Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userAn ultracompact X-ray binary in the globular cluster M15 (NGC7078)
91
0
0.0
(
0
)
Authors
A. Dieball
Ask ChatGPT about the research
No Arabic abstract
We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) waveband. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semi-amplitude and we clearly detect a period of 22.5806+/-0.0002 min. We have carried out extensive Monte Carlo simulations which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV bright (approx. 17 mag) and is characterized by an extremely blue spectral energy distribution (F_lambda ~ lambda^{-2.0}). We also find evidence for an excess of flux between 1500 and 1600 AA and probably between 1600 and 2000 AA, which might be due to CIV 1550 and HeII 1640 emission lines. We also show that M15 X-2s X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is approx. 34 degree. We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.
rate research
Read More
The globular cluster M15 was observed on three occasions with the High Resolution Camera on board Chandra in 2001 in order to investigate the X-ray source population in the cluster centre. After subtraction of the two bright central sources, four faint sources were identified within 50 arcsec of the core. One of these sources is probably the planetary nebula, K648, making this the first positive detection of X-rays from a planetary nebula inside a globular cluster. Another two are identified with UV variables (one previously known), which we suggest are cataclysmic variables (CVs). The nature of the fourth source is more difficult to ascertain, and we discuss whether it is possibly a quiescent soft X-ray transient (qSXT) or also a CV.
Results are discussed of an X-ray and optical observation campaign of the low-mass X-ray binary A1246-58 performed with instruments on Satellite per Astronomia X (BeppoSAX), the Rossi X-ray Timing Explorer (RXTE), the X-ray Multi-mirror Mission (XMM-Newton), the Swift mission, and the Very Large Telescope. Spectra and flux time histories are studied. The most important results are the lack of hydrogen spectral features in the optical spectrum, supporting the proposition that this is an ultracompact X-ray binary (UCXB), the determination of a 4.3 kpc distance from time-resolved spectroscopy of thermonuclear X-ray bursts, and the detection of intermediately long thermonuclear bursts as seen in a number of other UCXBs. There is evidence for a Ne/O abundance ratio in the line of sight that is higher than solar and variable. This may be due to different changes in the ionization degrees of Ne and O, which may be related to the variable irradiating flux. We discuss the spectral variability and the peculiarities of the long-term light curve.
We report the discovery of OGLE-UCXB-01, a 12.8 minute variable object located in the central field of Galactic bulge globular cluster Djorg 2. The presence of frequent, short-duration brightenings at such an ultrashort period in long-term OGLE photometry together with the blue color of the object in Hubble Space Telescope images and the detection of moderately hard X-rays by Chandra observatory point to an ultracompact X-ray binary system. The observed fast period decrease makes the system a particularly interesting target for gravitational-wave detectors such as the planned Laser Interferometer Space Antenna.
The bright X-ray binary X2127+119 in the core of the globular cluster M15 has long been thought to be in an unusual evolutionary state, in which the binary is embedded in a common envelope. Support for this idea comes from X2127+119s absorption lines, which are blue shifted at all orbital phases, indicating the existence of outflows from the system. A common-envelope scenario implies that the absorption lines should exhibit maximum blue shift near mid-eclipse (binary phase 0.0). We have re-analysed INT spectra of X2127+119 obtained in 1986, 1987 and 1988 using the latest orbital ephemeris (substantially different from that used in the original analysis), and find that the orbital phase at which the absorption lines show a maximum blue shift is not 0.0, but rather 0.25 -- 0.3. These results indicate that a common-envelope scenario for X2127+119 may not work. In addition, from spectrograms of the He II 4686 line, we report the first tentative detection of X2127+119s companion star.
Neutron stars are thought to be born rapidly rotating and then exhibit a phase of a rotation-powered pulsations as they slow down to 1-10 s periods. The significant population of millisecond pulsars observed in our Galaxy is explained by the recycling concept: during an epoch of accretion from a donor star in a binary system, the neutron star is spun up to millisecond periods. However, only a few pulsars are observed during this recycling process, with relatively high rotational frequencies. Here we report the detection of an X-ray pulsar with $P_{rm spin} = 1.20$ s in the globular cluster B091D in the Andromeda galaxy, the slowest pulsar ever found in a globular cluster. This bright (up-to 30% of the Eddington luminosity) spinning-up pulsar, persistent over the 12 years of observations, must have started accreting less than 1 Myr ago and has not yet had time to accelerate to hundreds of Hz. The neutron star in this unique wide binary with an orbital period $P_{rm orb} = 30.5$ h in a 12 Gyr old, metal rich star cluster, accretes from a low mass, slightly evolved post-main sequence companion. We argue that we are witnessing a binary formed at relatively recent epoch by getting a $sim 0.8M_odot$ star in a dynamical interaction -- a viable scenario in a massive dense globular cluster like B091D with high global and specific stellar encounter rates. This intensively accreting non-recycled X-ray pulsar provides therefore a long-sought missing piece in the standard pulsar recycling picture.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
