E. Armstrong

Columbia University, New York City, New York, United States

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Publications (21)51.59 Total impact

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    ABSTRACT: We report photometric detections of orbital and superorbital signals, and negative orbital sidebands, in the light curves of the nova-like cataclysmic variables AQ Mensae and IM Eridani. The frequencies of the orbital, superorbital, and sideband signals are 7.0686 (3), 0.263 (3), and 7.332 (3) cycles per day (c/d) in AQ Mensae, and 6.870 (1), 0.354 (7), and 7.226 (1) c/d in IM Eridani. We also find a spectroscopic orbital frequency in IM Eridani of 6.86649 (2) c/d. These observations can be reproduced by invoking an accretion disc that is tilted with respect to the orbital plane. This model works well for X-ray binaries, in which irradiation by a primary neutron star can account for the disc's tilt. A likely tilt mechanism has yet to be identified in CVs, yet the growing collection of observational evidence indicates that the phenomenon of tilt is indeed at work in this class of object. The results presented in this paper bring the number of CVs known to display signals associated with retrograde disc precession to twelve. We also find AQ Mensae to be an eclipsing system. The eclipse depths are highly variable, which suggests that the eclipses are grazing. This finding raises the possibility of probing variations in disc tilt by studying systematic variations in the eclipse profile.
    Monthly Notices of the Royal Astronomical Society 07/2013; 435(1). · 5.52 Impact Factor
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    Eve Armstrong
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    ABSTRACT: We report a non-detection, to a limiting magnitude of V = 18.4 (9), of the elusive entity commonly described as the Tooth Fairy. We review various physical models and conclude that follow-up observations must precede an interpretation of our result.
    04/2012;
  • E. Armstrong, J. Patterson, J. Kemp
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    ABSTRACT: We report photometric periodicities in the AM CVn system CP Eridani (CP Eri) of 1716.2 ± 0.2 and 1701.4 ± 0.2 s, obtained while the system was in quiescence. From a second observation obtained during superoutburst, we interpret the 1716-s signal as a superhump period (Psh) and the 1701-s signal as the orbital period (Porb) of the binary. The derived fractional superhump period excess ɛ adds CP Eri to a small collection of AM CVn stars with ɛ measured via time-series photometry of superhump and orbital periods. Plotting ɛ(Porb) for these systems, we find that AM CVn systems may, as expected, be evolving towards longer Porb. We discuss a technique of using ɛ to determine the degree of degeneracy of the donor star in a contact binary, and we show that for the AM CVn systems, the donor stars are well described by mass-radius relations of partially-degenerate objects.
    Monthly Notices of the Royal Astronomical Society 04/2012; 421(3):2310-2315. · 5.52 Impact Factor
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    ABSTRACT: We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620s orbital period. The mass transfer in systems such as this one is thought to be driven by gravitational radiation, which causes the binary to evolve to longer periods since the semi-degenerate donor star expands in size as it loses mass. We supplement these ULTRACAM+WHT data with observations made with smaller telescopes around the world over a nine year baseline. All of the observations show variation on the orbital period, and by timing this variation we track the period evolution of this system. We do not detect any significant departure from a linear ephemeris, implying a donor star that is of small mass and close to a fully degenerate state. This finding favours the double white dwarf formation channel for this AM CVn star. An alternative explanation is that the system is in the relatively short-lived phase in which the mass transfer rate climbs towards its long-term value.
    Monthly Notices of the Royal Astronomical Society 01/2011; 413. · 5.52 Impact Factor
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    ABSTRACT: Nonradial pulsations had ceased in the accreting white dwarf SDSS J074531.92+453829.6 subsequent to its October 2006 outburst. We recently acquired optical high‐speed time‐series photometry on this cataclysmic variable more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre‐outburst periods within the uncertainties of 1–2 s. This discovery is both remarkable and significant because it indicates that the outburst did not affect the interior stellar structure, which dictates the observed pulsation frequencies. Using this discovery in addition to an HST ultra‐violet temperature measurement obtained one year after outburst, we have also been able to constrain the matter accreted during the 2006 outburst.
    AIP Conference Proceedings. 11/2010; 1273(1):520-525.
  • J. P. Halpern, E. Armstrong
    GRB Coordinates Network. 01/2007;
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    ABSTRACT: We obtained 98 R-band and 18 B, r', i' images of the optical afterglow of GRB 060526 (z=3.21) with the MDM 1.3m, 2.4m, and the PROMPT telescopes in Cerro Tololo over the 5 nights following the burst trigger. Combining these data with other optical observations reported in GCN and the Swift-XRT observations, we compare the optical and X-ray afterglow light curves of GRB 060526. Both the optical and X-ray afterglow light curves show rich features, such as flares and breaks. The densely sampled optical observations provide very good coverage at T>1.e4 sec. We observed a break at 2.4e5 sec in the optical afterglow light curve. Compared with the X-ray afterglow light curve, the break is consistent with an achromatic break supporting the beaming models of GRBs. However, the pre-break and post-break temporal decay slopes are difficult to explain in simple afterglow models. We estimated a jet angle of \theta_j ~ 7 degrees and a prompt emission size of R_{prompt} ~ 2e14 cm. In addition, we detected several optical flares with amplitudes of \Delta m ~ 0.2, 0.6, and 0.2 mag. The X-ray afterglows detected by Swift have shown complicated decay patterns. Recently, many well-sampled optical afterglows also show decays with flares and multiple breaks. GRB 060526 provides an additional case of such a complex, well observed optical afterglow. The accumulated well-sampled afterglows indicate that most of the optical afterglows are complex.
    The Astrophysical Journal 10/2006; · 6.73 Impact Factor
  • J. P. Halpern, N. Mirabal, E. Armstrong
    GRB Coordinates Network. 01/2006;
  • J. P. Halpern, E. Armstrong
    GRB Coordinates Network. 01/2006;
  • GRB Coordinates Network. 01/2006;
  • J. P. Halpern, N. Mirabal, E. Armstrong
    GRB Coordinates Network. 01/2006;
  • J. P. Halpern, E. Armstrong, N. Mirabal
    GRB Coordinates Network. 01/2006;
  • GRB Coordinates Network. 01/2006;
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    ABSTRACT: We report on successes and failures in searching for positive superhumps in cataclysmic variables, and show the superhumping fraction as a function of orbital period. Basically, all short-period system do, all long-period systems do not, and a 50% success rate is found at Porb=3.1+/-0.2 hr. We can use this to measure the critical mass ratio for the creation of superhumps. With a mass-radius relation appropriate for cataclysmic variables, and an assumed mean white-dwarf mass of 0.75 Msolar, we find a mass ratio qcrit=0.35+/-0.02. We also report superhump studies of several stars of independently known mass ratio: OU Vir, XZ Eri, UU Aqr, and KV UMa (=XTE J1118+480). The latter two are of special interest, because they represent the most extreme mass ratios for which accurate superhump measurements have been made. We use these to improve the ɛ(q) calibration, by which we can infer the elusive q from the easy-to-measure ɛ (the fractional period excess of Psuperhump over Porb). This relation allows mass and radius estimates for the secondary star in any cataclysmic variable (CV) showing superhumps. The consequent mass-radius law shows an apparent discontinuity in radius near 0.2 Msolar, as predicted by the disrupted magnetic braking model for the 2.1-2.7 hr period gap. This is effectively the ``empirical main sequence'' for CV secondaries.
    Publications of the Astronomical Society of the Pacific 11/2005; 117(837):1204-1222. · 3.69 Impact Factor
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    ABSTRACT: We report a photometric study of the WZ Sagittae-type dwarf nova PQ Andromedae. The light curve shows strong (0.05 mag full amplitude) signals with periods of 1263(1) and 634(1) s, and a likely double-humped signal with P=80.6(2) min. We interpret the first two as nonradial pulsation periods of the underlying white dwarf, and the last as the orbital period of the underlying binary. We estimate a distance of 150(50) pc from proper motions and the two standard candles available: the white dwarf and the dwarf-nova outburst. At this distance, the K magnitude implies that the secondary is probably fainter than any star on the main sequence -- indicating a mass below the Kumar limit at 0.075 M_sol. PQ And may be another "period bouncer", where evolution now drives the binary out to longer period. Comment: PDF, 13 pages, 2 figures; accepted, in press, to appear September 2005, PASP; more info at http://cba.phys.columbia.edu/
    Publications of the Astronomical Society of the Pacific 06/2005; · 3.69 Impact Factor
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    J. R. Thorstensen, E. Armstrong
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    ABSTRACT: The radio source FIRST J102347.6+003841 was presented as the first radio-selected cataclysmic. In the discovery paper, Bond et al. (2002) show a spectrum consistent with a magnetic AM Her-type system and a light curve with rapid, irregular flickering. In contrast, Woudt, Warner, and Pretorius (2004) found a smoothly-varying light curve with a period near 4.75 h and one minimum per orbit, indicating a dramatic change. We present time-resolved spectra showing a superficially normal, mid-G type photosphere, with no detectable emission lines. The absorption-line radial velocity varies sinusoidally, with semi-amplitude 268 +- 4 km/s, on the orbital period, which is refined to 0.198094(2) d. At this orbital period the secondary's spectral type is atypically early, suggesting an unusual evolutionary history. We also obtained BVI photometry around the orbit. The light curve resembles that given by Woudt et al., and the color modulation is consistent with a heating effect. A simple illumination model matches the observations strikingly well with a Roche-lobe filling secondary near 5650 kelvin being illuminated by a primary giving out around 2 solar luminosities. The modest amplitude of the observed modulation constrains the orbital inclination to be less than about 55 deg, unless the gravity darkening is artificially reduced. The resulting primary star mass is above the Chandrasekhar limit (assuming conventional gravity darkening). We examine the possibility that the compact object in this system is not a white dwarf, in which case this is not actually a cataclysmic variable. On close examination, FIRST J102347.6+003841 defies easy classification.
    The Astronomical Journal 05/2005; · 4.97 Impact Factor
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    ABSTRACT: We report photometry and spectroscopy of the novalike variable DW Cancri. The spectra show the usual broad H and He emission lines, with an excitation and continuum slope characteristic of a moderately high accretion rate. A radial-velocity search yields strong detections at two periods, 86.1015(3) min and 38.58377(6) min. We interpret these as respectively the orbital period P_orb of the binary, and the spin period P_spin of a magnetic white dwarf. The light curve also shows the spin period, plus an additional strong signal at 69.9133(10) min, which coincides with the difference frequency 1/P_spin-1/P_orb. These periods are stable over the 1 year baseline of measurement. This triply-periodic structure mimics the behavior of several well-credentialed members of the "DQ Herculis" (intermediate polar) class of cataclysmic variables. DQ Her membership is also suggested by the mysteriously strong sideband signal (at nu_spin-nu_orb), attesting to a strong pulsed flux at X-ray/EUV/UV wavelengths. DW Cnc is a new member of this class, and would be an excellent target for extended observation at these wavelengths. Comment: PDF, 28 pages, 6 tables, 9 figures; accepted, in press, to appear June 2004, PASP; more info at http://cba.phys.columbia.edu/
    Publications of the Astronomical Society of the Pacific 04/2004; · 3.69 Impact Factor
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    ABSTRACT: We present high-speed photometry of the Sloan Digital Sky Survey cataclysmic variable SDSS J013701.06 - 091234.9 in quiescence and during its 2003 December superoutburst. The orbital modulation at 79.71 +/- 0.01 min is double-humped; the superhump period is 81.702 +/- 0.007 min. Towards the end of the outburst late superhumps with a period of 81.29 +/- 0.01 min were observed. We argue that this is a system of very low mass transfer rate, and that it probably has a long outburst interval.
    Monthly Notices of the Royal Astronomical Society 01/2004; 352(3):1056-1060. · 5.52 Impact Factor
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    ABSTRACT: The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3=2.328 for its host galaxy, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers at z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s respectively relative to the host galaxy Lyman-alpha emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than 0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a WCL progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jet-like optical decay further supports this interpretation since fluctuations must be present at some level due to inhomogeneities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor. Comment: Replaced with version accepted by ApJ; 40 pages, 9 figures
    The Astrophysical Journal 03/2003; · 6.73 Impact Factor
  • J. P. Halpern, E. Armstrong

Publication Stats

285 Citations
51.59 Total Impact Points

Institutions

  • 2003–2013
    • Columbia University
      • Columbia Astrophysics Laboratory
      New York City, New York, United States
  • 2005–2006
    • University of California, San Diego
      • Department of Physics
      San Diego, California, United States