The Future of X-ray Time Domain Surveys

Modern X-ray observatories yield unique insight into the astrophysical time domain. Each X-ray photon can be assigned an arrival time, an energy and a sky position, yielding sensitive, energy-dependent light curves and enabling time-resolved spectra down to millisecond time-scales. Combining those with multiple views of the same patch of sky (e.g., in the Chandra and XMM-Newton deep fields) so as to extend variability studies over longer baselines, the spectral timing capacity of X-ray observatories then stretch over 10 orders of magnitude at spatial resolutions of arcseconds, and 13 orders of magnitude at spatial resolutions of a degree. A wealth of high-energy time-domain data already exists, and indicates variability on timescales ranging from microseconds to years in a wide variety of objects, including numerous classes of AGN, high-energy phenomena at the Galactic centre, Galactic and extra-Galactic X-ray binaries, supernovae, gamma-ray bursts, stellar flares, tidal disruption flares, and as-yet unknown X-ray variables. This workshop explored the potential of strategic X-ray surveys to probe a broad range of astrophysical sources and phenomena. Here we present the highlights, with an emphasis on the science topics and mission designs that will drive future discovery in the X-ray time domain.Comment: 8 pages, 1 figure, Conference proceedings for IAU Symposium 285, "New Horizons in Time Domain Astronomy," Oxford, UK, Sep 19-23, 2011. To be published by IA

Signatures of Massive Black Hole Merger Host Galaxies from Cosmological Simulations I: Unique Galaxy Morphologies in Imaging

Low-frequency gravitational wave experiments such as the Laser Interferometer Space Antenna and pulsar timing arrays are expected to detect individual massive black hole (MBH) binaries and mergers. However, secure methods of identifying the exact host galaxy of each MBH merger amongst the large number of galaxies in the gravitational wave localization region are currently lacking. We investigate the distinct morphological signatures of MBH merger host galaxies, using the Romulus25 cosmological simulation. We produce mock telescope images of 201 simulated galaxies in Romulus25 hosting recent MBH mergers, through stellar population synthesis and dust radiative transfer. Based on comparisons to mass- and redshift-matched control samples, we show that combining multiple morphological statistics via a linear discriminant analysis enables identification of the host galaxies of MBH mergers, with accuracies that increase with chirp mass and mass ratio. For mergers with high chirp masses (>10^8.2 Msun) and high mass ratios (>0.5), the accuracy of this approach reaches >80%, and does not decline for at least >1 Gyr after numerical merger. We argue that these trends arise because the most distinctive morphological characteristics of MBH merger and binary host galaxies are prominent classical bulges, rather than relatively short-lived morphological disturbances from their preceding galaxy mergers. Since these bulges are formed though major mergers of massive galaxies, they lead to (and become permanent signposts for) MBH binaries and mergers that have high chirp masses and mass ratios. Our results suggest that galaxy morphology can aid in identifying the host galaxies of future MBH binaries and mergers.Comment: 19 pages, 10 figures. Submitted to Ap

Far Ultra-Violet Insights Into NGC 1399's Globular Cluster Population

We investigate archival Hubble Space Telescope ACS/SBC F140LP observations of NGC~1399 to search for evidence of multiple stellar populations in extragalactic globular clusters. Enhanced FUV populations are thought to be indicators of He-enhanced second generation populations in globular clusters, specifically extreme/blue horizontal branch stars. Out of 149 globular clusters in the field of view, 58 have far ultraviolet (FUV) counterparts with magnitudes brighter than 28.5. Six of these FUV-deteced globular clusters are also detected in X-rays, including one ultraluminous X-ray source ($L_X > 10^{39}$ erg/s). While optically bright clusters corresponded to brighter FUV counterparts, we observe FUV emission from both metal-rich and metal-poor clusters, which implies that the FUV excess is not dependent on optical colour. We also find no evidence that the cluster size influences the FUV emission. The clusters with X-ray emission are not unusually FUV bright, which suggests that even the ultraluminous X-ray source does not provide significant FUV contributions. NGC 1399 is only the fourth galaxy to have its globular cluster system probed for evidence of FUV-enhanced populations, and we compare these clusters to previous studies of the Milky Way, M31, M87, and the brightest cluster in M81. These sources indicate that many globular clusters likely host extreme HB stars and/or second generation stars, and highlight the need for more complete FUV observations of extragalactic globular cluster systems.Comment: accepted to MNRA

New Black Hole Spin Values for Sagittarius A* Obtained with the Outflow Method

Six archival Chandra observations are matched with eight sets of radio data and studied in the context of the outflow method to measure and study the spin properties of $\rm{Sgr ~A^*}$. Three radio and X-ray data sets obtained simultaneously, or partially simultaneously, are identified as preferred for the purpose of measuring the spin properties of $\rm{Sgr ~A^*}$. Similar results are obtained with other data sets. Results obtained with the preferred data sets are combined and indicate a weighted mean value of the spin function of $\rm{F} = 0.62 \pm 0.10$ and dimensionless spin angular momentum of $\rm{a_*} = 0.90 \pm 0.06$. The spin function translates into measurements of the black hole rotational mass, $\rm{M_{rot}}$, irreducible mass, $\rm{M_{irr}}$, and spin mass-energy available for extraction, $\rm{M_{spin}}$, relative to the total black hole dynamical mass, $\rm{M_{dyn}}$. Weighted mean values of $\rm{(M_{rot}/M_{dyn}) = (0.53 \pm 0.06)}$, $\rm{({M_{irr}/M_{dyn})} = (0.85 \pm 0.04)}$, $\rm{({M_{spin}/M_{dyn})} = (0.15 \pm 0.04)}$, $\rm{{M_{rot}} = (2.2 \pm 0.3) \times 10^6 ~M_{\odot}}$, $\rm{{M_{irr}} = (3.5 \pm 0.2) \times 10^6 ~M_{\odot}}$, and $\rm{{M_{spin}} = (6.2 \pm 1.6) \times 10^5 ~M_{\odot}}$ are obtained; of course $\rm{{(M_{rot}/M_{irr})} = (0.62 \pm 0.10)}$ since $\rm{{(M_{rot}/M_{irr})} = F}$. Values obtained for $\rm{Sgr ~A^*}$ are compared with those obtained for M87 based on the published spin function which indicate that M87 carries substantially more rotational energy and spin mass-energy relative to the total (i.e., dynamical) black hole mass, the irreducible black hole mass, and in absolute terms.Comment: Accepted for publication in MNRAS on October 16, 202