P. Esposito

Relativistic jet activity from the tidal disruption of a star by a massive black hole

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.D. N. Burrows , J. A. Kennea , G. Ghisellini , V. Mangano , B. Zhang , K. L. Page , M. Eracleous , P. Romano , T. Sakamoto , A. D. Falcone , J. P. Osborne , S. Campana , A. P. Beardmore , A. A. Breeveld , M. M. Chester , R. Corbet , S. Covino , J. R. Cummings , P. D’Avanzo , V. D’Elia , P. Esposito , P. A. Evans , D. Fugazza, J. M. Gelbord , K. Hiroi , S. T. Holland , K. Y. Huang , M. Im, G. Israel , Y. Jeon , Y.-B. Jeon , N. Kawai , H. A. Krimm , P. Mészáros , H. Negoro , N. Omodei , W.K. Park , J. S. Perkins , M. Sugizaki , H.-I. Sung , G. Tagliaferri , E. Troja , Y. Ueda, Y. Urata, R. Usui , L. A. Antonelli , S. D. Barthelmy , G. Cusumano , P. Giommi , F. E. Marshall , A. Melandri , M. Perri , J. L. Racusin , B. Sbarufatti , M. H. Siegel , & N. Gehrels 21

Relativistic jet activity from the tidal disruption of a star by a massive black hole [Discovery of the onset of rapid accretion by a dormant massive black hole]

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.D. N. Burrows , J. A. Kennea , G. Ghisellini , V. Mangano , B. Zhang , K. L. Page , M. Eracleous , P. Romano , T. Sakamoto , A. D. Falcone , J. P. Osborne , S. Campana , A. P. Beardmore , A. A. Breeveld , M. M. Chester , R. Corbet , S. Covino , J. R. Cummings , P. D’Avanzo , V. D’Elia , P. Esposito , P. A. Evans , D. Fugazza, J. M. Gelbord , K. Hiroi , S. T. Holland , K. Y. Huang , M. Im, G. Israel , Y. Jeon , Y.-B. Jeon , N. Kawai , H. A. Krimm , P. Mészáros , H. Negoro , N. Omodei , W.K. Park , J. S. Perkins , M. Sugizaki , H.-I. Sung , G. Tagliaferri , E. Troja , Y. Ueda, Y. Urata, R. Usui , L. A. Antonelli , S. D. Barthelmy , G. Cusumano , P. Giommi , F. E. Marshall , A. Melandri , M. Perri , J. L. Racusin , B. Sbarufatti , M. H. Siegel , & N. Gehrels 21

A Low-Magnetic-Field Soft Gamma Repeater

Odd Magnetar Magnetars are neutron stars that are widely thought to be powered by extremely high magnetic fields. Using data from three different x-ray observatories, Rea et al. (p. 944, published online 14 October) show that a previously known magnetar has a magnetic field that is much smaller than those of other magnetars. A strong magnetic field is thus not a necessary requirement for a neutron star to show magnetar-like behavior, implying that this population is wider than was previously thought. A neutron star lacking a strong magnetic field can still emit short bursts of high-energy radiation. Soft gamma repeaters (SGRs) and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, that is, neutron stars powered by extreme magnetic fields, B ~ 1014 to 1015 gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5 × 1012 gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages, and evolutionary stages than observed so far.

A New Low Magnetic Field Magnetar: The 2011 Outburst of Swift J1822.3-1606

We report on the long-term X-ray monitoring withSwift,RXTE,Suzaku,Chandra, andXMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3−1606 (SGR 1822−1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3−1606’s radiopulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative ˙ P = 8.3(2) × 10 −14 ss −1 , which implies an inferred dipolar surface magnetic field of B � 2.7 × 10 13 G at the equator. This measurement makes Swift J1822.3−1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3−1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ∼ 1.5 × 10 14 G and Btor ∼ 7 × 10 14 G, respectively, and if its current age

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Spinning up an extragalactic neutron star Ultraluminous x-ray sources (ULXs) are strange objects in other galaxies that cannot be explained by conventional accretion onto stellar-mass objects. This has led to exotic interpretations, such as the long-sought intermediate-mass black holes. Israel et al. observed a ULX in the nearby galaxy NGC 5907 and found that it is instead a neutron star. The spinning neutron star is accreting material so fast that its spin period is quickly accelerating. The only way that it can consume enough material to explain these properties is if it has a strong multipolar magnetic field. Science, this issue p. 817 An ultraluminous x-ray source in NGC 5907 is a spinning neutron star with a complex magnetic field. Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of ~1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥ 1041 erg second−1) might harbor NSs.

Magnetar outbursts: an observational review

Transient outbursts from magnetars have shown to be a key property of their emission, and one of the main way to discover new sources of this class. From the discovery of the first transient event around 2003,we now count about a dozen of outbursts, which increased the number of these strongly magnetic neutron stars by a third in six years. Magnetars’ outbursts might involve their multi-band emission resulting in an increased activity from radio to hard X-ray, usually with a soft Xray flux increasing by a factor of 10–1000 with respect to the quiescent level. A connected X-ray spectral evolution is also often observed, with a spectral softening during the outburst decay. The flux decay times vary a lot from source to source, ranging from a few weeks to several years, as also the decay law which can be exponential-like, a power-law or even multiple power-laws can be required to model the flux decrease. We review here on the latest observational results on the multiband emission of magnetars, and summarize one by one all the transient events which could be studied to date from these sources.

THE EVOLUTION OF THE γ- AND X-RAY LUMINOSITIES OF PULSAR WIND NEBULAE

Pulsar wind nebulae are a prominent class of very high energy (E > 0.1 TeV) Galactic sources. Their -ray spectra are interpreted as due to inverse Compton scattering of ultrarelativistic electrons on the ambient photons, whereas the X-ray spectra are due to synchrotron emission. We investigate the relation between the - and-X-ray emission and the pulsars’ spin-down luminosity and characteristic age. We find that the distance-independent - to X-ray flux ratio of the nebulae is inversely proportional to the spin-down luminosity, (∝ u E −1.9 ), while it appears proportional to the characteristic age, (∝ � 2.2 c ), of the parent pulsar. We interpret these results as due to the evolution of the electron energy distribution and the nebular dynamics, supporting the idea of so-called relic pulsar wind nebulae. These empirical relations provide a new tool to classify unidentified diffuse -ray sources and to estimate the spin-down luminosity and characteristic age of rotation powered pulsars with no detected pulsation from the X- and −ray properties of the associated pulsar wind nebulae. We apply these relations to predict the spin-down luminosity and characteristic age of four (so far unpulsing) candidate pulsars associated to wind nebulae. Subject headings: pulsars : general — radiation mechanisms: non-thermal — supernova remnants — X-rays : stars — gamma rays: observations

The first outburst of the new magnetar candidate SGR 0501+4516

We report here on the outburst onset and evolution of the new soft gamma-ray repeater SGR 0501+4516. We monitored the new SGR with XMM- Newton starting on 2008 August 23, 1 day after the source became burst active, and continuing with four more observations in the following month, with the last one on 2008 September 30. Combining the data with the Swift X-ray telescope (Swift-XRT) and Suzaku data, we modelled the outburst decay over a 3-month period, and we found that the source flux decreased exponentially with a time-scale of t(c) = 23.8 d. In the first XMM-Newton observation, a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behaviour of the source during the first month of the outburst decay, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our second and fourth XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to similar to 100 keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component [(sic) = -1.6(4) x 10(-19) s s(-2)]. Thanks to the phase connection, we were able to study the phase-resolved spectral evolution of SGR 0501+ 4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux similar to 80 times lower than that measured during the outburst, and a rather soft, thermal spectrum.

Magnetars as persistent hard X-ray sources : INTEGRAL discovery of a hard tail in SGR 1900+14

Using 2.5 Ms of data obtained by the INTEGRAL satellite in 2003-2004, we discovered persistent hard X-ray emission from the soft gamma-ray repeater SGR 1900+14. Its 20-100 keV spectrum is well described by a steep power law with photon index r = 3.1 ± 0.5 and flux 1.5 x 10 -11 erg cm -2 s -1 . Contrary to SGR 1806-20, the only other soft gamma-ray repeater for which persistent emission above 20 keV was reported, SGR 1900+14 has been detected in the hard X-ray range while it was in a quiescent state (the last bursts from this source were observed in 2002). By comparing the broad band spectra (1-100 keV) of all the magnetars detected by INTEGRAL (the two SGRs and three anomalous X-ray pulsars) we find evidence for a different spectral behaviour of these two classes of sources.

A variable absorption feature in the X-ray spectrum of a magnetar

Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays. This behaviour can be understood in the magnetar model, according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties of SGRs and AXPs are greater than—or at the high end of the range of—those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters, whereas a strong field has been proposed to reside in the stellar interior and in multipole components on the surface. Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star’s rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2 × 1014 gauss to more than 1015 gauss.