A helium-burning white dwarf binary as a supersoft X-ray source.

Type Ia supernovae are cosmic distance indicators1,2, and the main source of iron in the Universe3,4, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed5 and suggested as Type Ia supernovae progenitors6-9. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion10, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs. 7,11,12), including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae13, within this scenario.

Detection of large-scale X-ray bubbles in the Milky Way halo.

The halo of the Milky Way provides a laboratory to study the properties of the shocked hot gas that is predicted by models of galaxy formation. There is observational evidence of energy injection into the halo from past activity in the nucleus of the Milky Way1-4; however, the origin of this energy (star formation or supermassive-black-hole activity) is uncertain, and the causal connection between nuclear structures and large-scale features has not been established unequivocally. Here we report soft-X-ray-emitting bubbles that extend approximately 14 kiloparsecs above and below the Galactic centre and include a structure in the southern sky analogous to the North Polar Spur. The sharp boundaries of these bubbles trace collisionless and non-radiative shocks, and corroborate the idea that the bubbles are not a remnant of a local supernova5 but part of a vast Galaxy-scale structure closely related to features seen in γ-rays6. Large energy injections from the Galactic centre7 are the most likely cause of both the γ-ray and X-ray bubbles. The latter have an estimated energy of around 1056 erg, which is sufficient to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way.

An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre.

Evidence has mounted in recent decades that outflows of matter and energy from the central few parsecs of our Galaxy have shaped the observed structure of the Milky Way on a variety of larger scales1. On scales of 15 parsecs, the Galactic Centre has bipolar lobes that can be seen in both the X-ray and radio parts of the spectrum2,3, indicating broadly collimated outflows from the centre, directed perpendicular to the Galactic plane. On larger scales, approaching the size of the Galaxy itself, γ-ray observations have revealed the so-called 'Fermi bubble' features4, implying that our Galactic Centre has had a period of active energy release leading to the production of relativistic particles that now populate huge cavities on both sides of the Galactic plane. The X-ray maps from the ROSAT all-sky survey show that the edges of these cavities close to the Galactic plane are bright in X-rays4-6. At intermediate scales (about 150 parsecs), radio astronomers have observed the Galactic Centre lobe, an apparent bubble of emission seen only at positive Galactic latitudes7,8, but again indicative of energy injection from near the Galactic Centre. Here we report prominent X-ray structures on these intermediate scales (hundreds of parsecs) above and below the plane, which appear to connect the Galactic Centre region to the Fermi bubbles. We propose that these structures, which we term the Galactic Centre 'chimneys', constitute exhaust channels through which energy and mass, injected by a quasi-continuous train of episodic events at the Galactic Centre, are transported from the central few parsecs to the base of the Fermi bubbles4.

Synchronization of environmentally coupled, passively mode-locked fiber lasers.

We have synchronized a pair of femtosecond, passively mode-locked Er-doped fiber lasers by active adjustment of the cavity length of one laser with an electronic servo. While each laser delivers pulses with durations of several hundred femtoseconds, they have been synchronized to within a relative timing jitter of 5 ps rms, as measured by cross correlation. This corresponds to a phase error of 30 arcsec. Low drift and jitter were accomplished by thermal and mechanical coupling of the two lasers.

ROSAT Observations of X-ray Emissions from Jupiter During the Impact of Comet Shoemaker-Levy 9.

Röntgensatellit (ROSAT) observations made shortly before and during the collision of comet Shoemaker-Levy 9 with Jupiter show enhanced x-ray emissions from the planet's northern high latitudes. These emissions, which occur at System III longitudes where intensity enhancements have previously been observed in Jupiter's ultraviolet aurora, appear to be associated with the comet fragment impacts in Jupiter's southern hemisphere and may represent brightenings of the jovian x-ray aurora caused either by the fragment impacts themselves or by the passage of the fragments and associated dust clouds through Jupiter's inner magnetosphere.

Regenerative Nd:glass amplifier seeded with a Nd:fiber laser.

Wavelength tuning and broad-bandwidth operation of a passively mode-locked Nd:fiber laser is demonstrated at 1060 microm. The oscillator pulses are used to seed a bulk regenerative Nd:glass amplifier, and 300-fs transformlimited pulses with an energy of 10 microJ are obtained after 31 round trips at a repetition rate of 500 Hz.

Additive-pulse-compression mode locking of a neodymium fiber laser.

The generation of bandwidth-limited shoulder-free 125-fsec pulses by additive-pulse-compression mode locking of a neodymium glass laser is described. An all-fiber nonlinear amplifying loop mirror is employed as a fast saturable absorber and permits stable pulse generation under the condition of large pulse shaping in the cavity.

Mode locking with cross-phase and self-phase modulation.

Cross-phase and self-phase modulation are used for self-sustained mode locking of a high-power neodymium glass fiber laser. Stable pulses with a FWHM as short as 70 fs and pulse energies of as much as 1 nJ are generated at a wavelength of 1.064 microm.

Nonlinear amplifying loop mirror.

A novel device arrangement for all-optical switching that permits efficient exploitation of waveguide nonlinearities is discussed. It is based on a long optical fiber loop mirror with an integral short asymmetrically located optical amplifier. The device performance is demonstrated by using a Nd(3+)-doped fiber amplifier. Switching is obtained for peak signal powers of less than 1 W and an amplifier pump power of 10 mW.

Active mode locking of a neodymium-doped fiber laser using intracavity pulse compression.

Active mode locking of a Nd(3+)-doped fiber laser with piezoelectrically induced Raman-Nath diffraction modulation is demonstrated. By using intracavity pulse compression, stable pulses of 2.4-psec length are generated at a wavelength of 1054 nm.