Person: Cowperthwaite, Philip
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Cowperthwaite
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Philip
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Cowperthwaite, Philip
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Publication A Comprehensive Study of Detectability and Contamination in Deep Rapid Optical Searches for Gravitational Wave Counterparts(IOP Publishing, 2015) Cowperthwaite, Philip; Berger, EdoThe first direct detection of gravitational waves (GW) by the ground-based Advanced LIGO/Virgo interferometers is expected to occur within the next few years. These interferometers are designed to detect the mergers of compact object binaries composed of neutron stars and/or black holes to a fiducial distance of ∼ 200 Mpc and a localization region of ∼ 100 deg2. To maximize the science gains from such GW detections it is essential to identify electromagnetic (EM) counterparts. Among the wide range of proposed counterparts, the most promising is optical/IR emission powered by the radioactive decay of r-process elements synthesized in the neutron-rich merger ejecta – a “kilonova”. Here we present detailed simulated observations that encompass a range of strategies for kilonova searches during GW follow-up. We utilize these simulations to assess both the detectability of kilonovae and our ability to distinguish them from a wide range of contaminating transients in the large GW localization regions. We find that if pre-existing deep template images for the GW localization region are available, then nightly observations to a depth of i ≈ 24 mag and z ≈ 23 mag are required to achieve a 95% detection rate; observations that commence within ∼ 12 hours of trigger will also capture the kilonova peak and provide stronger constraints on the ejecta properties. We also find that kilonovae can be robustly separated from other known and hypothetical types of transients utilizing cuts on color (i − z & 0.3 mag) and rise time (trise . 4 days). In the absence of a pre-existing template the observations must reach ∼ 1 mag deeper to achieve the same kilonova detection rate, but robust rejection of contaminants can still be achieved. Motivated by the results of our simulations we discuss the expected performance of current and future wide-field telescopes in achieving these observational goals, and find that prior to LSST the Dark Energy Camera (DECam) on the Blanco 4-m telescope and Hyper Suprime-Cam (HSC) on the Subaru 8-m telescope offer the best kilonova discovery potential.Publication A Decam Search for an Optical Counterpart to the Ligo Gravitational-Wave Event Gw151226(American Astronomical Society, 2016) Cowperthwaite, Philip; Berger, Edo; Soares-Santos, M.; Annis, J.; Brout, D.; Brown, D. A.; Buckley-Geer, E.; Cenko, S. B.; Chen, H. Y.; Chornock, R.; Diehl, H. T.; Doctor, Z.; Drlica-Wagner, A.; Drout, M. R.; Farr, B.; Finley, D. A.; Foley, R. J.; Fong, W.; Fox, D. B.; Frieman, J.; Garcia-Bellido, J.; Gill, M. S. S.; Gruendl, R. A.; Herner, K.; Holz, D. E.; Kasen, D.; Kessler, R.; Lin, H.; Margutti, R.; Marriner, J.; Matheson, T.; Metzger, B. D.; Neilsen Jr., E. H.; Quataert, E.; Rest, A.; Sako, M.; Scolnic, D.; Smith, N.; Sobreira, F.; Strampelli, G. M.; Villar, V. A.; Walker, A. R.; Wester, W.; Williams, Preston; Yanny, B.; Abbott, T. M. C.; Abdalla, F. B.; Allam, S.; Armstrong, R.; Bechtol, K.; Benoit-Lévy, A.; Bertin, E.; Brooks, D.; Burke, D. L.; Rosell, A. Carnero; Kind, M. Carrasco; Carretero, J.; Castander, F. J.; Cunha, C. E.; D’Andrea, C. B.; Costa, L. N. da; Desai, S.; Dietrich, J. P.; Evrard, A. E.; Neto, A. Fausti; Fosalba, P.; Gerdes, D. W.; Giannantonio, T.; Goldstein, D. A.; Gruen, D.; Gutierrez, G.; Honscheid, K.; James, D. J.; Johnson, M. W. G.; Johnson, M. D.; Krause, E.; Kuehn, K.; Kuropatkin, N.; Lima, M.; Maia, M. A. G.; Marshall, J. L.; Menanteau, F.; Miquel, R.; Mohr, J. J.; Nichol, R. C.; Nord, B.; Ogando, R.; Plazas, A. A.; Reil, K.; Romer, A. K.; Sanchez, E.; Scarpine, V.; Sevilla-Noarbe, I.; Smith, R. C.; Suchyta, E.; Tarle, G.; Thomas, D.; Thomas, R. C.; Tucker, D. L.; Weller, J.We report the results of a Dark Energy Camera (DECam) optical follow-up of the gravitational wave (GW) event GW151226, discovered by the Advanced LIGO detectors. Our observations cover 28.8 deg2 of the localization region in the i and z bands (containing 3% of the BAYESTAR localization probability), starting 10 hours after the event was announced and spanning four epochs at 2 −24 days after the GW detection. We achieve 5σ point-source limiting magnitudes of i ≈ 21.7 and z ≈ 21.5, with a scatter of 0.4 mag, in our difference images. Given the two day delay, we search this area for a rapidly declining optical counterpart with & 3σ significance steady decline between the first and final observations. We recover four sources that pass our selection criteria, of which three are cataloged AGN. The fourth source is offset by 5.8 arcsec from the center of a galaxy at a distance of 187 Mpc, exhibits a rapid decline by 0.5 mag over 4 days, and has a red color of i−z ≈ 0.3 mag. These properties could satisfy a set of cuts designed to identify kilonovae. However, this source was detected several times, starting 94 days prior to GW151226, in the Pan-STARRS Survey for Transients (dubbed as PS15cdi) and is therefore unrelated to the GW event. Given its long-term behavior, PS15cdi is likely a Type IIP supernova that transitioned out of its plateau phase during our observations, mimicking a kilonova-like behavior. We comment on the implications of this detection for contamination in future optical follow-up observations.Publication SN 2015bn: A Detailed Multi-wavelength View of a Nearby Superluminous Supernova(American Astronomical Society, 2016) Nicholl, Matthew; Berger, Edo; Smartt, S. J.; Margutti, R.; Kamble, Atish; Alexander, Kate; Chen, T.-W.; Inserra, C.; Arcavi, I.; Blanchard, Peter; Cartier, R.; Chambers, K. C.; Childress, M. J.; Chornock, R.; Cowperthwaite, Philip; Drout, M.; Flewelling, H. A.; Fraser, M.; Gal-Yam, A.; Galbany, L.; Harmanen, J.; Holoien, T. W.-S.; Hosseinzadeh, G.; Howell, D. A.; Huber, M. E.; Jerkstrand, A.; Kankare, E.; Kochanek, C. S.; Lin, Z.-Y.; Lunnan, R.; Magnier, E. A.; Maguire, K.; McCully, C.; McDonald, M.; Metzger, B. D.; Milisavljevic, D.; Mitra, A.; Reynolds, T.; Saario, J.; Shappee, B. J.; Smith, K. W.; Valenti, S.; Villar, V. A.; Waters, C.; Young, D. R.We present observations of SN 2015bn (=PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z = 0.1136. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brighter (${M}_{U}\approx -23.1$) and in a fainter galaxy (${M}_{B}\approx -16.0$) than other SLSNe at $z\sim 0.1$. We used this opportunity to collect the most extensive data set for any SLSN I to date, including densely sampled spectroscopy and photometry, from the UV to the NIR, spanning −50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30–50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20–30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a $\gtrsim 10$ M ${}_{\odot }$ stripped progenitor exploding with $\sim {10}^{51}$ erg kinetic energy, forming a magnetar with a spin-down timescale of ~20 days (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario—interaction with ~20 M ${}_{\odot }$ of dense, inhomogeneous circumstellar material—can be tested with continuing radio follow-up.Publication PS1-14bj: A Hydrogen-poor Superluminous Supernova with a Long Rise and Slow Decay.(American Astronomical Society, 2016) Lunnan, R.; Chornock, R.; Berger, Edo; Milisavljevic, Danny; Jones, D. O.; Rest, A.; Fong, W.; Fransson, C.; Margutti, R.; Drout, M. R.; Blanchard, Peter; Challis, P.; Cowperthwaite, Philip; Foley, R. J.; Kirshner, Robert; Morrell, N.; Riess, A. G.; Roth, K. C.; Scolnic, D.; Smartt, S. J.; Smith, K. W.; Villar, Ashley; Chambers, K. C.; Draper, P. W.; Huber, M. E.; Kaiser, N.; Kudritzki, R.-P.; Magnier, E. A.; Metcalfe, N.; Waters, C.We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift z = 0.5215 discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its extremely slow evolution, with an observed rise of ∼ > 125 restframe days, and exponential decline out to ∼ 250 days past peak at a measured rate of 0.01 mag day−1, consistent with fully-trapped 56Co decay. This is the longest rise time measured in a SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly-evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, though lower in luminosity (Lpeak ≃ 4.6×1043ergs−1) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for > 200 days past peak, and strong emission lines from [O III] λ5007 and [O III] λ4363 with a velocity width of ∼3400 km s−1 in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple 56Ni-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O III] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.