RESEARCH NEWS & VIEWS
1. Tsang, W. Y. & Dynlacht, B. D. Cilia 2, 9 (2013). 2. Song, R. et al. Nature 510, 115–120 (2014).
3. Lizé, M., Herr, C., Klimke, A., Bals, R. & Dobbelstein, M. Cell Cycle 9, 4579–4583 (2010). 4. Martinez-Anton, A. et al. Am. J. Respir. Cell Mol. Biol. 49, 384–395 (2013). 5. Marcet, B. et al. Nature Cell Biol. 13, 693–699 (2011). 6. Spektor, A., Tsang, W. Y., Khoo, D. & Dynlacht, B. D. Cell 130, 678–690 (2007). 7. Flynt, A. S. & Lai, E. C. Nature Rev. Genet. 9, 831–842 (2008).
AST ROPHYSICS
The MAD world of black holes An analysis of optical and radio observations has revealed how powerful jets are launched from the centres of active galaxies, where supermassive black holes accrete matter through magnetically arrested disks, or MADs. See Letter p.126 DENISE GABUZDA
A
lthough the light given off by most galaxies is due to stars and glowing gas, some galaxies have extremely bright centres, or nuclei, with luminosities about 100,000 times greater than those of normal galaxies. In such active galactic nuclei, energy is liberated when matter spirals inwards and is captured by a supermassive black hole — billions of times more massive than the Sun — sitting at the galactic centre. In about 10% of these active nuclei, some of the in-spiralling matter is pushed into two a jets of matter and radiation that shoot out in opposite directions at close to the speed of light (Fig. 1). However, the forces causing these out flows have remained unknown. In this issue, Zamaninasab et al.1 (page 126) report direct evidence that the jets are launched on their a
journey by a kind of gigantic electromagnetic generator, in which magnetic fields in the vicinity of the black hole are twisted by the black hole’s spin, with the energy of this spin being transformed into the energy of the jets’ outward motion2. It has long been suspected that this mecha nism might explain how jets in active galactic nuclei (AGN) are produced. However, direct observational evidence has been elusive, because the scales on which this mechanism should operate are tiny compared with the smallest scales that can be observed directly using the highest-resolution technique available, which are typically about 1 par sec (3 × 1016 metres) from the central black hole. This scale might seem huge, but at the extremely large distances of AGN (billions of parsecs), it translates to a tiny angular distance as observed on the sky.
8. Cao, J. et al. Nature Cell Biol. 14, 697–706 (2012). 9. Chen, Z., Indjeian, V. B., McManus, M., Wang, L. & Dynlacht, B. D. Dev. Cell 3, 339–350 (2002). 10. Li, J. et al. Nature 495, 255–259 (2013). 11. D’Angiolella, V. et al. Nature 466, 138–142 (2010). 12. Lai, Y. et al. J. Allergy Clin. Immunol. 128, 1207–1215.e1 (2011).
Images of such small scales can be obtained using very-long-baseline interferometry (VLBI), an elegant technique in which radio telescopes around the world observe in syn chrony to imitate a single radio telescope with a diameter the size of Earth. The larger the telescope used, the finer the detail that can be seen; accordingly, VLBI yields radio images with phenomenally high resolution, equivalent to being able to peer across the Atlantic Ocean from the western edge of Europe and identify a coin held by someone standing on the eastern coast of the United States. Unfortunately, however, the scales imaged with VLBI are still tens to thousands of times larger than those on which the powerful jets of AGN are launched. Zamaninasab et al. have found a clever way to bridge this gap, by considering the magnetic flux in the jets — essentially, the product of the magnetic field pointing along the jet and the jet’s crosssection. The magnetic flux near the black hole cannot be measured directly, but should be proportional to the luminosity of the matter in the accretion disk surrounding the black hole. The disk’s luminosity can be estimated from observations of optical lines in the spectrum of the AGN. The authors have found a tight linear cor relation between the estimated accretion-disk luminosities for 76 AGN and the magnetic fluxes in their jets measured with VLBI. This correlation demonstrates that the magnetic flux near the black hole is proportional to the
b Twisted magnetic field
Jet
Black hole Rotating accretion disk
Figure 1 | Jets emerging from the centre of an active galactic nucleus. a, Composite image of the optical (true colour), X-ray (blue) and radio (orange) emission of the giant elliptical galaxy NGC 5128, showing its powerful radio jets extending far beyond the optical galaxy. b, Zamaninasab et al.1 have found evidence that jets emerging from the centres of active galaxies are launched from deep in the galactic nucleus (square in a) by twisted magnetic fields2, and that these fields are dynamically important in the vicinity of the galaxy’s central black hole and surrounding accretion disk. 4 2 | NAT U R E | VO L 5 1 0 | 5 J U N E 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
A: ESO/WFI (OPTICAL); MPIFR/ESO/APEX/A. WEISS ET AL. (SUBMILLIMETRE); NASA/CXC/CFA/R. KRAFT ET AL. (X-RAY)
Irma Sánchez and Brian D. Dynlacht are at the New York University School of Medicine, New York, New York 10016, USA. e-mails: [email protected]; [email protected]
NEWS & VIEWS RESEARCH magnetic flux far down the jets, as would be expected if the electromagnetic jet-launching mechanism referred to above is at work. The authors’ results thus provide direct observa tional evidence that this is the case. Theoretical simulations3,4 of accretion disks have shown that, under certain conditions, the magnetic flux in the vicinity of the black hole naturally reaches a maximum equilibrium value. When this happens, forces exerted by the magnetic field dominate in the inner part of the disk. Disks for which this is true are called magnetically arrested disks, or MADs3,4. Zamaninasab et al. find that the derived slope of the linear correlation between the accretiondisk luminosity and the jet magnetic flux is precisely the value predicted for such disks, strongly suggesting that this MAD scenario is operating in the hearts of AGN. These results indicate that the jets of AGN are launched electromagnetically by magnetic fields twisted by the black hole’s spin, that these magnetic fields have a dominant role in deter mining the dynamics of the disk and jets in the vicinity of the central black hole, and that this may remain true at least out to VLBI scales, several parsecs from the black hole. It will therefore be important to consider the influ ence of the magnetic field, for example, when inferring the properties of the central black hole and accretion disk from high-resolution studies made with millimetre-wavelength, ground-based VLBI5–7, or with ‘space VLBI’, in which one or more antennas orbiting Earth are used with ground antennas8,9. Zamaninasab and colleagues’ findings also radically change the way astronomers view the jets emanating from the centres of AGN. These jets are not just outflows of matter car rying tremendous amounts of energy, but are also intrinsically magnetic structures. Many of their properties are probably determined by the magnetic fields embedded in them and travelling outwards with them. The twisting of the central magnetic fields that launches the jets should give rise to helical jet mag netic fields, which may be manifest in the jets’ magnetic-field structure and morphol ogy10,11. Because a fundamental relationship exists between magnetic fields and electrical currents, jet outflows should be regarded as systems of magnetic fields and currents. This is essential if we are to understand these enormous structures: how they propagate, why they remain so narrow as they traverse enormous distances, and how they inter act with the material through which they are moving. As the jets travel beyond their host galaxy and into intergalactic space, effects other than magnetic forces will probably also come into play, making the jets and the surrounding gas more turbulent and reducing the magnetic field’s effects. Further detailed studies of the jets of AGN and their magnetic fields, from VLBI scales out to the ends of the jets many
thousands of parsecs from the central black hole, should help to determine whether such a transition occurs, and where. ■ Denise Gabuzda is in the Department of Physics, University College Cork, Cork, Ireland. e-mail: [email protected] 1. Zamaninasab, M., Clausen-Brown, E., Savolainen, T. & Tschekhovskoy, A. Nature 510, 126–128 (2014). 2. Blandford, R. D. & Znajek, R. L. Mon. Not. R. Astron. Soc. 179, 433–456 (1977). 3. Tchekhovskoy, A., Narayan, R. & McKinney, J. C.
Mon. Not. R. Astron. Soc. 418, L79–L83 (2011). 4. McKinney, J. C., Tchekhovskoy, A. & Blandford, R. D. Mon. Not. R. Astron. Soc. 423, 3083–3117 (2012). 5. Dexter, J. & Fragile, P. C. Mon. Not. R. Astron. Soc. 432, 2252–2272 (2013). 6. Doeleman, S. S. et al. Science 338, 355–358 (2012). 7. Johannsen, T. et al. Astrophys. J. 758, 30 (2012). 8. Kardashev, N. S. et al. Astron. Rep. 57, 153–194 (2013). 9. Takahashi, R. & Mineshige, S. Astrophys. J. 729, 86 (2011). 10. Molina, S. N. et al. Astron. Astrophys. (in the press); preprint at http://arXiv.org/abs/1404.5961 (2014). 11. Gabuzda, D. C., Cantwell, T. M. & Cawthorne, T. V. Mon. Not. R. Astron. Soc. 438, L1–L5 (2014).
H EPATI TI S C
Treatment triumphs A stampede of recent clinical studies suggests that we are on the cusp of developing well-tolerated, orally delivered drugs that can effectively eradicate hepatitis C virus from most, if not all, infected individuals. CHARLES M. RICE & MOHSAN SAEED
T
he story of hepatitis C began in the 1970s, when it was recognized that something other than hepatitis A or hepatitis B infections was causing liver inflam mation following blood transfusions1,2. In 1989, the troublemaker was identified as a small RNA virus, named hepatitis C (HCV)3. Although there are now effective diagnostic procedures that allow a safe blood supply in most developed countries, intravenous drug abuse continues to lead to new infections. An estimated 185 million people are chronically infected with HCV and are at risk of develop ing life-threatening liver diseases, including cirrhosis and cancer4. But a recent series of clin ical trials, reported in the New England Journal of Medicine5–11, demonstrate drastic increases in the effectiveness of anti-HCV drugs. Historically, HCV-infected patients have
1980s
Mystery virus
1989
HCV identified, diagnostics developed
~5% cure
been treated by intravenous injections with type I interferons — secreted cellular proteins that elicit potent antiviral responses12. The success rates for interferon-based regimens improved from single digits in the 1970s to around 50% by 2002, accomplished by increas ing dose, lengthening treatment, chemically stabilizing the interferon (by PEGylation) and adding ribavirin, an RNA-nucleoside analogue. Ribavirin has poor anti-HCV activity when used alone but significantly increased treat ment success when combined with interferon (by mechanisms that are still unsettled). How ever, this treatment required a 24- or 48-week course and was plagued by awful side effects, including nausea, depression and anaemia. Hence, the goal remained to develop highly effective, orally administered and well-toler ated regimens that work for all patient groups. Two enzymes encoded by HCV that are essential for viral replication — a serine
2002
PEG-IFN + ribavirin ~50% cure
2011
PEG-IFN + ribavirin + DAA ~75% cure
2014
All-oral DAA >95% cure
Figure 1 | HCV trajectory. In the 1980s, mysterious cases of liver inflammation following blood transfusions that were not explained by hepatitis A or hepatitis B viral infections were treated using type I interferon proteins, with a success rate of around 5%. The cause of these infections was identified in 1989 as RNA virus hepatitis C (HCV). The combination of PEGylated interferon (PEG-IFN) and ribavirin, approved in 2002, improved cure rates to around 50%. By 2011, drug cocktails containing HCV-specific direct-acting antivirals (DAAs) were being used to treat patients, with around 75% cure rates, and recent clinical trials5–11 of all-oral, interferon-free, DAA-based regimens have increased treatment success rates to more than 95%. 5 J U N E 2 0 1 4 | VO L 5 1 0 | NAT U R E | 4 3
© 2014 Macmillan Publishers Limited. All rights reserved
1. Tsang, W. Y. & Dynlacht, B. D. Cilia 2, 9 (2013). 2. Song, R. et al. Nature 510, 115–120 (2014).
3. Lizé, M., Herr, C., Klimke, A., Bals, R. & Dobbelstein, M. Cell Cycle 9, 4579–4583 (2010). 4. Martinez-Anton, A. et al. Am. J. Respir. Cell Mol. Biol. 49, 384–395 (2013). 5. Marcet, B. et al. Nature Cell Biol. 13, 693–699 (2011). 6. Spektor, A., Tsang, W. Y., Khoo, D. & Dynlacht, B. D. Cell 130, 678–690 (2007). 7. Flynt, A. S. & Lai, E. C. Nature Rev. Genet. 9, 831–842 (2008).
AST ROPHYSICS
The MAD world of black holes An analysis of optical and radio observations has revealed how powerful jets are launched from the centres of active galaxies, where supermassive black holes accrete matter through magnetically arrested disks, or MADs. See Letter p.126 DENISE GABUZDA
A
lthough the light given off by most galaxies is due to stars and glowing gas, some galaxies have extremely bright centres, or nuclei, with luminosities about 100,000 times greater than those of normal galaxies. In such active galactic nuclei, energy is liberated when matter spirals inwards and is captured by a supermassive black hole — billions of times more massive than the Sun — sitting at the galactic centre. In about 10% of these active nuclei, some of the in-spiralling matter is pushed into two a jets of matter and radiation that shoot out in opposite directions at close to the speed of light (Fig. 1). However, the forces causing these out flows have remained unknown. In this issue, Zamaninasab et al.1 (page 126) report direct evidence that the jets are launched on their a
journey by a kind of gigantic electromagnetic generator, in which magnetic fields in the vicinity of the black hole are twisted by the black hole’s spin, with the energy of this spin being transformed into the energy of the jets’ outward motion2. It has long been suspected that this mecha nism might explain how jets in active galactic nuclei (AGN) are produced. However, direct observational evidence has been elusive, because the scales on which this mechanism should operate are tiny compared with the smallest scales that can be observed directly using the highest-resolution technique available, which are typically about 1 par sec (3 × 1016 metres) from the central black hole. This scale might seem huge, but at the extremely large distances of AGN (billions of parsecs), it translates to a tiny angular distance as observed on the sky.
8. Cao, J. et al. Nature Cell Biol. 14, 697–706 (2012). 9. Chen, Z., Indjeian, V. B., McManus, M., Wang, L. & Dynlacht, B. D. Dev. Cell 3, 339–350 (2002). 10. Li, J. et al. Nature 495, 255–259 (2013). 11. D’Angiolella, V. et al. Nature 466, 138–142 (2010). 12. Lai, Y. et al. J. Allergy Clin. Immunol. 128, 1207–1215.e1 (2011).
Images of such small scales can be obtained using very-long-baseline interferometry (VLBI), an elegant technique in which radio telescopes around the world observe in syn chrony to imitate a single radio telescope with a diameter the size of Earth. The larger the telescope used, the finer the detail that can be seen; accordingly, VLBI yields radio images with phenomenally high resolution, equivalent to being able to peer across the Atlantic Ocean from the western edge of Europe and identify a coin held by someone standing on the eastern coast of the United States. Unfortunately, however, the scales imaged with VLBI are still tens to thousands of times larger than those on which the powerful jets of AGN are launched. Zamaninasab et al. have found a clever way to bridge this gap, by considering the magnetic flux in the jets — essentially, the product of the magnetic field pointing along the jet and the jet’s crosssection. The magnetic flux near the black hole cannot be measured directly, but should be proportional to the luminosity of the matter in the accretion disk surrounding the black hole. The disk’s luminosity can be estimated from observations of optical lines in the spectrum of the AGN. The authors have found a tight linear cor relation between the estimated accretion-disk luminosities for 76 AGN and the magnetic fluxes in their jets measured with VLBI. This correlation demonstrates that the magnetic flux near the black hole is proportional to the
b Twisted magnetic field
Jet
Black hole Rotating accretion disk
Figure 1 | Jets emerging from the centre of an active galactic nucleus. a, Composite image of the optical (true colour), X-ray (blue) and radio (orange) emission of the giant elliptical galaxy NGC 5128, showing its powerful radio jets extending far beyond the optical galaxy. b, Zamaninasab et al.1 have found evidence that jets emerging from the centres of active galaxies are launched from deep in the galactic nucleus (square in a) by twisted magnetic fields2, and that these fields are dynamically important in the vicinity of the galaxy’s central black hole and surrounding accretion disk. 4 2 | NAT U R E | VO L 5 1 0 | 5 J U N E 2 0 1 4
© 2014 Macmillan Publishers Limited. All rights reserved
A: ESO/WFI (OPTICAL); MPIFR/ESO/APEX/A. WEISS ET AL. (SUBMILLIMETRE); NASA/CXC/CFA/R. KRAFT ET AL. (X-RAY)
Irma Sánchez and Brian D. Dynlacht are at the New York University School of Medicine, New York, New York 10016, USA. e-mails: [email protected]; [email protected]
NEWS & VIEWS RESEARCH magnetic flux far down the jets, as would be expected if the electromagnetic jet-launching mechanism referred to above is at work. The authors’ results thus provide direct observa tional evidence that this is the case. Theoretical simulations3,4 of accretion disks have shown that, under certain conditions, the magnetic flux in the vicinity of the black hole naturally reaches a maximum equilibrium value. When this happens, forces exerted by the magnetic field dominate in the inner part of the disk. Disks for which this is true are called magnetically arrested disks, or MADs3,4. Zamaninasab et al. find that the derived slope of the linear correlation between the accretiondisk luminosity and the jet magnetic flux is precisely the value predicted for such disks, strongly suggesting that this MAD scenario is operating in the hearts of AGN. These results indicate that the jets of AGN are launched electromagnetically by magnetic fields twisted by the black hole’s spin, that these magnetic fields have a dominant role in deter mining the dynamics of the disk and jets in the vicinity of the central black hole, and that this may remain true at least out to VLBI scales, several parsecs from the black hole. It will therefore be important to consider the influ ence of the magnetic field, for example, when inferring the properties of the central black hole and accretion disk from high-resolution studies made with millimetre-wavelength, ground-based VLBI5–7, or with ‘space VLBI’, in which one or more antennas orbiting Earth are used with ground antennas8,9. Zamaninasab and colleagues’ findings also radically change the way astronomers view the jets emanating from the centres of AGN. These jets are not just outflows of matter car rying tremendous amounts of energy, but are also intrinsically magnetic structures. Many of their properties are probably determined by the magnetic fields embedded in them and travelling outwards with them. The twisting of the central magnetic fields that launches the jets should give rise to helical jet mag netic fields, which may be manifest in the jets’ magnetic-field structure and morphol ogy10,11. Because a fundamental relationship exists between magnetic fields and electrical currents, jet outflows should be regarded as systems of magnetic fields and currents. This is essential if we are to understand these enormous structures: how they propagate, why they remain so narrow as they traverse enormous distances, and how they inter act with the material through which they are moving. As the jets travel beyond their host galaxy and into intergalactic space, effects other than magnetic forces will probably also come into play, making the jets and the surrounding gas more turbulent and reducing the magnetic field’s effects. Further detailed studies of the jets of AGN and their magnetic fields, from VLBI scales out to the ends of the jets many
thousands of parsecs from the central black hole, should help to determine whether such a transition occurs, and where. ■ Denise Gabuzda is in the Department of Physics, University College Cork, Cork, Ireland. e-mail: [email protected] 1. Zamaninasab, M., Clausen-Brown, E., Savolainen, T. & Tschekhovskoy, A. Nature 510, 126–128 (2014). 2. Blandford, R. D. & Znajek, R. L. Mon. Not. R. Astron. Soc. 179, 433–456 (1977). 3. Tchekhovskoy, A., Narayan, R. & McKinney, J. C.
Mon. Not. R. Astron. Soc. 418, L79–L83 (2011). 4. McKinney, J. C., Tchekhovskoy, A. & Blandford, R. D. Mon. Not. R. Astron. Soc. 423, 3083–3117 (2012). 5. Dexter, J. & Fragile, P. C. Mon. Not. R. Astron. Soc. 432, 2252–2272 (2013). 6. Doeleman, S. S. et al. Science 338, 355–358 (2012). 7. Johannsen, T. et al. Astrophys. J. 758, 30 (2012). 8. Kardashev, N. S. et al. Astron. Rep. 57, 153–194 (2013). 9. Takahashi, R. & Mineshige, S. Astrophys. J. 729, 86 (2011). 10. Molina, S. N. et al. Astron. Astrophys. (in the press); preprint at http://arXiv.org/abs/1404.5961 (2014). 11. Gabuzda, D. C., Cantwell, T. M. & Cawthorne, T. V. Mon. Not. R. Astron. Soc. 438, L1–L5 (2014).
H EPATI TI S C
Treatment triumphs A stampede of recent clinical studies suggests that we are on the cusp of developing well-tolerated, orally delivered drugs that can effectively eradicate hepatitis C virus from most, if not all, infected individuals. CHARLES M. RICE & MOHSAN SAEED
T
he story of hepatitis C began in the 1970s, when it was recognized that something other than hepatitis A or hepatitis B infections was causing liver inflam mation following blood transfusions1,2. In 1989, the troublemaker was identified as a small RNA virus, named hepatitis C (HCV)3. Although there are now effective diagnostic procedures that allow a safe blood supply in most developed countries, intravenous drug abuse continues to lead to new infections. An estimated 185 million people are chronically infected with HCV and are at risk of develop ing life-threatening liver diseases, including cirrhosis and cancer4. But a recent series of clin ical trials, reported in the New England Journal of Medicine5–11, demonstrate drastic increases in the effectiveness of anti-HCV drugs. Historically, HCV-infected patients have
1980s
Mystery virus
1989
HCV identified, diagnostics developed
~5% cure
been treated by intravenous injections with type I interferons — secreted cellular proteins that elicit potent antiviral responses12. The success rates for interferon-based regimens improved from single digits in the 1970s to around 50% by 2002, accomplished by increas ing dose, lengthening treatment, chemically stabilizing the interferon (by PEGylation) and adding ribavirin, an RNA-nucleoside analogue. Ribavirin has poor anti-HCV activity when used alone but significantly increased treat ment success when combined with interferon (by mechanisms that are still unsettled). How ever, this treatment required a 24- or 48-week course and was plagued by awful side effects, including nausea, depression and anaemia. Hence, the goal remained to develop highly effective, orally administered and well-toler ated regimens that work for all patient groups. Two enzymes encoded by HCV that are essential for viral replication — a serine
2002
PEG-IFN + ribavirin ~50% cure
2011
PEG-IFN + ribavirin + DAA ~75% cure
2014
All-oral DAA >95% cure
Figure 1 | HCV trajectory. In the 1980s, mysterious cases of liver inflammation following blood transfusions that were not explained by hepatitis A or hepatitis B viral infections were treated using type I interferon proteins, with a success rate of around 5%. The cause of these infections was identified in 1989 as RNA virus hepatitis C (HCV). The combination of PEGylated interferon (PEG-IFN) and ribavirin, approved in 2002, improved cure rates to around 50%. By 2011, drug cocktails containing HCV-specific direct-acting antivirals (DAAs) were being used to treat patients, with around 75% cure rates, and recent clinical trials5–11 of all-oral, interferon-free, DAA-based regimens have increased treatment success rates to more than 95%. 5 J U N E 2 0 1 4 | VO L 5 1 0 | NAT U R E | 4 3
© 2014 Macmillan Publishers Limited. All rights reserved
