We have used Minor Planet Center data and tools to explore the discovery circumstances and properties of the currently known population of over 10,000 NEAs, and to quantify the challenges for follow-up from ground-based telescopes. The increasing rat
e of discovery has grown to ~1,000/year as surveys have become more sensitive, by 1mag every ~7.5 years. However, discoveries of large (H =< 22) NEAs have remained stable at ~365/year over the past decade, at which rate the 2005 Congressional mandate to find 90% of 140m NEAs will not be met before 2030. Meanwhile, characterization is falling farther behind: Fewer than 10% of NEAs are well characterized in terms of size, rotation periods, and spectra, and at current rates of follow-up it will take about a century to determine them even for the known population. Over 60% of NEAs have an orbital uncertainty parameter, U >= 4, making reacquisition more than a year following discovery difficult; for H > 22 this fraction is over 90%. We argue that rapid follow-up will be essential to characterize newly-discovered NEAs. Most new NEAs are found within 0.5mag of peak brightness and fade quickly, typically by 0.5/3.5/5mag after 1/4/6 weeks. About 80% have synodic periods of <3 years that bring them close to Earth several times a decade. However, follow-up observations on subsequent apparitions will be near impossible for the bulk of new discoveries, as these will be H > 22 NEAs that tend to return 100 times fainter. We show that for characterization to keep pace with discovery would require: Visible spectroscopy within days with a dedicated >2m telescope; long-arc astrometry, used also for phase curves, with a >4m telescope; and fast-cadence (<min) lightcurves obtained within days with a >= 4m telescope. For the already-known large (H =< 22) NEAs, subsequent-apparition spectroscopy, astrometry, and photometry could be done with 1-2m telescopes.
In this paper we report on Chandra observations of the starburst galaxy NGC 922. NGC 922 is a drop-through ring galaxy with an expanding ring of star formation, similar in many respects to the Cartwheel galaxy. The Cartwheel galaxy is famous for host
ing 12 ULX, most of which are in the star forming ring. This is the largest number of ULX seen in a single system, and has led to speculation that the low metallicity of the Cartwheel (0.3 solar) may optimize the conditions for ULX formation. In contrast, NGC 922 has metallicity near solar. The Chandra observations reveal a population of bright X-ray sources, including 7 ULX. The number of ULX in NGC 922 and the Cartwheel scales with the star formation rate: we do not find any evidence for an excess of sources in the Cartwheel. Simulations of the binary population in these galaxies suggest that the ULX population in both systems is dominated by systems with strong wind accretion from supergiant donors onto direct-collapse BHs. The simulations correctly predict the ratio of the number of sources in NGC 922 and the Cartwheel. Thus it would appear that the the metallicity of the Cartwheel is not low enough to see a difference in the ULX population compared to NGC 922.
Almost all Galactic black hole binaries with low mass donor stars are transient X-ray sources; we expect most of the X-ray transients observed in external galaxies to be black hole binaries also. Obtaining period estimates for extra-galactic transien
ts is challenging, but the resulting period distribution is an important tool for modeling the evolution history of the host galaxy. We have obtained periods, or upper limits, for 12 transients in M31, using an updated relation between the optical and X-ray luminosities. We have monitored the central region of M31 with Chandra for the last ~12 years, and followed up promising transients with HST; 4sigma B magnitude limits for optical counterparts are ~26--29, depending on crowding. We obtain period estimates for each transient for both neutron star and black hole accretors. Periods range from <0.4 to 490+/-90 hours (<0.97 to <175 hrs if all are BH systems). These M31 transients appear to be somewhat skewed towards shorter periods than the Milky Way (MW) transients; indeed, comparing the M31 and MW transients with survival analysis techniques used to account for some data with only upper limits yield probabilities of ~0.02--0.08 that the two populations are drawn from the same distribution. We also checked for a correlation between orbital period and distance from the nucleus, finding a 12% probability of no correlation. Further observations of M31 transients will strengthen these results.
We have monitored 41 Be/X-ray binary systems in the Small Magellanic Cloud over ~9 years using PCA-RXTE data from a weekly survey program. The resulting light curves were analysed in search of orbital modulations with the result that 10 known orbital
ephemerides were confirmed and refined, while 10 new ones where determined. A large number of X-ray orbital profiles are presented for the first time, showing similar characteristics over a wide range of orbital periods. Lastly, three pulsars: SXP46.4, SXP89.0 and SXP165 were found to be misidentifications of SXP46.6, SXP91.1 and SXP169, respectively.
