In 1975, just a few years after the UHURU X-ray satellite had identified a large population of X-ray binaries in the Galaxy, Hansen & van Horn (1975) argued that gas accreting on the surface of a neutron star would undergo thermonuclear reactions, and that the burning would be unstable. The very next year, Grindlay et al. (1976) discovered Type I X-ray bursts; the bursts were immediately associated with thermonuclear instability on the surface of a neutron star (Woosley & Taam 1976).
In the years that followed, many observational studies expanded our knowledge of burst phenomenology (e.g. Lewin et al. 1993, and references therein), including a recent explosion of data on burst oscillations. At the same time, a number of theoretical investigations provided the basic physical explanation for many of the observed phenomena (see Bildsten 1998, for reviews). Today, the Type I burst phenomenon represents a mature and exciting branch of high-energy astrophysics that continues to thrive.
The launch of the Chandra and XMM-Newton observatories along with the earlier launch of the Rossi X-ray Timing Explorer (RXTE) have opened a new era of our understanding of these ob jects; both the quantity and quality of data have increased manifold. It is indeed a golden age of high-energy astrophysics. The influx of discovery has fueled much of my recent research especially in the physics of neutron stars and their atmospheres. An excellent way to learn about neutron stars is to observe how matter that we understand well interacts with them. Because many if not most stars are in binaries, many neutron stars and black holes have companions from which they accrete material. How the compact object transforms the accreted material and how this material affects the compact ob ject provides a unique diagnostic on the properties of the neutron star or black hole.
A neutron star with a low-mass companion may accrete from its companion for a billion years or longer. During this time a portion of the angular momentum of the orbit is transferred to the neutron star, spinning it up. Later after accretion ceases, the neutron star may become a millisecond radio pulsar. Theoretically, accretion could spin up a neutron star to a period of less than a millisecond, but the fastest spinning neutron star has a significantly larger period of 1.6 ms. One solution to this puzzle is that the neutron star could radiate away the accreted angular momentum as gravitational radiation. How and whether this process occurs depends on the properties of the neutron- star interior. Specifically, the angular momentum could be radiated away steadily as material is accreted, it could be radiated in bursts, or not at all. Using the current understanding of the r-mode oscillations within the neutron star, the neutron star will spin up to a critical frequency and than radiate the accumulated angular momentum in a burst lasting several thousand years (Heyl 2002). This has two important implications. First, several neutron stars in the galaxy will emit gravitational waves strong enough to be observed with LIGO. Second, these neutron stars will have surfaces as hot as a neutron star a couple of decades after the supernova. Both of these phenomena may be observed in the next few years.
As material accumulates and is compressed on the surface of a neutron star, it releases nuclear energy. If the accretion rate is sufficiently high the energy is released steadily. Otherwise, it is released explosively. The explosions appear as Type-I X-ray Bursts. Although our understanding of Type-I bursts has increased dramatically since they were first observed about twenty-five years ago, many puzzles remain.