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Search Results: 1 - 10 of 167637 matches for " E. Bellm "
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RHESSI Tests of Quasi-Thermal Gamma-Ray Burst Spectral Models
Eric C. Bellm
Physics , 2009, DOI: 10.1088/0004-637X/714/1/881
Abstract: Prompt gamma-ray burst spectra evolve on short time scales, suggesting that time-resolved spectral fits may help diagnose the still unknown prompt emission mechanism. We use broad-band gamma-ray data from the RHESSI spacecraft to test quasi-thermal models with high signal-to-noise time-resolved spectra of nine bright gamma-ray bursts. In contrast to results reported in more narrow energy bands, the quality of the fits of quasi-thermal models is poor in relation to fits of the phenomenological Band function. Moreover, the best-fit parameters for the simplest quasi-thermal model, a black body plus a nonthermal power law, show significant dependence on the fit band. Models that replace the power law with more complicated nonthermal functions are not robust for the data considered here and decrease the physical relevance of the fit black body.
The Zwicky Transient Facility
Eric C. Bellm
Physics , 2014,
Abstract: The Zwicky Transient Facility (ZTF) is a next-generation optical synoptic survey that builds on the experience and infrastructure of the Palomar Transient Factory (PTF). Using a new 47 deg$^2$ survey camera, ZTF will survey more than an order of magnitude faster than PTF to discover rare transients and variables. I describe the survey and the camera design. Searches for young supernovae, fast transients, counterparts to gravitational-wave detections, and rare variables will benefit from ZTF's high cadence, wide area survey.
Confirmation of a High Magnetic Field in GRO J1008-57
Eric C. Bellm,Felix Fuerst,Katja Pottschmidt,John A. Tomsick,Steven E. Boggs,Deepto Chakrabarty,Finn E. Christensen,William W. Craig,Charles J. Hailey,Fiona A. Harrison,Daniel Stern,Dominic J. Walton,Joern Wilms,William W. Zhang
Physics , 2014, DOI: 10.1088/0004-637X/792/2/108
Abstract: GRO J1008$-$57 is a high-mass X-ray binary for which several claims of a cyclotron resonance scattering feature near 80 keV have been reported. We use NuSTAR, Suzaku, and Swift data from its giant outburst of November 2012 to confirm the existence of the 80 keV feature and perform the most sensitive search to date for cyclotron scattering features at lower energies. We find evidence for a 78$^{+3}_{-2}$ keV line in the NuSTAR and Suzaku data at $> 4\sigma$ significance, confirming the detection using Suzaku alone by Yamamoto et al. (2014). A search of both the time-integrated and phase-resolved data rules out a fundamental at lower energies with optical depth larger than 5% of the 78 keV line. These results indicate that GRO J1008$-$57 has a magnetic field of $6.7\times10^{12} (1+z)$ G, the highest of known accreting pulsars.
Broadband X-ray Properties of the Gamma-ray Binary 1FGL J1018.6-5856
Hongjun An,Eric Bellm,Varun Bhalerao,Steven E. Boggs,Finn E. Christensen,William W. Craig,Felix Fuerst,Charles J. Hailey,Fiona A. Harrison,Victoria M. Kaspi,Lorenzo Natalucci,Daniel Stern,John A. Tomsick,William W. Zhang
Physics , 2015,
Abstract: We report on NuSTAR, XMM-Newton and Swift observations of the gamma-ray binary 1FGL J1018.6-5856. We measure the orbital period to be 16.544+/-0.008 days using Swift data spanning 1900 days. The orbital period is different from the 2011 gamma-ray measurement which was used in the previous X-ray study of An et al. (2013) using ~400 days of Swift data, but is consistent with a new gamma-ray solution reported in 2014. The light curve folded on the new period is qualitatively similar to that reported previously, having a spike at phase 0 and broad sinusoidal modulation. The X-ray flux enhancement at phase 0 occurs more regularly in time than was previously suggested. A spiky structure at this phase seems to be a persistent feature, although there is some variability. Furthermore, we find that the source flux clearly correlates with the spectral hardness throughout all orbital phases, and that the broadband X-ray spectra measured with NuSTAR, XMM-Newton, and Swift are well fit with an unbroken power-law model. This spectrum suggests that the system may not be accretion-powered.
A Hard X-Ray Study of Ultraluminous X-ray Source NGC 5204 X-1 with NuSTAR and XMM-Newton
E. S. Mukherjee,D. J. Walton,M. Bachetti,F. A. Harrison,D. Barret,E. Bellm,S. E. Boggs,F. E. Christensen,W. W. Craig,A. C. Fabian,F. Fuerst,B. W. Grefenstette,C. J. Hailey,K. K. Madsen,M. J. Middleton,J. M. Miller,V. Rana,D. Stern,W. Zhang
Physics , 2015, DOI: 10.1088/0004-637X/808/1/64
Abstract: We present the results from coordinated X-ray observations of the ultraluminous X-ray source NGC 5204 X-1 performed by NuSTAR and XMM-Newton in early 2013. These observations provide the first detection of NGC 5204 X-1 above 10 keV, extending the broadband coverage to 0.3-20 keV. The observations were carried out in two epochs separated by approximately 10 days, and showed little spectral variation, with an observed luminosity of Lx = (4.95+/-0.11)e39 erg/s. The broadband spectrum confirms the presence of a clear spectral downturn above 10 keV seen in some previous observations. This cutoff is inconsistent with the standard low/hard state seen in Galactic black hole binaries, as would be expected from an intermediate mass black hole accreting at significantly sub-Eddington rates given the observed luminosity. The continuum is apparently dominated by two optically thick thermal-like components, potentially accompanied by a faint high energy tail. The broadband spectrum is likely associated with an accretion disk that differs from a standard Shakura & Sunyaev thin disk.
NuSTAR Observations of GRB130427A establish a single component synchrotron afterglow origin for the late optical to multi-GeV emission
C. Kouveliotou,J. Granot,J. L. Racusin,E. Bellm,G. Vianello,S. Oates,C. L. Fryer,S. E. Boggs,F. E. Christensen,W. W. Craig,C. D. Dermer,N. Gehrels,C. J. Hailey,F. A. Harrison,A. Melandri,J. E. McEnery,C. G. Mundell,D. K. Stern,G. Tagliaferri,W. W. Zhang
Physics , 2013, DOI: 10.1088/2041-8205/779/1/L1
Abstract: GRB 130427A occurred in a relatively nearby galaxy; its prompt emission had the largest GRB fluence ever recorded. The afterglow of GRB 130427A was bright enough for the Nuclear Spectroscopic Telescope ARray (NuSTAR) to observe it in the 3-79 keV energy range long after its prompt emission (~1.5 and 5 days). This range, where afterglow observations were previously not possible, bridges an important spectral gap. Combined with Swift, Fermi and ground-based optical data, NuSTAR observations unambiguously establish a single afterglow spectral component from optical to multi-GeV energies a day after the event, which is almost certainly synchrotron radiation. Such an origin of the late-time Fermi/LAT > 10 GeV photons requires revisions in our understanding of collisionless relativistic shock physics.
Detection and Imaging of the Crab Nebula with the Nuclear Compton Telescope
M. S. Bandstra,E. C. Bellm,S. E. Boggs,D. Perez-Becker,A. Zoglauer,H. -K. Chang,J. -L. Chiu,J. -S. Liang,Y. -H. Chang,Z. -K. Liu,W. -C. Hung,M. -H. A. Huang,S. J. Chiang,R. -S. Run,C. -H. Lin,M. Amman,P. N. Luke,P. Jean,P. von Ballmoos,C. B. Wunderer
Physics , 2011, DOI: 10.1088/0004-637X/738/1/8
Abstract: The Nuclear Compton Telescope (NCT) is a balloon-borne Compton telescope designed for the study of astrophysical sources in the soft gamma-ray regime (200 keV--20 MeV). NCT's ten high-purity germanium crossed-strip detectors measure the deposited energies and three-dimensional positions of gamma-ray interactions in the sensitive volume, and this information is used to restrict the initial photon to a circle on the sky using the Compton scatter technique. Thus NCT is able to perform spectroscopy, imaging, and polarization analysis on soft gamma-ray sources. NCT is one of the next generation of Compton telescopes --- so-called compact Compton telescopes (CCTs) --- which can achieve effective areas comparable to COMPTEL's with an instrument that is a fraction of the size. The Crab Nebula was the primary target for the second flight of the NCT instrument, which occurred on 17--18 May 2009 in Fort Sumner, New Mexico. Analysis of 29.3 ks of data from the flight reveals an image of the Crab at a significance of 4-sigma. This is the first reported detection of an astrophysical source by a CCT.
The smooth cyclotron line in Her X-1 as seen with NuSTAR
Felix Fuerst,Brian W. Grefenstette,Ruediger Staubert,John A. Tomsick,Matteo Bachetti,Didier Barret,Eric C. Bellm,Steven E. Boggs,Jerome Chenevez,Finn E. Christensen,William W. Craig,Charles J. Hailey,Fiona Harrison,Dmitry Klochkov,Kristin K. Madsen,Katja Pottschmidt,Daniel Stern,Dominic J. Walton,Joern Wilms,William Zhang
Physics , 2013,
Abstract: Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35d super-orbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broad-band spectra can be fitted with a powerlaw with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape, in all observations and at all pulse-phases. We compare the residuals of a line with a Gaussian optical depth profile to a Lorentzian optical depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum is only changing marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot corona of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date.
X-ray Spectral Components in the Afterglow of GRB 130925A
Eric C. Bellm,Nicolas M. Barriere,Varun Bhalerao,Steven E. Boggs,S. Bradley Cenko,Finn E. Christensen,William W. Craig,Karl Forster,Chris L. Fryer,Charles J. Hailey,Fiona A. Harrison,Assaf Horesh,Chryssa Kouveliotou,Kristin K. Madsen,Jon M. Miller,Eran O. Ofek,Daniel A. Perley,Vikram R. Rana,Stephen P. Reynolds,Daniel Stern,John A. Tomsick,William W. Zhang
Physics , 2014, DOI: 10.1088/2041-8205/784/2/L19
Abstract: We have identified spectral features in the late-time X-ray afterglow of the unusually long, slow-decaying GRB 130925A using NuSTAR, Swift-XRT, and Chandra. A spectral component in addition to an absorbed power-law is required at $>4\sigma$ significance, and its spectral shape varies between two observation epochs at $2\times10^5$ and $10^6$ seconds after the burst. Several models can fit this additional component, each with very different physical implications. A broad, resolved Gaussian absorption feature of several keV width improves the fit, but it is poorly constrained in the second epoch. An additive black body or second power-law component provide better fits. Both are challenging to interpret: the blackbody radius is near the scale of a compact remnant ($10^8$ cm), while the second powerlaw component requires an unobserved high-energy cutoff in order to be consistent with the non-detection by Fermi-LAT.
NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619
Varun Bhalerao,Patrizia Romano,John Tomsick,Lorenzo Natalucci,David M. Smith,Eric Bellm,Steven E. Boggs,Deepto Chakrabarty,Finn E. Christensen,William W. Craig,Felix Fuerst,Charles J. Hailey,Fiona A. Harrison,Roman A. Krivonos,Ting-Ni Lu,Kristin Madsen,Daniel Stern,George Younes,William Zhang
Physics , 2014, DOI: 10.1093/mnras/stu2495
Abstract: We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544-2619 is indeed a neutron star. This is the first measurement of the magnetic field in a SFXT. The inferred magnetic field strength, B = (1.45 +/- 0.03) * 10^12 G * (1+z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We do not find any significant pulsations in the source on time scales of 1-2000 s.
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