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Register a new userIdentification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
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J.J.M. in t Zand
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After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.
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The globular cluster NGC 6440 is known to harbor a bright neutron-star X-ray transient. We observed the globular cluster with Chandra on two occasions when the bright transient was in its quiescent state in July 2000 and June 2003 (both observations were made nearly 2 years after the end of their preceding outbursts). The quiescent spectrum during the first observation is well represented by a two component model (a neutron-star atmosphere model plus a power-law component which dominates at energies above 2 keV). During the second observation (which was roughly of equal duration to the first observation) we found that the power-law component could no longer be detected. Our spectral fits indicate that the effective temperature of the neutron-star surface was consistent between the two observations. We conclude that the effect of the change in power-law component caused the 0.5-10 keV flux to be a factor of ~2 lower during the second observation compared to the first observation. We discuss plausible explanations for the variations, including variable residual accretion onto the neutron star magnetosphere or some variation in the interaction of the pulsar wind with the matter still outflowing from the companion star.
We present the results of optical and infrared photometry of the quiescent X-ray transient V404 Cyg during the period 1992-2003. The ellipsoidal modulations extracted from the most complete databases (years 1992, 1998 and 2001) show unequal maxima and minima with relative strength varying from year to year although their peak to peak amplitudes remain roughly constant at 0.24+-0.01 magnitudes. Fast optical variations superimposed on the secondary stars double-humped ellipsoidal modulation were detected every year with a mean amplitude of ~0.07 mags. We have not found significant changes in the activity during this decade which indicates that this variability is probably not connected to the 1989 outburst. We have found periodicities in the 1998 and 2001 data near the 6 hr quasi-periodicity observed in 1992, although we interpret it as consequence of the appearance of a flare event almost every night. Significant variability is also present in the I and near infrared (J and K_short) bands and this decreases slightly or remains approximately constant at longer wavelengths. A cross correlation analysis shows that both the R and I emission are simultaneous down to 40 s.
We report the search for low-mass X-ray binaries in quiescence (qLMXBs) in the globular cluster NGC 6304 using XMM observations. We present the spectral analysis leading to the identification of three candidate qLMXBs within the field of this globular cluster (GC), each consistent with the X-ray spectral properties of previously identified qLMXBs in the field and in other globular clusters -- specifically, with a hydrogen atmosphere neutron star with radius between 5--20km. One (source 4, with R=11.7^{+8.3}_{-0.4} (D/5.97 kpc) km and kT_eff=117^{+59}_{-44} eV) is located within one core radius (r_c) of the centre of NGC 6304. This candidate also presents a spectral power-law component contributing 49 per cent of the 0.5-10 keV flux. A second one (source 9 with R=15.3^{+11.2}_{-6.5} (D/5.97 kpc) km and kT_eff=100^{+24}_{-19} eV) is found well outside the optical core (at 32 r_c) but still within the tidal radius. From spatial coincidence, we identify a bright 2MASS infrared counterpart which, at the distance of NGC 6304, seems to be a post-asymptotic giant branch star. The third qLMXB (source 5 with R=23^{+38}_{-14} (D/5.97 kpc) km and kT_eff=70^{+28}_{-20} eV) is a low signal-to-noise candidate for which we also identify from spatial coincidence a bright 2MASS infrared counterpart, with 99.916 per cent confidence. Three qLMXBs from this GC is marginally consistent with that expected from the encounter rate of NGC 6304. We also report a low signal-to-noise source with an unusually hard photon index (alpha=-2.0^{+1.2}_{-2.2}). Finally, we present an updated catalogue of the X-ray sources lying in the field of NGC 6304, and compare this with the previous catalogue compiled from ROSAT observations.
During five ROSAT HRI observations in July 1995, a bright X-ray source (designated 1RXH J132519.8-430312) was detected 2.5 south-west of the nucleus of NGC 5128 (Centaurus A) at the outer regions of the elliptical galaxy. At that time it was the brightest point source in the ROSAT HRI field-of-view. All other ROSAT observations made before and after these observations show no trace of the source suggesting that this is a highly variable object. Chandra observations of Cen A made in September 1999 show a source at the ROSAT position which may be the quiescent counterpart of the transient. It is very probable that the transient is located in NGC 5128. Given the existence of a transient source that close to the direction of NGC 5128, all observations (at least in soft X-rays) of Cen A, with instruments of low spatial resolution, must take the presence and variability of this object into account.
A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded a large number of X-ray sources. This is the second in a series of papers with the aim of identifying the optical counterparts of these X-ray sources. We have obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece. For 17 of the 20 X-ray sources, we have identified the optical counterpart as a normal late type star (of type F or later) in the foreground (i.e. in the Milky Way). For two more sources there were two possible optical counterparts in each case, while two more objects have X-ray properties that are not compatible with the spectral characteristics of late type non-flaring stars.
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