Erik Andrew Hoversten

A statistical study of gamma-ray burst afterglows measured by the Swift Ultraviolet Optical Telescope

We present the first statistical analysis of 27 Ultraviolet Optical Telescope (UVOT) optical/ultraviolet light curves of gamma-ray burst (GRB) afterglows. We have found, through analysis of the light curves in the observers frame, that a significant fraction rise in the first 500 s after the GRB trigger, all light curves decay after 500 s, typically as a power law with a relatively narrow distribution of decay indices, and the brightest optical afterglows tend to decay the quickest. We find that the rise could be either produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising light curve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8 per cent confidence, there is a correlation, in the observed frame, between the apparent magnitude of the light curves at 400 s and the rate of decay after 500 s. However, in the rest frame, a Spearman rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle. or within a core of a uniform energy density theta(c). We also produced logarithmic luminosity distributions for three rest-frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT light curves with the X-ray Telescope (XRT) light-curve canonical model. The range in decay indices seen in UVOT light curves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model, there is no indication of the rising behaviour observed in the UVOT light curves.

Further calibration of the Swift ultraviolet/optical telescope

The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard the Swift observatory. The photometric calibration has been published, and this paper follows up with details on other aspects of the calibration including a measurement of the point spread function with an assessment of the orbital variation and the effect on photometry. A correction for large-scale variations in sensitivity over the field of view is described, as well as a model of the coincidence loss which is used to assess the coincidence correction in extended regions. We have provided a correction for the detector distortion and measured the resulting internal astrometric accuracy of the UVOT, also giving the absolute accuracy with respect to the International Celestial Reference System. We have compiled statistics on the background count rates, and discuss the sources of the background, including instrumental scattered light. In each case, we describe any impact on UVOT measurements, whether any correction is applied in the standard pipeline data processing or whether further steps are recommended.

The Absolute Magnitudes of Type Ia Supernovae in the Ultraviolet

We examine the absolute magnitudes and light-curve shapes of 14 nearby (redshift z = 0.004-0.027) Type Ia supernovae (SNe Ia) observed in the ultraviolet (UV) with the Swift Ultraviolet/Optical Telescope. Colors and absolute magnitudes are calculated using both a standard Milky Way extinction law and one for the Large Magellanic Cloud that has been modified by circumstellar scattering. We find very different behavior in the near-UV filters (uvw1_(rc) covering ~2600-3300 A after removing optical light, and u ≈3000-4000 A) compared to a mid-UV filter (uvm2 ≈2000-2400 A). The uvw1_(rc) – b colors show a scatter of ~0.3 mag while uvm2–b scatters by nearly 0.9 mag. Similarly, while the scatter in colors between neighboring filters is small in the optical and somewhat larger in the near-UV, the large scatter in the uvm2 – uvw1 colors implies significantly larger spectral variability below 2600 A. We find that in the near-UV the absolute magnitudes at peak brightness of normal SNe Ia in our sample are correlated with the optical decay rate with a scatter of 0.4 mag, comparable to that found for the optical in our sample. However, in the mid-UV the scatter is larger, ~1 mag, possibly indicating differences in metallicity. We find no strong correlation between either the UV light-curve shapes or the UV colors and the UV absolute magnitudes. With larger samples, the UV luminosity might be useful as an additional constraint to help determine distance, extinction, and metallicity in order to improve the utility of SNe Ia as standardized candles.

Two years of monitoring supergiant fast X-ray transients with Swift

We present results based on 2 yr of intense Swift monitoring of three supergiant fast X-ray transients (SFXTs), IGR J16479-4514, XTE J1739-302 and IGR J17544-2619, which we started in 2007 October. Our out-of-outburst intensity-based X-ray (0.3-10 keV) spectroscopy yields absorbed power laws characterized by hard photon indices (Γ ~ 1-2). The broad-band (0.3-150 keV) spectra of these sources, obtained while they were undergoing new outbursts observed during the second year of monitoring, can be fitted well with models typically used to describe the X-ray emission from accreting neutron stars in high-mass X-ray binaries. We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. By considering our monitoring as a casual sampling of the X-ray light curves, we can infer that the time these sources spend in bright outbursts is between 3 and 5 per cent of the total. The most probable X-ray flux for these sources is ~(1-2) × 10 -11 erg cm -2 s -1 (2-10 keV, unabsorbed), corresponding to luminosities of the order of a few 10 33 to a few 10 34 erg s -1 (two orders of magnitude lower than the bright outbursts). In particular, the duty-cycle of inactivity is ~19, 39 and 55 per cent (~5 per cent uncertainty) for IGR J16479-4514, XTE J1739-302 and IGR J17544-2619, respectively. We present a complete list of BAT onboard detections, which further confirm the continued activity of these sources. This demonstrates that true quiescence is a rare state and that these transients accrete matter throughout their life at different rates. Variability in the X-ray flux is observed at all time-scales and intensity ranges we can probe. Superimposed on the day-to-day variability is intraday flaring, which involves flux variations up to one order of magnitude that can occur down to time-scales as short as ~1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses M cl ~ (0.3-2) × 10 19 g. Thanks to the Swift observations, the general picture we obtain is that, despite individual differences, common X-ray characteristics of this class are now well defined, such as outburst lengths well in excess of hours, with a multiple peaked structure, and a high dynamic range (including bright outbursts), up to approximately four orders of magnitude.

Multi-Wavelength Properties of the Type IIb SN 2008ax

We present the UV, optical, X-ray, and radio properties of the Type IIb SN 2008ax discovered in NGC 4490. The observations in the UV are one of the earliest of a Type IIb supernova (SN). On approximately day 4 after the explosion, a dramatic upturn in the u and uvw1 (λ_c = 2600 A) light curves occurred after an initial rapid decline which is attributed to adiabatic cooling after the initial shock breakout. This rapid decline and upturn is reminiscent of the Type IIb SN 1993J on day 6 after the explosion. Optical/near-IR spectra taken around the peak reveal prominent Hα, He I, and Ca II absorption lines. A fading X-ray source is also located at the position of SN 2008ax, implying an interaction of the SN shock with the surrounding circumstellar material and a mass-loss rate of the progenitor of M (overdot) = (9 ± 3) × 10^(−6) M_☉ yr^(−1). The unusual time evolution (14 days) of the 6 cm peak radio luminosity provides further evidence that the mass-loss rate is low. Combining the UV, optical, X-ray, and radio data with models of helium exploding stars implies the progenitor of SN 2008ax was an unmixed star in an interacting binary. Modeling of the SN light curve suggests a kinetic energy (E_k) of 0.5 × 10^(51) erg, an ejecta mass (M_(ej)) of 2.9 M_☉, and a nickel mass (M_(Ni)) of 0.06 M_☉.

Monitoring supergiant fast X-ray transients with Swift: results from the first year

The advent of Swift has allowed, for the first time, the possibility to give Super giant Fast X‐ray Transients (SFXTs), the new class of High Mass X‐ray Binaries discovered by INTEGRAL, non serendipitous attention throughout most phases of their life. In this paper we present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479 4514, XTE J1739 302, IGR J17544 2619, and AX J1841.0 0536. We obtain the first assessment of how long each source spends i n each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is � 17, 28, 39, 55 % (�5 % uncertainty), for IGR J16479 4514, AX J1841.0 0536, XTE J1739‐ 302, and IGR J17544 2619, respectively, so that true quiescence, which is below our detection ability even with the exposures we collected in one year, is a rare state, when compared with estimates from less sensitive instruments. This demonstrates that these transients accrete matter throughout their lifetime at different rates. AX J1841.0 0536 is the only source which has not undergone a bright outburst during our monitoring campaign. Although individual sources behave somewhat differently, common X‐ray characteristics of this class are emerging such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of � 4 orders of magnitude have been observed in IGR J17544 2619 and XTE J1739 302, of �3 in IGR J16479 4514, and of about 2 in AX J1841.0 0536 (this lowest range is due to the lack of bright flares). We also present a complete list of B AT on-board detections, which complements our previous work, and further confirms the cont inuous activity of these sources. We performed out-of-outburst intensity-based spectroscopy. In particular, spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred meters, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We used the whole BAT dataset, since the beginning of the mission, to search for periodicities due to orbital motion and found Porb = 3.32 d for IGR J16479 4514, confirming previous findings. We also present the UVOT data of these sour ces; we show the UVOT light curves of AX J1841.0 0536 and the ones of XTE J1739 302 before, during, and after the outbursts.

GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst

We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was ≈13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, Lyβ and the Lyman continuum break at a redshift z = 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH I = 22.0 ± 0.1 cm −2 , which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v ≈ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4–6 min from the time of the Swift-BAT trigger.

Ultraviolet Number Counts of Galaxies from Swift Ultraviolet/Optical Telescope Deep Imaging of the Chandra Deep Field South

Deep Swift UV/Optical Telescope (UVOT) imaging of the Chandra Deep Field South is used to measure galaxy number counts in three near-ultraviolet (NUV) filters (uvw2: 1928 A, uvm2: 2246 A, and uvw1: 2600 A) and the u band (3645 A). UVOT observations cover the break in the slope of the NUV number counts with greater precision than the number counts by the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Galaxy Evolution Explorer, spanning a range 21 approx< m{sub AB} approx< 25. Model number counts confirm earlier investigations in favoring models with an evolving galaxy luminosity function.

A SEARCH FOR LYMAN BREAK GALAXIES IN THE CHANDRA DEEP FIELD SOUTH USING SWIFT ULTRAVIOLET/OPTICAL TELESCOPE

While the Swift satellite is primarily designed to study gamma-ray bursts, its ultraviolet and optical imaging and spectroscopy capabilities are also being used for a variety of scientific programs. In this study, we use the UV/Optical Telescope (UVOT) instrument aboard Swift to discover 0.5 < z < 2 Lyman break galaxies (LBGs). UVOT has covered ∼ 266 arcmin at >60ks exposure time, achieving a limiting magnitude of u < 24.5, in the Chandra Deep Field South (CDF-S). Applying UVOT near-ultraviolet color selection, we select 50 UV-dropouts from this UVOT CDF-S data. We match the selected sources with available multiwavelength data from GOODS-South, MUSYC, and COMBO-17 to characterize the spectral energy distributions for these galaxies and determine stellar masses, star formation rates (SFRs), and dust attenuations. We compare these properties for LBGs selected in this paper versus z ∼ 3 LBGs and other CDF-S galaxies in the same redshift range (0.5 < z < 2), identified using photometric redshift techniques. The z ∼ 1 LBGs have stellar masses of 〈Log M∗/M⊙〉 = 9.4± 0.6, which is slightly lower than z ∼ 3 LBGs (〈Log M∗/M⊙〉 = 10.2± 0.4) and slightly higher compared to the z ∼ 1 CDF-S galaxies (〈Log M∗/M⊙〉 = 8.7± 0.7). Similarly, our sample of z ∼ 1 LBGs has SFRs (derived using both ultraviolet and infrared data, where available) of 〈Log SFR/(M⊙ yr )〉 = 0.7 ± 0.6, which is nearly an order of magnitude lower than z ∼ 3 LBGs (〈Log SFR/M⊙ yr 〉 = 1.5 ± 0.4), but slightly higher than the comparison z ∼ 1 sample of CDF-S galaxies (〈Log SFR/M⊙ yr 〉 = 0.2 ± 0.7). We find that our z ∼ 1 UV-dropouts have 〈AFUV〉 = 2.0 ± 1.0, which is higher than z ∼ 3 LBGs (〈AFUV〉 = 1.0 ± 0.5), but is similar to the distribution of dust attenuations in the other CDF-S galaxies (〈AFUV〉 ∼ 2.8 ± 1.5). Using the GOODS-South multiwavelength catalog of galaxies, we simulate a larger and fainter sample of LBGs to compare their properties with those of the UVOT-selected LBG sample. We conclude that UVOT can be useful for finding and studying the bright end of 0.5< z <2.0 LBGs.

The RESOLVE Survey Atomic Gas Census and Environmental Influences on Galaxy Gas Reservoirs

We present the H i mass inventory for the REsolved Spectroscopy Of a Local VolumE (RESOLVE) survey, a volume-limited, multi-wavelength census of >1500 z = 0 galaxies spanning diverse environments and complete in baryonic mass down to dwarfs of ~109