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Jet launching of M87 | Nature Astronomy
A simulated hybrid emission model to mimic the morphology of the jet launching region of M87 reproduces the observed shape of the innermost jet and favours a high spin of the central black hole. You have full access to this article via your institution. Download PDF Download PDF NCG 4486, also known as Messier 87 or M87, lies at the heart of the Virgo cluster at approximately 16.8 Mpc. It is one of the most studied active galactic nuclei (AGNs) across all spectral bands. AGNs are in general the most luminous sources in the Universe because of their powerful central engines that not only accrete the matter around them but also, in some cases, drive a powerful jet. M87 is of particular interest to astronomers because of its powerful jet, which is observed not only across all radio bands but also in the X-ray (Fig. 1 ). Moreover, the jet of M87 displays interesting features such as limb brightening and a wide opening angle at sub-parsec scales. These features also make it a useful source for theoretical studies where the focus of investigation lies in simulating such observed accretion and jet activity. Fig. 1: M87 in different wavelengths (from radio to gamma rays) observed at different resolutions. The EHT Multi-wavelength Science Working Group; the EHT Collaboration; ALMA (ESO/NAOJ/NRAO); the EVN; the EAVN Collaboration; VLBA (NRAO); the GMVA; the Hubble Space Telescope; the Neil Gehrels Swift Observatory; the Chandra X-ray Observatory; the Nuclear Spectroscopic Telescope Array; the Fermi-LAT Collaboration; the H.E.S.S collaboration; the MAGIC collaboration; the VERITAS collaboration; NASA and ESA; compiled by J. C. Algaba At radio wavelengths the images of the large-scale jet are observed at low frequencies, whereas the core is visible at sub-mm wavelengths. The highest-resolution images in the optical and X-ray bands can at best image the large-scale jet as shown here. Full size image Now, writing in Nature Astronomy , Alejandro Cruz-Osorio and collaborators 1 report a model combining three-dimensional (3D) general-relativistic magnetohydrodynamic (GRMHD) simulations with a hybrid thermal–non-thermal particle distribution function to model the observed innermost structure of the relativistic jet in M87. The authors can reproduce the innermost jet morphology to great accuracy in comparison with that observed by the Global 3-mm VLBI array (GMVA) at 86 GHz (ref. 2 ). In common with other AGNs, M87 was predicted to have a supermassive black hole at its centre with a mass of approximately M BH = 6.5 × 10 9 M ⊙ estimated from its observed luminosity and also from its jet power 3 . In April of 2019, the Event Horizon Telescope Collaboration (EHTC) finally published the first images of the black hole shadow of M87 (ref. 4 ), a true breakthrough moment in the history of astronomy. The luminosity inferred from this observation favours models where the synchrotron photons are emitted by thermal electrons from the accretion flow around the black hole. The aim of the observation was to image the innermost accretion regions to resolve the black hole shadow, which the EHTC successfully accomplished. Due to the low dynamic range of these observations, this dataset alone does not seem to suggest the existence of a jet base. However, previously other lower-resolution VLBI observations and X-ray detectors had clearly established the existence of a powerful jet. Cruz-Osorio et al. explore the intermediate regime where an accretion disk with thermal synchrotron emission and a jet that is dominated by non-thermal synchrotron emission can co-exist. They accomplish this by considering a hybrid emission model that includes emission from both synchrotron processes: a kappa-distribution 5 with a thermal low-energy core and a non-thermal high-energy tail to include heating of particles by the magnetic energy. In general, AGN jets are believed to be powered by a combination of magnetic fields and rotational energy extracted from a spinning black hole (Blandford–Znajek mechanism) 6 . Most of the work in the literature model such systems using GRMHD simulations assuming either a magnetically arrested disk (MAD) 7 or a standard and normal evolution (SANE) 8 disk. In MAD models, the accreting gas drags in a strong poloidal magnetic field towards the central supermassive black hole. The MAD model simulations are successful in generating relativistic jets with large opening angles as opposed to the SANE models. The presence of magnetic fields induces the production of synchrotron emission from the accretion flow as well as the jet. As the matter ejected in the relativistic jet does not have enough time to thermalize, the emission from the jets can be best modelled by a non-thermal synchrotron emission model. The spin of the black holes in AGNs, although not known directly from observations, are nonetheless important for powering the jet. However, constraints on the spin parameter a can be obtained through the X-ray luminosity or jet power. For M87, these observations lead to an estimate on the spin parameter a > 0.5. Cruz-Osorio et al. 1 used 3D MAD GRMHD simulations combined with a general relativistic radiative transfer code incorporating a hybrid thermal–non-thermal synchrotron emission model. They use the MAD model because the M87 jet displays a wide opening angle at sub-parsec scales as observed by GMVA at 86 GHz (ref. 2 ). Their simulations are able to both generate a spectrum that fits the available M87 data accurately for frequencies between 10 10 –10 16 Hz, as well as recreate the milliarcsecond-scale morphological features seen in previous VLBI observations at millimetre wavelengths with GMVA. Their work clearly shows that to explain the shape of the observed spectra an emission model with a positive spin parameter of the black hole is required. However, a tighter constraint on the spin is obtained by comparing the simulated morphology of the emission feature at 86 GHz to the observation, which appears to favour models with high spin values. To quantify the extent to which their results match the data, the authors have incorporated a technique to measure the jet diameter. The diameter measurement is done by slicing the jet in a direction orthogonal to that of the jet propagation and fitting the flux-density profile with up to three different Gaussians. The jet width is then defined as the distance between the two outermost Gaussians. The jet diameter is thus evaluated for various spin models at each point from the core to distances up to the milliarcsecond scale from the core and compared to available observations. Whereas at smaller distances from the core, all simulated models deviate from the data at 86 GHz, at larger distances there is a good match to the data for models with high spin values. This result thus constrains models favouring high spin values. The success of this innovative hybrid radiative model for post processing the results from 3D MAD GRMHD simulations to obtain a good match to the jet emission data will pave the way for investigating the jet emission from other such systems with similarly high-resolution good quality observations. Due to the high sensitivity and resolution of the EHT and other current and next-generation VLBI facilities, we are now able to obtain images of the cores of many such AGNs. Indeed, recently the EHT has obtained the images of Centaurus A’s limb-brightened jet 9 and also the jet base of 3C 279 (ref. 10 ). The work by Cruz-Osorio et al. can be extended to study the launching of the jet in the cores of these systems and place independent constraints on the spins of their black holes. Thus, this approach would be important to test models favouring or disfavouring the standard theory of general relativity. References . 1. Cruz-Osorio, A. et al. Nat. Astron . https://doi.org/10.1038/s41550-021-01506-w (2021). 2. Kim, J.-Y. et al. Astron. Astrophys. 616 , A188 (2018). ADS ? Article ? Google Scholar ? 3. Broderick, A. E. et al. Astrophys. J. 805 , 179 (2015). ADS ? Article ? Google Scholar ? 4. Event Horizon Telescope Collaboration et al. Astrophys. J. Lett. 875 , L1 (2019). 5. Xiao, F. Fusion 48 , 203–213 (2006). Article ? Google Scholar ? 6. Blandford, R. D. & Znajek, R. L. Mon. Not. R. Astron. Soc. 179 , 433–456 (1977). ADS ? Article ? Google Scholar ? 7. Narayan, R., Igumenshchev, I. V. & Abramowicz, M. A. Publ. Astron. Soc. Japan 55 , L69–L72 (2003). ADS ? Article ? Google Scholar ? 8. Narayan, R., Sa?dowski, A., Penna, R. F. & Kulkarni, A. K. Mon. Not. R. Astron. Soc. 426 , 3241–3259 (2012). ADS ? Article ? Google Scholar ? 9. Janssen, M. et al. Nat. Astron. 5 , 1017–1028 (2021). ADS ? Article ? Google Scholar ? 10. Kim, J.-Y. et al. Astron. Astrophys 640 , A69 (2020). Article ? Google Scholar ? Download references Author information . Affiliations . Departamento de Astronomía, Universidad de Concepción, Concepción, Chile Bidisha Bandyopadhyay Authors Bidisha Bandyopadhyay View author publications You can also search for this author in PubMed ? Google Scholar Corresponding author . Correspondence to Bidisha Bandyopadhyay . Ethics declarations . Competing interests . The author declares no competing interests. Rights and permissions . Reprints and Permissions About this article . Cite this article . Bandyopadhyay, B. Jet launching of M87. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01535-5 Download citation Published : 25 November 2021 DOI : https://doi.org/10.1038/s41550-021-01535-5 Share this article . Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative .
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