extended mass onto a stellar orbit is known as mass precession, and it runs counter to the Schwarzschild precession.Aims.We explore a strategy for how the Schwarzschild and mass precessions can be separated from each other despite their secular interference, by pinpointing their signatures within a single orbit. From these insights, we then seek to assess the prospects for improving the dark mass constraints in the coming years.Methods.We analysed the dependence of the osculating orbital elements and of the observables on true anomaly, and we compared these functions for models with and without extended mass. We then translated the maximum astrometric impacts within one orbit to detection thresholds given hypothetical data of different accuracies. These theoretical investigations were then supported and complemented by an extensive mock-data fitting analysis.Results.We have four main results. 1. While the mass precession almost exclusively impacts the orbit in the apocentre half, the Schwarzschild precession almost exclusively impacts it in the pericentre half, allowing for a clear separation of the effects. 2. Data that are limited to the pericentre half are not sensitive to a dark mass, while data limited to the apocentre half are, but only to a limited extent. 3. A full orbit of data is required to substantially constrain a dark mass. 4. For a full orbit of astrometric and spectroscopic data, the astrometric component in the pericentre halff plays the stronger role in constraining the dark mass than the astrometric data in the apocentre half. Furthermore, we determine the 1σdark mass detection thresholds given different datasets on one full orbit. In particular, with a full orbit of data of 50 microarcsec (VLTI/GRAVITY) and 10 km s−1(VLT/SINFONI) precision, the 1σbound would improve to ∼1000 M⊙, for example.Conclusions.The current upper dark mass bound of ∼4000 M⊙has mainly been obtained from a combination of GRAVITY and VLT/NACO astrometric data, as well as from SINFONI spectroscopic data, where the GRAVITY data were limited to the pericentre half. From our results 3 and 4, we know that all components were thereby crucial, but also that the GRAVITY data were dominant in the astrometric components in constraining the dark mass. From results 1 and 2, we deduce that a future population of the apocentre half with GRAVITY data points will substantially further improve the dark mass sensitivity of the dataset, and we note that at the time of publication, we already entered this regime. In the context of the larger picture, our analysis demonstrates how precession effects that interfere on secular timescales can clearly be distinguished from each other based on their distinct astrometric signatures within a single orbit. The extension of our analysis to the Lense-Thirring precession should thus be of value in order to assess future spin detection prospects for the galactic centre massive black hole.

The spins of the Galactic black holes in MAXI J1535–571 and 4U 1630–472 from Insight-HXMT

ABSTRACT Insight-HXMT is the first Chinese X-ray astronomical mission. With a 1–250 keV bandpass, an energy resolution of 150 eV at 6 keV, and without being subject to pile-up distortions, it is suitable to study bright X-ray sources like Galactic black holes. In this paper, we study Insight-HXMT observations of the X-ray binaries MAXI J1535–571 and 4U 1630–472 during their outbursts in 2017 and 2020, respectively. From the analysis of the reflection features, we are able to infer the black hole spin parameter in the two sources. For MAXI J1535–571, we find a spin parameter very close to 1, a* = 0.9916 ± 0.0012 (90 per cent C.L., statistical error), which is consistent with the previous NuSTAR measurement. For 4U 1630–472, we find a moderately high value of the black hole spin parameter, a* = 0.817 ± 0.014 (90 per cent C.L., statistical error), which is lower than the almost extremal value found with NuSTAR data.

Galactic dynamo seeds and black hole singularities driven by Einstein–Cartan QCD walls

Torsion spin polarised wall solution of Einstein–Cartan equations as torsion defects (Garcia de Andrade, Ann Phys 432 (2021)) where spin is distributed orthogonally to the wall has recently been obtained. Moreover, torsional inflating defect [Class and Quantum Gravity, 16 (1999) 2097] in the form of a dilatonic domain wall has also been found. Our goal in this paper is to obtain thin torsion wall solutions as teleparallel and Riemann–Cartan (RC) spacetime walls, with Einstein–Cartan stress–energy tensor, similar to Kalb–Ramond torsion fields. In thin teleparallel wall, we show that torsion is proportional to a time Dirac delta function which possesses a time singularity at the wall like Big Bang and black hole GR ones. This magnetic field has a torsional contribution of BQCD∼1017G in RC walls, which is suitable for dynamo amplification from a seed field of Bseed∼1012G as Big Bang Nucleosynthesis (BBN). From QCD cosmic time scale of tQCD∼10−6s torsion of the order of J0∼10−11cm−1 is obtained. From a 1 MeV torsion and seed field of Bseed∼10−32G, found by Barrow et al [PRD] in the framework of general relativity (GR) which is suitable to seed galactic dynamo field. In this case a domain wall magnetic field of BDW∼10−28G which is stronger, and more suitable as a galactic dynamo seed, is obtained. Magnetic field seed of 10−14G, quite close to orders of magnitude of O(10−15−10−18G) obtained by Atrayed et al. (EPJC (2018) 78: 1027) on a torsionless GR domain wall, generates a wall (DW) magnetic field of order of BDW∼1018G obtained at 1 kpc scales. These similarities between GR results and teleparallel one were expected due to the fact that the teleparallel theory of gravity considered in this paper is the teleparallel equivalent to GR or (TEGR). Chiral dynamos in dilatonic conformally flat metric are also discussed.

Discovery and Long-term Broadband X-Ray Monitoring of Galactic Black Hole Candidate MAXI J1803–298

Abstract We report the results from the broadband X-ray monitoring of the new Galactic black hole candidate MAXI J1803−298 with MAXI/GSC and Swift/BAT during its outburst. After the discovery on 2021 May 1, the soft X-ray flux below 10 keV rapidly increased for ∼10 days, then gradually decreased over five months. In the brightest phase, the source exhibited the state transition from the low/hard state to the high/soft state via the intermediate state. The broadband X-ray spectrum during the outburst is well described with a disk blackbody plus its thermal or nonthermal Comptonization. Before the transition, the source spectrum is described by a thermal Comptonization component with a photon index of ∼1.7 and an electron temperature of ∼30 keV, while a strong disk blackbody component is observed after the transition. The spectral properties in these periods are consistent with the low/hard state and the high/soft state, respectively. A sudden flux drop with a duration of a few days, unassociated with a significant change in the hardness ratio, was found in the intermediate state. A possible cause of this variation is that the mass accretion rate rapidly increased at the disk transition, which induced a strong Compton-thick outflow and scattered out the X-ray flux. Assuming a nonspinning black hole, we estimate the black hole mass of MAXI J1803−298 to be 5.8 ± 0.4 ( cos i / cos 70 ° ) − 1 / 2 ( D / 8 kpc ) M ⊙ (where i and D are the inclination angle and the distance, respectively) from the inner disk radius obtained in the high/soft state.