D. Pooley

The galaxy hosts and large-scale environments of short-hard (gamma)-ray bursts

The rapid succession of discoveries of short-duration hard-spectrum gamma-ray bursts (GRBs) has led to unprecedented insights into the energetics of the explosion and nature of the progenitors. Yet short of the detection of a smoking gun, such as a burst of coincident gravitational radiation or a Li-Paczynski minisupernova, it is unlikely that a definitive claim can be made for the progenitors. As was the case with long-duration soft-spectrum GRBs, however, the expectation is that a systematic study of the hosts and locations of short GRBs could begin to yield fundamental clues as to their nature. We present an aggregate study of the host galaxies of short-duration hard-spectrum GRBs. In particular, we present the Gemini-North and Keck discovery spectra of the galaxies that hosted three short GRBs and a moderate-resolution (R ≈ 6000) spectrum of a fourth host. We find that these short-hard GRBs originate in a variety of low-redshift (z < 1) environments that differ substantially from those of long-soft GRBs, both on individual galaxy scales and on galaxy-cluster scales. Specifically, three of the bursts are found to be associated with old and massive galaxies with no current (<0.1 M☉ yr-1) or recent star formation. Two of these galaxies are located within a cluster environment. These observations support an origin from the merger of compact stellar remnants, such as double neutron stars or a neutron star-black hole binary. The fourth event, in contrast, occurred within a dwarf galaxy with a star formation rate exceeding 0.3 M☉ yr-1. Therefore, it appears that like supernovae of Type Ia, the progenitors of short-hard bursts are created in all galaxy types, suggesting a corresponding class with a wide distribution of delay times between formation and explosion.

Faint X-Ray Sources in the Globular Cluster Terzan 5

We report our analysis of a Chandra X-ray observation of the rich globular cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray luminosity of 3 × 1031 ergs s-1 within the half-mass radius of the cluster. Thirty-three of these have LX > 1032 ergs s-1, the largest number yet seen in any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB; identified by Wijnands et al.), another 12 relatively soft sources may be quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5 to the Galactic center sources studied by Muno and collaborators and find the Galactic center sources to have harder X-ray colors, indicating a possible difference in the populations. We cannot clearly identify a metallicity dependence in the production of low-luminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordan et al. for extragalactic LMXBs is consistent with our data.

Exciting the magnetosphere of the magnetar CXOU J164710.2-455216 in Westerlund 1

We describe XMM-Newton observations taken 4.3 days prior to and 1.5 days subsequent to two remarkable events that were detected with Swift on 2006 September 21 from the candidate magnetar CXOU J164710.2-455216: (1) a 20 ms burst with an energy of 1e37 erg (15-150 keV), and (2) a rapid spin-down (glitch) with a fractionap period change of 1e-4. We find that the luminosity of the pulsar increased by a factor of 100 in the interval between observations, from 1e33 to 1e35 erg/s (0.5-8.0 keV), and that its spectrum hardened. The pulsed count rate increased by a factor of 10 (0.5-8.0 keV), but the fractional rms amplitude of the pulses decreased from 65 to 11 per cent, and their profile changed from being single-peaked to exhibiting three peaks. Similar changes have been observed from other magnetars in response to outbursts, such as that of 1E 2259+586 in 2002 June. We suggest that a plastic deformation of the neutron stars crust induced a very slight twist in the external magnetic field, which in turn generated currents in the magnetosphere that were the direct cause of the X-ray outburst.

X-Ray Observations of Type Ia Supernovae with Swift: Evidence of Circumstellar Interaction for SN 2005ke

We present a study of the early (days to weeks) X-ray and UV properties of eight Type Ia supernovae (SNe Ia) that have been extensively observed with the X-Ray Telescope (XRT) and UV/Optical Telescope (UVOT) on board Swift, ranging from 5 to 132 days after the outburst. SN 2005ke is tentatively detected (at a 3–3.6 j level of significance) in X-rays based on deep monitoring with the XRT ranging from 8 to 120 days after the outburst. The inferred X-ray luminosity [ ergs s 1 ; 0.3–2 keV band] is likely caused by 38 L p (2 1) # 10 0.3–2 interaction of the SN shock with circumstellar material (CSM) deposited by a stellar wind from the progenitor’s companion star with a mass-loss rate of . Evidence of CSM interaction 6 1 1 ˙ M ≈ 3 # 10 M yr (v /10 km s ) , w in X-rays is independently confirmed by an excess of UV emission, as observed with the UVOT on board Swift, starting around 35 days after the explosion. The nondetection of SN 2005ke with Chandra 105 days after the outburst implies a rate of decline steeper than , consistent with the decline expected from the interaction 0.75 L ∝ t X of the SN shock with a spherically symmetric CSM ( ). None of the other seven SNe Ia is detected in X-rays 1 t

SWIFT AND CHANDRA DETECTIONS OF SUPERNOVA 2006jc : EVIDENCE FOR INTERACTION OF THE SUPERNOVA SHOCK WITH A CIRCUMSTELLAR SHELL

The peculiar Type Ib supernova (SN) 2006jc has been observed with the UV/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on board the Swift observatory over a period of 19-183 days after the explosion. Signatures of interaction of the outgoing SN shock with dense circumstellar material (CSM) are detected, such as strong X-ray emission (L_(0.2–10) > 10^39 erg s^−1) and the presence of Mg II 2800 A line emission visible in the UV spectra. In combination with a Chandra observation obtained on day 40 after the explosion, the X-ray light curve is constructed, which shows a unique rise of the X-ray emission by a factor of ~5 over a period of ~4 months, followed by a rapid decline. We interpret the unique X-ray and UV properties as a result of the SN shock interacting with a shell of material that was deposited by an outburst of the SN progenitor 2 years prior to the explosion. Our results are consistent with the explosion of a Wolf-Rayet star that underwent an episodic mass ejection qualitatively similar to those of luminous blue variable stars prior to its explosion. This led to the formation of a dense (~10^7 cm^−3) shell at a distance of ~10^16 cm from the site of the explosion, which expands with the WR wind at a velocity of 1300 ± 300 km s^−1.

X-Ray and Optical Flux Anomalies in the Quadruply Lensed QSO 1RXS J1131?1231

Optical and X-ray observations of the quadruply imaged quasar 1RXS J1131-1231 show flux ratio anomalies among the images of factors of ~2 in the optical and ~3-9 in X-rays. Temporal variability of the quasar seems an unlikely explanation for the discrepancies between the X-ray and optical flux ratio anomalies. The negative parity of the most affected image and the decreasing trend of the anomalies with wavelength suggest microlensing as a possible explanation; this would imply that the source of optical radiation in RXS J1131 is ~104Rg in size for a black hole mass of ~108 M?. We also present evidence for different X-ray spectral hardness ratios among the four images.

Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio

We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as L x oc t -0.4 . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of ∼2 x 10 -6 M ⊙ yr -1 . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of 56 Ni to be 0.06 ± 0.03 M ⊙ . Using the expressions of Litvinova & Nadezhin we estimate an explosion energy of (0.98 ± 0.25) x 10 51 erg. We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of ∼20 M ⊙ for SN 2004et.

GRB 050408: A Bright Gamma-Ray Burst Probing an Atypical Galactic Environment

The bright ORB 050408 was localized by HETE-2 near local midnight in the Western Hemisphere, enabling an impressive ground-based follow-up effort, as well as space-based follow-up from Swift. The Swift data from the XRT and our own optical photometry and spectrum of the afterglow provide the cornerstone for our analysis. Under the traditional assumption that the visible wave band was above the peak synchrotron frequency and below the cooling frequency, the optical photometry of 0.03-5.03 days shows an afterglow decay corresponding to an electron energy index of plc = 2.05±0.04, without a jet break as suggested by others. A break is seen in the X-ray data at early times (at ∼ 12,600 s after the GRB). The spectral slope of the optical spectrum is consistent with plc assuming a host galaxy extinction of A v = 1.18 mag. The optical-NIR broadband spectrum is also consistent with p = 2.05 but prefers Av = 0.57 mag. The X-ray afterglow shows a break at 1.26×104 s, which may be the result of a refreshed shock. This burst stands out in that the optical and X-ray data suggest a large H I column density of NH I ≈ 1022 cm-2; it is very likely a damped Lyα system, so the faintness of the host galaxy (Mv > -18 mag) is noteworthy. Moreover, we detect extraordinarily strong Ti II absorption lines with a column density through the GRB host that exceeds the largest values observed for the Milky Way by 1 order of magnitude. Furthermore, the Ti II equivalent width is in the top 1% of Mg II absorption-selected QSOs. This suggests that the large-scale environment of GRB 050408 has significantly lower Ti depletion than our Galaxy and a large velocity width (δv > 150 km s-1).

A Chandra X-ray observation of the globular cluster Terzan 1

We present a ∼19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-S observation of the globular cluster Terzan 1. 14 sources are detected within 1.4 arcmin of the cluster centre with two of these sources predicted to be not associated with the cluster (background active galactic nuclei or foreground objects). The neutron star X-ray transient, X1732−304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 yr. Here, we find four sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the Very Large Array that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5‐10 keV luminosity of 2.6 × 10 32 erg s −1 , quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5‐10 keV luminosity upper limit of 8 × 10 31 erg s −1 . We also discuss other X-ray sources within the globular cluster. From the estimated stellar encounter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al.

SPATIALLY RESOLVED STAR FORMATION IMAGE AND THE ULTRALUMINOUS X-RAY SOURCE POPULATION IN NGC 2207/IC 2163

The colliding galaxy pair NGC 2207/IC 2163, at a distance of 39 Mpc, was observed with Chandra, and an analysis reveals 28 well resolved X-ray sources, including 21 ultraluminous X-ray sources (ULXs) with LX & 10 39 erg s 1 , as well as the nucleus of NGC 2207. The number of ULXs is comparable with the largest numbers of ULXs per unit mass in any galaxy yet reported. In this paper we report on these sources, and quantify how their locations correlate with the local star formation rates seen in spatially-resolved star formation rate density images that we have constructed using combinations of Galex FUV and Spitzer 24 m images. We show that the numbers of ULXs are strongly correlated with the local star formation rate densities surrounding the sources, but that the luminosities of these sources are not strongly correlated with star formation rate density. Subject headings: stars: binaries: general | stars: formation | stars: luminosity function, mass function | stars: neutron | galaxies: individual (NGC 2207/IC 2163) | galaxies: interactions | galaxies: nuclei | galaxies: starburst | galaxies: structure | Xrays: binaries | infrared: galaxies