arXiv subjects

C. Akerlof

Publications and source records attributed to C. Akerlof.

At least 19 recordsLinked to original sources

arXiv2018

Study of radon reduction in gases for rare event search experiments

The noble elements, argon and xenon, are frequently employed as the target and event detector for weakly interacting particles such as neutrinos and Dark Matter. For such rare processes, background radiation must be carefully minimized. Radon provides one of the most significant contaminants since it is an inevitable product of trace amounts of natural uranium. To design a purification system for reducing such contamination, the adsorption characteristics of radon in nitrogen, argon, and xenon carrier gases on various types of charcoals with different adsorbing properties and intrinsic radioactive purities have been studied in the temperature range of 190-295 K at flow rates of 0.5 and 2 standard liters per minute. Essential performance parameters for the various charcoals include the average breakthrough times ($τ$), dynamic adsorption coefficients (k$_a$) and the number of theoretical stages (n). It is shown that the k$_a$-values for radon in nitrogen, argon, and xenon increase as the temperature of the charcoal traps decreases, and that they are significantly larger in nitrogen and argon than in xenon gas due to adsorption saturation effects. It is found that, unlike in xenon, the dynamic adsorption coefficients for radon in nitrogen and argon strictly obey the Arrhenius law. The experimental results strongly indicate that nitric acid etched Saratech is the best candidate among all used charcoal brands. It allows reducing total radon concentration in the LZ liquid Xe detector to meet the ultimate goal in the search for Dark Matter.

physics.ins-det↗

arXiv2016

Extensive Spectroscopy and Photometry of the Type IIP Supernova 2013ej

We present extensive optical ($UBVRI$, $g'r'i'z'$, and open CCD) and near-infrared ($ZYJH$) photometry for the very nearby Type IIP SN ~2013ej extending from +1 to +461 days after shock breakout, estimated to be MJD $56496.9\pm0.3$. Substantial time series ultraviolet and optical spectroscopy obtained from +8 to +135 days are also presented. Considering well-observed SNe IIP from the literature, we derive $UBVRIJHK$ bolometric calibrations from $UBVRI$ and unfiltered measurements that potentially reach 2\% precision with a $B-V$ color-dependent correction. We observe moderately strong Si II $\lambda6355$ as early as +8 days. The photospheric velocity ($v_{\rm ph}$) is determined by modeling the spectra in the vicinity of Fe II $\lambda5169$ whenever observed, and interpolating at photometric epochs based on a semianalytic method. This gives $v_{\rm ph} = 4500\pm500$ km s$^{-1}$ at +50 days. We also observe spectral homogeneity of ultraviolet spectra at +10--12 days for SNe IIP, while variations are evident a week after explosion. Using the expanding photosphere method, from combined analysis of SN 2013ej and SN 2002ap, we estimate the distance to the host galaxy to be $9.0_{-0.6}^{+0.4}$ Mpc, consistent with distance estimates from other methods. Photometric and spectroscopic analysis during the plateau phase, which we estimated to be $94\pm7$ days long, yields an explosion energy of $0.9\pm0.3\times10^{51}$ ergs, a final pre-explosion progenitor mass of $15.2\pm4.2$~M$_\odot$ and a radius of $250\pm70$~R$_\odot$. We observe a broken exponential profile beyond +120 days, with a break point at +$183\pm16$ days. Measurements beyond this break time yield a $^{56}$Ni mass of $0.013\pm0.001$~M$_\odot$.

astro-ph.SR↗

arXiv2015

The optical rebrightening of GRB100814A: an interplay of forward and reverse shocks?

We present a wide dataset of gamma-ray, X-ray, UVOIR, and radio observations of the Swift GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, this burst shows a sudden and prominent rebrightening in the optical band only, followed by a fast decay in both bands. The optical rebrightening also shows chromatic evolution. Such a puzzling behaviour cannot be explained by a single component model. We discuss other possible interpretations, and we find that a model that incorporates a long-lived reverse shock and forward shock fits the temporal and spectral properties of GRB100814A the best.

astro-ph.HE↗

arXiv2014

A Luminous, Fast Rising UV-Transient Discovered by ROTSE: a Tidal Disruption Event?

We present follow-up observations of an optical transient (OT) discovered by ROTSE on Jan. 21, 2009. Photometric monitoring was carried out with ROTSE-IIIb in the optical and Swift in the UV up to +70 days after discovery. The light curve showed a fast rise time of ~10 days followed by a steep decline over the next 60 days, which was much faster than that implied by 56Ni - 56Co radioactive decay. The SDSS DR10 database contains a faint, red object at the position of the OT, which appears slightly extended. This and other lines of evidence suggest that the OT is of extragalactic origin, and this faint object is likely the host galaxy. A sequence of optical spectra obtained with the 9.2-m Hobby-Eberly Telescope (HET) between +8 and +45 days after discovery revealed a hot, blue continuum with no visible spectral features. A few weak features that appeared after +30 days probably originated from the underlying host. Fitting synthetic templates to the observed spectrum of the host galaxy revealed a redshift of z = 0.19. At this redshift the peak magnitude of the OT is close to -22.5, similar to the brightest super-luminous supernovae; however, the lack of identifiable spectral features makes the massive stellar death hypothesis less likely. A more plausible explanation appears to be the tidal disruption of a sun-like star by the central super-massive black hole. We argue that this transient likely belongs to a class of super-Eddington tidal disruption events.

astro-ph.HE↗

arXiv2013

Large-Scale Image Processing with the ROTSE Pipeline for Follow-Up of Gravitational Wave Events

Electromagnetic (EM) observations of gravitational-wave (GW) sources would bring unique insights into a source which are not available from either channel alone. However EM follow-up of GW events presents new challenges. GW events will have large sky error regions, on the order of 10-100 square degrees, which can be made up of many disjoint patches. When searching such large areas there is potential contamination by EM transients unrelated to the GW event. Furthermore, the characteristics of possible EM counterparts to GW events are also uncertain. It is therefore desirable to be able to assess the statistical significance of a candidate EM counterpart, which can only be done by performing background studies of large data sets. Current image processing pipelines such as that used by ROTSE are not usually optimised for large-scale processing. We have automated the ROTSE image analysis, and supplemented it with a post-processing unit for candidate validation and classification. We also propose a simple ad hoc statistic for ranking candidates as more likely to be associated with the GW trigger. We demonstrate the performance of the automated pipeline and ranking statistic using archival ROTSE data. EM candidates from a randomly selected set of images are compared to a background estimated from the analysis of 102 additional sets of archival images. The pipeline's detection efficiency is computed empirically by re-analysis of the images after adding simulated optical transients that follow typical light curves for gamma-ray burst afterglows and kilonovae. We show that the automated pipeline rejects most background events and is sensitive to simulated transients to limiting magnitudes consistent with the limiting magnitude of the images.

astro-ph.IM↗

arXiv2012

GRB100814A as a member of the growing set of bursts with sudden optical rebrightening

We present the gamma-ray, X-ray, optical and radio data for GRB100814A. At the end of the slow decline phase of the X-ray and optical afterglow, a sudden and prominent rebrightening in the optical band occurs followed by a fast decay in both bands. This optical rebrightening is accompanied by possible chromatic variations. We discuss possible interpretations, such as double component scenarios and internal dissipation mechanism, with their virtues and drawbacks. We also compare GRB100814A with other Swift bursts that show optical rebrightenings with similar properties.

astro-ph.HE↗

arXiv2012

The GRB 071112C: A Case Study of Different Mechanisms in X-ray and Optical Temporal Evolution

We present the study on GRB 071112C X-ray and optical light curves. In these two wavelength ranges, we have found different temporal properties. The R-band light curve showed an initial rise followed by a single power-law decay, while the X-ray light curve was described by a single power-law decay plus a flare-like feature. Our analysis shows that the observed temporal evolution cannot be described by the external shock model in which the X-ray and optical emission are produced by the same emission mechanism. No significant color changes in multi-band light curves and a reasonable value of the initial Lorentz factor (Γ0 = 275 \pm 20) in a uniform ISM support the afterglow onset scenario as the correct interpretation for the early R-band rise. The result suggests the optical flux is dominated by afterglow. Our further investigations show that the X-ray flux could be created by an additional feature related to energy injection and X-ray afterglow. Different theoretical interpretations indicate the additional feature in X-ray can be explained by either late internal dissipation or local inverse-Compton scattering in the external shock.

astro-ph.HE↗

arXiv2012

The origin of the early time optical emission of Swift GRB 080310

We present broadband multi-wavelength observations of GRB 080310 at redshift z = 2.43. This burst was bright and long-lived, and unusual in having extensive optical and near IR follow-up during the prompt phase. Using these data we attempt to simultaneously model the gamma-ray, X-ray, optical and IR emission using a series of prompt pulses and an afterglow component. Initial attempts to extrapolate the high energy model directly to lower energies for each pulse reveal that a spectral break is required between the optical regime and 0.3 keV to avoid over predicting the optical flux. We demonstrate that afterglow emission alone is insufficient to describe all morphology seen in the optical and IR data. Allowing the prompt component to dominate the early-time optical and IR and permitting each pulse to have an independent low energy spectral indices we produce an alternative scenario which better describes the optical light curve. This, however, does not describe the spectral shape of GRB 080310 at early times. The fit statistics for the prompt and afterglow dominated models are nearly identical making it difficult to favour either. However one enduring result is that both models require a low energy spectral index consistent with self absorption for at least some of the pulses identified in the high energy emission model.

astro-ph.HE↗

arXiv2011

SN2008am: A Super-Luminous Type IIn Supernova

We present observations and interpretation of the Type IIn supernova SN 2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN 2008am peaked at approximately -22.3 mag at a redshift of z=0.2338, giving it a peak luminosity of 3 x 10^{44}erg/s and making it one of the most luminous supernovae ever observed. The total radiated energy is ~ 2 x 10^{51} erg. Photometric observations in the ultraviolet, optical and infrared bands (J,H,Ks) constrain the SED evolution. We obtained six optical spectra of the supernova, five on the early decline from maximum light and a sixth nearly a year later plus a very late-time spectrum (~2 yr) of the host galaxy. The spectra of SN 2008am show strong Balmer-line and He I lambda 5876A emission with intermediate widths (~25A) in the first ~40 days after optical maximum. We examine a variety of models for the line wings and conclude that multiple scattering is most likely, implying that our spectra contain no specific information on the bulk flow velocity. We examine a variety of models for the ROTSE light curve subject to the rise time and the nature of the spectra, including radioactive decay, shocks in optically-thick and optically-thin circumstellar media (CSM) and a magnetar. The most successful model is one for which the CSM is optically-thick and in which diffusion of forward shock-deposited luminosity gives rise to the observed light curve. Diffusion of the shock-deposited energy from the forward shock is found to be important to account for the rising part of the light curve. Although there are differences in detail, SN 2008am appears to be closely related to other super-luminous Type IIn supernovae, SN 2006gy, SN 2006tf and perhaps SN 2008iy, that may represent the deaths of very massive LBV-type progenitors and for which the luminosity is powered by the interaction of the ejecta with a dense circumstellar medium.

astro-ph.HE↗

arXiv2010

GRB 090902B: afterglow observations and implications

The optical-infrared afterglow of the LAT-detected long duration burst, GRB 090902B, has been observed by several instruments. The earliest detection by ROTSE-IIIa occurred 80 minutes after detection by the GBM instrument onboard the Fermi Gamma-Ray Space Telescope, revealing a bright afterglow and a decay slope suggestive of a reverse shock origin. Subsequent optical-IR observations followed the light curve for 6.5 days. The temporal and spectral behavior at optical-infrared frequencies is consistent with synchrotron fireball model predictions; the cooling break lies between optical and XRT frequencies ~ 1.9 days after the burst. The inferred electron energy index is $p = 1.8 \pm 0.2$, which would however imply an X-ray decay slope flatter than observed. The XRT and LAT data have similar spectral indices and the observed steeper value of the LAT temporal index is marginally consistent with the predicted temporal decay in the radiative regime of the forward shock model. Absence of a jet break during the first 6 days implies a collimation-corrected $γ$-ray energy $E_γ > 2.2\times10^{52}\rm$ ergs, one of the highest ever seen in a long-duration GRBs. More events combining GeV photon emission with multi-wavelength observations will be required to constrain the nature of the central engine powering these energetic explosions and to explore the correlations between energetic quanta and afterglow emission.

astro-ph.HE↗

arXiv2009

Optical follow-up of high-energy neutrinos detected by IceCube

Three-quarters of the 1 cubic kilometer neutrino telescope IceCube is currently taking data. Current models predict high-energy neutrino emission from transient objects like supernovae (SNe) and gammaray bursts (GRBs). To increase the sensitivity to such transient objects we have set up an optical follow-up program that triggers optical observations on multiplets of high-energy muon-neutrinos. We define multiplets as a minimum of two muon-neutrinos from the same direction (within 4 deg) that arrive within a 100 s time window. When this happens, an alert is issued to the four ROTSE-III telescopes, which immediately observe the corresponding region in the sky. Image subtraction is applied to the optical data to find transient objects. In addition, neutrino multiplets are investigated online for temporal and directional coincidence with gamma-ray satellite observations issued over the Gamma-Ray Burst Coordinate Network. An overview of the full program is given, from the online selection of neutrino events to the automated follow-up, and the resulting sensitivity to transient neutrino sources is presented for the first time.

astro-ph.HE↗

arXiv2009

Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution

GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ROTSE and a host of other telescopes and was detected in the radio by the VLA. The redshift of z= 3.355 +/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasithermal model shifting from ~60 keV to ~3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this originated from the afterglow component, not the pulsed/flaring prompt emission. Broadband optical and X-ray coverage of the afterglow at the start of the final X-ray decay (~8 ks) reveals a spectral break between the optical and X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there is no evidence for a jet break in the afterglow up to six days following the burst.

astro-ph.HE↗

arXiv2008

Discovery of the Ultra-Bright Type II-L Supernova 2008es

We report the discovery by the Robotic Optical Transient Experiment (ROTSE-IIIb) telescope of SN 2008es, an overluminous supernova (SN) at z=0.205 with a peak visual magnitude of -22.2. We present multiwavelength follow-up observations with the Swift satellite and several ground-based optical telescopes. The ROTSE-IIIb observations constrain the time of explosion to be 23+/-1 rest-frame days before maximum. The linear decay of the optical light curve, and the combination of a symmetric, broad Hαemission line profile with broad P Cygni Hβand Na I \lambda5892 profiles, are properties reminiscent of the bright Type II-L SNe 1979C and 1980K, although SN 2008es is greater than 10 times more luminous. The host galaxy is undetected in pre-supernova Sloan Digital Sky Survey images, and similar to Type II-L SN 2005ap (the most luminous SN ever observed), the host is most likely a dwarf galaxy with M_r > -17. Swift Ultraviolet/Optical Telescope observations in combination with Palomar photometry measure the SED of the SN from 200 to 800 nm to be a blackbody that cools from a temperature of 14,000 K at the time of the optical peak to 6400 K 65 days later. The inferred blackbody radius is in good agreement with the radius expected for the expansion speed measured from the broad lines (10,000 km/s). The bolometric luminosity at the optical peak is 2.8 x 10^44 erg/s, with a total energy radiated over the next 65 days of 5.6 x 10^50 erg. We favor a model in which the exceptional peak luminosity is a consequence of the core-collapse explosion of a progenitor star with a low-mass extended hydrogen envelope and a stellar wind with a density close to the upper limit on the mass-loss rate measured from the lack of an X-ray detection by the Swift X-Ray Telescope. (Abridged).

astro-ph↗

arXiv2008

GRB 071003: Broadband Follow-up Observations of a Very Bright Gamma-Ray Burst in a Galactic Halo

The optical afterglow of long-duration GRB 071003 is among the brightest yet to be detected from any GRB, with R ~ 12 mag in KAIT observations starting 42 s after the GRB trigger, including filtered detections during prompt emission. However, our high S/N ratio afterglow spectrum displays only extremely weak absorption lines at what we argue is the host redshift of z = 1.60435 - in contrast to the three other, much stronger Mg II absorption systems observed at lower redshifts. Together with Keck adaptive optics observations which fail to reveal a host galaxy coincident with the burst position, our observations suggest a halo progenitor and offer a cautionary tale about the use of Mg II for GRB redshift determination. We present early through late-time observations spanning the electromagnetic spectrum, constrain the connection between the prompt emission and early variations in the light curve (we observe no correlation), and discuss possible origins for an unusual, marked rebrightening that occurs a few hours after the burst: likely either a late-time refreshed shock or a wide-angle secondary jet. Analysis of the late-time afterglow is most consistent with a wind environment, suggesting a massive star progenitor. Together with GRB 070125, this may indicate that a small but significant portion of star formation in the early universe occurred far outside what we consider a normal galactic disk.

astro-ph↗

arXiv2007

Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages

We report on follow-up observations of the GRB 060927 using the ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at lambda ~ 8070 A produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A which we interpret as SiII at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a column density with log(N_HI/cm^-2) ~ 22.5. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from z>6 should be relatively easy to detect from the ground, but rapid NIR monitoring is necessary to ensure that they are found; ii) The presence of large HI column densities in some GRBs host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at z>5 and therefore the type of galaxies responsible for the reionization of the universe.

astro-ph↗

arXiv2002

The ROTSE-IIIa Telescope System

We report on the current operating status of the ROTSE-IIIa telescope, currently undergoing testing at Los Alamos National Laboratories in New Mexico. It will be shipped to Siding Spring Observatory, Australia, in first quarter 2002. ROTSE-IIIa has been in automated observing mode since early October, 2001, after completing several weeks of calibration and check-out observations. Calibrated lists of objects in ROTSE-IIIa sky patrol data are produced routinely in an automated pipeline, and we are currently automating analysis procedures to compile these lists, eliminate false detections, and automatically identify transient and variable objects. The manual application of these procedures has already led to the detection of a nova that rose over six magnitudes in two days to a maximum detected brightness of m_R~13.9 and then faded two magnitudes in two weeks. We also readily identify variable stars, includings those suspected to be variables from the Sloan Digital Sky Survey. We report on our system to allow public monitoring of the telescope operational status in real time over the WWW.

astro-ph↗

arXiv2002

Discovery of the CV ROTSE3 J015118.59-022300.1

We report on the discovery of an outburst from a previously unknown high- latitude CV we designate ROTSE3 J015118.59-022300.1. The object was first detected on 2001 Oct 13.291 at m-ROTSE3=14.71+-0.06, after which it faded by more than 2 magnitudes over 13 d. Spectral analysis confirms a Galactic location, despite unusual properties that render classification difficult.

astro-ph↗

arXiv2001

Observations of the Optical Counterpart to XTE J1118+480 During Outburst by the ROTSE-I Telescope

The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of 2000. As detected by the Rossi X-ray Timing Explorer (RXTE), the first outburst began in early January and the second began in early March. Routine imaging of the northern sky by the Robotic Optical Transient Search Experiment (ROTSE) shows the optical counterpart to XTE J1118+480 during both outbursts. These data include over 60 epochs from January to June 2000. A search of the ROTSE data archives reveal no previous optical outbursts of this source in selected data between April 1998 and January 2000. While the X-ray to optical flux ratio of XTE J1118+480 was low during both outbursts, we suggest that they were full X-ray novae and not mini-outbursts based on comparison with similar sources. The ROTSE measurements taken during the March 2000 outburst also indicate a rapid rise in the optical flux that preceded the X-ray emission measured by the RXTE by approximately 10 days. Using these results, we estimate a pre-outburst accretion disk inner truncation radius of 1.2 x 10^4 Schwarzschild radii.

astro-ph↗