An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.     

A powerful bursting radio source towards the Galactic Centre

Transient astronomical sources are typically powered by compact objects and usually signify highly explosive or dynamic events. Although high-time-resolution observations are often possible in radio astronomy, they are usually limited to quite narrow fields of view. The dynamic radio sky is therefore poorly sampled, in contrast to the situation in the X-ray and gamma-ray bands in which wide-field instruments routinely detect transient sources. Here we report a transient radio source, GCRT J1745-3009, which was detected during a moderately wide-field monitoring programme of the Galactic Centre region at 0.33 GHz. The characteristics of its bursts are unlike those known for any other class of radio transient. If located in or near the Galactic Centre, its brightness temperature (approximately 10(16) K) and the implied energy density within GCRT J1745-3009 vastly exceed those observed in most other classes of radio astronomical sources, and are consistent with coherent emission processes that are rarely observed. We conclude that it represents a hitherto unknown class of transient radio sources, the first of possibly many new classes that may be discovered by emerging wide-field radio telescopes.     

Near-infrared flares from accreting gas around the supermassive black hole at the Galactic Centre

Recent measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* (refs 4, 5) at the Galactic Centre is a 3.6-million-solar-mass black hole. Sgr A* is remarkably faint in all wavebands other than the radio region, however, which challenges current theories of matter accretion and radiation surrounding black holes. The black hole's rotation rate is not known, and therefore neither is the structure of space-time around it. Here we report high-resolution infrared observations of Sgr A* that reveal 'quiescent' emission and several flares. The infrared emission originates from within a few milliarcseconds of the black hole, and traces very energetic electrons or moderately hot gas within the innermost accretion region. Two flares exhibit a 17-minute quasi-periodic variability. If the periodicity arises from relativistic modulation of orbiting gas, the emission must come from just outside the event horizon, and the black hole must be rotating at about half of the maximum possible rate.     

Infrared diffuse interstellar bands in the Galactic Centre region

The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called 'diffuse interstellar bands' (DIBs). The first DIBs were reported about 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micrometre (10,000 ?ngstr?ms), the longer of which is at 1.318?micrometres (ref. 6). Here we report 13 diffuse interstellar bands in the 1.5-1.8 micrometre interval on high-extinction sightlines towards stars in the Galactic Centre. We argue that they originate almost entirely in the Galactic Centre region, a considerably warmer and harsher environment than where DIBs have been observed previously. The relative strengths of these DIBs towards the Galactic Centre and the Cygnus OB2 diffuse cloud are consistent with their strengths scaling mainly with the extinction by diffuse material.     

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre.

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 x 106 solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.     

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of 37(+16)(-10) microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.     

A faint discrete source origin for the highly ionized iron emission from the Galactic Centre region.

The origin of the X-ray emission for the central region of our Galaxy has remained a mystery. In particular, the relative spectral contributions of the diffuse emission and discrete sources, which are critical to understanding the high-energy phenomena in this environment, have been unclear because of the lack of sufficient spatial resolution. Here we report the results of a large-scale imaging survey of the Galactic Centre that resolves these components. We find that the Kalpha emission from iron that has been highly ionized (so that it has only two electrons left), which has previously been attributed to the diffuse component, actually arises mainly from discrete sources. This suggests that the presence of a large amount of hot gas (T approximately 108 K) is no longer required to explain the iron line emission. The spectra of the discrete sources indicate the presence of numerous accreting white dwarfs, neutron stars, and/or black holes in the region. The diffuse emission dominates over the contribution from the faint point sources, and is shown to be associated globally with interstellar features that have been observed at radio and mid-infrared wavelengths, suggesting that it is the product of recent massive star formation.     

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Way's halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.     

Deuterium in the Galactic Centre as a result of recent infall of low-metallicity gas

The Galactic Centre is the most active and heavily processed region of the Milky Way, so it can be used as a stringent test for the abundance of deuterium (a sensitive indicator of conditions in the first 1,000 seconds in the life of the Universe). As deuterium is destroyed in stellar interiors, chemical evolution models predict that its Galactic Centre abundance relative to hydrogen is D/H = 5 x 10(-12), unless there is a continuous source of deuterium from relatively primordial (low-metallicity) gas. Here we report the detection of deuterium (in the molecule DCN) in a molecular cloud only 10 parsecs from the Galactic Centre. Our data, when combined with a model of molecular abundances, indicate that D/H = (1.7 +/- 0.3) x 10(-6), five orders of magnitude larger than the predictions of evolutionary models with no continuous source of deuterium. The most probable explanation is recent infall of relatively unprocessed metal-poor gas into the Galactic Centre (at the rate inferred by Wakker). Our measured D/H is nine times less than the local interstellar value, and the lowest D/H observed in the Galaxy. We conclude that the observed Galactic Centre deuterium is cosmological, with an abundance reduced by stellar processing and mixing, and that there is no significant Galactic source of deuterium.     

A magnetic torsional wave near the Galactic Centre traced by a 'double helix' nebula

The magnetic field in the central few hundred parsecs of the Milky Way has a dipolar geometry and is substantially stronger than elsewhere in the Galaxy, with estimates ranging up to a milligauss (refs 1-6). Characterization of the magnetic field at the Galactic Centre is important because it can affect the orbits of molecular clouds by exerting a drag on them, inhibit star formation, and could guide a wind of hot gas or cosmic rays away from the central region. Here we report observations of an infrared nebula having the morphology of an intertwined double helix about 100 parsecs from the Galaxy's dynamical centre, with its axis oriented perpendicular to the Galactic plane. The observed segment is about 25 parsecs in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional Alfvén wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the images show a possible meandering channel that warrants further investigation.     

A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ～3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.     

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earth's crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm x yr(-1)-which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6-1.3 cm x yr(-1)) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.     

A low level of extragalactic background light as revealed by gamma-rays from blazars

The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the gamma-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of gamma-ray emission from the blazars H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources-in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to gamma-rays than previously thought.     

Very fast optical flaring from a possible new Galactic magnetar

Highly luminous rapid flares are characteristic of processes around compact objects like white dwarfs, neutron stars and black holes. In the high-energy regime of X-rays and gamma-rays, outbursts with variabilities on timescales of seconds or less are routinely observed, for example in gamma-ray bursts or soft gamma-ray repeaters. At optical wavelengths, flaring activity on such timescales has not been observed, other than from the prompt phase of one exceptional gamma-ray burst. This is mostly due to the fact that outbursts with strong, fast flaring are usually discovered in the high-energy regime; most optical follow-up observations of such transients use instruments with integration times exceeding tens of seconds, which are therefore unable to resolve fast variability. Here we show the observation of extremely bright and rapid optical flaring in the Galactic transient SWIFT J195509.6+261406. Our optical light curves are phenomenologically similar to high-energy light curves of soft gamma-ray repeaters and anomalous X-ray pulsars, which are thought to be neutron stars with extremely high magnetic fields (magnetars). This suggests that similar processes are in operation, but with strong emission in the optical, unlike in the case of other known magnetars.     

Functions of FGF signalling from the apical ectodermal ridge in limb development

To determine the role of fibroblast growth factor (FGF) signalling from the apical ectodermal ridge (AER), we inactivated Fgf4 and Fgf8 in AER cells or their precursors at different stages of mouse limb development. We show that FGF4 and FGF8 regulate cell number in the nascent limb bud and are required for survival of cells located far from the AER. On the basis of the skeletal phenotypes observed, we conclude that these functions are essential to ensure that sufficient progenitor cells are available to form the normal complement of skeletal elements, and perhaps other limb tissues. In the complete absence of both FGF4 and FGF8 activities, limb development fails. We present a model to explain how the mutant phenotypes arise from FGF-mediated effects on limb bud size and cell survival.