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Espinoza C.M.,University of Manchester | Guillemot L.,Max Planck Institute for Radio Astronomy | celik O.,NASA | celik O.,Center for Research and Exploration in Space Science and Technology | And 32 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We report on the discovery of gamma-ray pulsations from five millisecond pulsars (MSPs) using the Fermi Large Area Telescope (LAT) and timing ephemerides provided by various radio observatories. We also present confirmation of the gamma-ray pulsations from a sixth source, PSR J2051-0827. Five of these six MSPs are in binary systems: PSRs J1713+0747, J1741+1351, J1600-3053 and the two black widow binary pulsars PSRs J0610-2100 and J2051-0827. The only isolated MSP is the nearby PSR J1024-0719, which is also known to emit X-rays. We present X-ray observations in the direction of PSRs J1600-3053 and J2051-0827. While PSR J2051-0827 is firmly detected, we can only give upper limits for the X-ray flux of PSR J1600-3053. There are no dedicated X-ray observations available for the other three objects.The MSPs mentioned above, together with most of the MSPs detected by Fermi, are used to put together a sample of 30 gamma-ray MSPs. This sample is used to study the morphology and phase connection of radio and gamma-ray pulse profiles. We show that MSPs with pulsed gamma-ray emission which is phase-aligned with the radio emission present the steepest radio spectra and the largest magnetic fields at the light cylinder among all MSPs. Also, we observe a trend towards very low, or undetectable, radio linear polarization levels. These properties could be attributed to caustic radio emission produced at a range of different altitudes in the magnetosphere. We note that most of these characteristics are also observed in the Crab pulsar, the only other radio pulsar known to exhibit phase-aligned radio and gamma-ray emission. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Guiriec S.,NASA | Guiriec S.,University of Maryland University College | Guiriec S.,Center for Research and Exploration in Space Science and Technology | Kouveliotou C.,NASA | And 19 more authors.
Astrophysical Journal | Year: 2015

Gamma-ray burst (GRB) prompt emission spectra in the keV-MeV energy range are usually considered to be adequately fitted with the empirical Band function. Recent observations with the Fermi Gamma-ray Space Telescope (Fermi) revealed deviations from the Band function, sometimes in the form of an additional blackbody (BB) component, while on other occasions in the form of an additional power law (PL) component extending to high energies. In this article we investigate the possibility that the three components may be present simultaneously in the prompt emission spectra of two very bright GRBs (080916C and 090926A) observed with Fermi, and how the three components may affect the overall shape of the spectra. While the two GRBs are very different when fitted to a single Band function, they look like "twins" in the three-component scenario. Through fine-time spectroscopy down to the 100 ms timescale, we follow the evolution of the various components. We succeed in reducing the number of free parameters in the three-component model, which results in a new semi-empirical model - but with physical motivations - to be competitive with the Band function in terms of number of degrees of freedom. From this analysis using multiple components, the Band function is globally the most intense component, although the additional PL can overpower the others in sharp time structures. The Band function and the BB component are the most intense at early times and globally fade across the burst duration. The additional PL is the most intense component at late time and may be correlated with the extended high-energy emission observed thousands of seconds after the burst with Fermi/Large Area Telescope. Unexpectedly, this analysis also shows that the additional PL may be present from the very beginning of the burst, where it may even overpower the other components at low energy. We investigate the effect of the three components on the new time-resolved luminosity-hardness relation in both the observer and rest frames and show that a strong correlation exists between the flux of the non-thermal Band function and its Epeak only when the three components are fitted simultaneously to the data (i.e., FiNT-Epeak,iNT relation). In addition, this result points toward a universal relation between those two quantities when transposed to the central engine rest frame for all GRBs (i.e., LiNT-Epeak,iNT relation). We discuss a possible theoretical interpretation of the three spectral components within this new empirical model. We suggest that (i) the BB component may be interpreted as the photosphere emission of a magnetized relativistic outflow, (ii) the Band component has synchrotron radiation in an optically thin region above the photosphere, either from internal shocks or magnetic field dissipation, and (iii) the extra PL component extending to high energies likely has an inverse Compton origin of some sort, even though its extension to a much lower energy remains a mystery. © 2015. The American Astronomical Society. All rights reserved.

Holland S.T.,Goddard Space Flight Center | Holland S.T.,Universities Space Research Association | Holland S.T.,Center for Research and Exploration in Space Science and Technology | Sbarufatti B.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 16 more authors.
Astrophysical Journal | Year: 2010

GRB 090417B was an unusually long burst with a T 90 duration of at least 2130s and a multi-peaked light curve at energies of 15-150keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst (GRB) and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5mag fainter than predicted by the X-ray flux. The Swift/XRT X-ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of AV ≳ 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a GRBs that is dark due to the localized dust structure in its host galaxy. © 2010. The American Astronomical Society. All rights reserved..

Ransom S.M.,U.S. National Radio Astronomy Observatory | Ray P.S.,U.S. Navy | Camilo F.,Columbia University | Roberts M.S.E.,Eureka Scientific | And 41 more authors.
Astrophysical Journal Letters | Year: 2011

We searched for radio pulsars in 25 of the non-variable, unassociated sources in the Fermi LAT Bright Source List with the Green Bank Telescope at 820 MHz. We report the discovery of three radio and γ-ray millisecond pulsars (MSPs) from a high Galactic latitude subset of these sources. All of the pulsars are in binary systems, which would have made them virtually impossible to detect in blind γ-ray pulsation searches. They seem to be relatively normal, nearby (≤2 kpc) MSPs. These observations, in combination with the Fermi detection of γ-rays from other known radio MSPs, imply that most, if not all, radio MSPs are efficient γ-ray producers. The γ-ray spectra of the pulsars are power law in nature with exponential cutoffs at a few GeV, as has been found with most other pulsars. The MSPs have all been detected as X-ray point sources. Their soft X-ray luminosities of ∼10 30-1031 erg s-1 are typical of the rare radio MSPs seen in X-rays. © 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Schimoia J.S.,Federal University of Rio Grande do Sul | Schimoia J.S.,Ohio State University | Storchi-Bergmann T.,Federal University of Rio Grande do Sul | Storchi-Bergmann T.,Harvard - Smithsonian Center for Astrophysics | And 12 more authors.
Astrophysical Journal | Year: 2015

Recent studies have suggested that the short-timescale (≲7 days) variability of the broad (∼10,000 km s-1) double-peaked Hα profile of the LINER nucleus of NGC 1097 could be driven by a variable X-ray emission from a central radiatively inefficient accretion flow. To test this scenario, we have monitored the NGC 1097 nucleus in X-ray and UV continuum with Swift and the Hα flux and profile in the optical spectrum using SOAR and Gemini-South from 2012 August to 2013 February. During the monitoring campaign, the Hα flux remained at a very low level - three times lower than the maximum flux observed in previous campaigns and showing only limited (∼20%) variability. The X-ray variations were small, only ∼13% throughout the campaign, while the UV did not show significant variations. We concluded that the timescale of the Hα profile variation is close to the sampling interval of the optical observations, which results in only a marginal correlation between the X-ray and Hα fluxes. We have caught the active galaxy nucleus in NGC 1097 in a very low activity state, in which the ionizing source was very weak and capable of ionizing just the innermost part of the gas in the disk. Nonetheless, the data presented here still support the picture in which the gas that emits the broad double-peaked Balmer lines is illuminated /ionized by a source of high-energy photons which is located interior to the inner radius of the line-emitting part of the disk. © 2015. The American Astronomical Society. All rights reserved.

Fertig D.,University of Maryland Baltimore County | Fertig D.,George Mason University | Mukai K.,University of Maryland Baltimore County | Mukai K.,Center for Research and Exploration in Space Science and Technology | And 7 more authors.
Publications of the Astronomical Society of the Pacific | Year: 2011

In a dwarf nova, the accretion disk around the white dwarf is a source of ultraviolet, optical, and infrared photons, but is never hot enough to emit X-rays. Observed X-rays instead originate from the boundary layer between the disk and the white dwarf. As the disk switches between quiescence and outburst states, the 2-10 keV X-ray flux is usually seen to be anticorrelated with the optical brightness. Here, we present RXTE monitoring observations of two dwarf novae, VW Hyi and WW Cet, confirming the optical/X-ray anticorrelation in these two systems. However, we do not detect any episodes of increased hard X-ray flux on the rise (out of two possible chances for WW Cet) or the decline (two for WW Cet and one for VW Hyi) from outburst, attributes that are clearly established in SS Cyg. The addition of these data to the existing literature establishes the fact that the behavior of SS Cyg is the exception, rather than the archetype as is often assumed. We speculate on the origin of the diversity of behaviors exhibited by dwarf novae, focusing on the role played by the white dwarf mass. © 2011. The Astronomical Society of the Pacific. All rights reserved.

Guiriec S.,NASA | Guiriec S.,University of Maryland University College | Guiriec S.,Center for Research and Exploration in Space Science and Technology | Gonzalez M.M.,National Autonomous University of Mexico | And 5 more authors.
Astrophysical Journal | Year: 2016

The paradigm for gamma-ray burst (GRB) prompt emission is changing. Since early in the Compton Gamma Ray Observatory (CGRO) era, the empirical Band function has been considered a good description of the keV-MeV γ-ray prompt emission spectra despite the fact that its shape was very often inconsistent with the theoretical predictions, especially those expected in pure synchrotron emission scenarios. We have recently established a new observational model analyzing data of the NASA Fermi Gamma-ray Space Telescope. In this model, GRB prompt emission would be a combination of three main emission components: (i) a thermal-like component that we have interpreted so far as emission from the jet photosphere, (ii) a non-thermal component that we have interpreted so far as either synchrotron radiation from the propagating and accelerated charged particles within the jet or reprocessed jet photospheric emission, and (iii) an additional non-thermal (cutoff) power law (PL) extending from low to high energies in γ-rays and most likely of inverse Compton origin. In this article we reanalyze some of the bright GRBs, namely GRBs 941017, 970111, and 990123, observed with the Burst And Transient Source Experiment (BATSE) on board CGRO with the new model. We conclude that BATSE data for these three GRBs are fully consistent with the recent results obtained with Fermi: some bright BATSE GRBs exhibit three separate components during the prompt phase with similar spectral parameters as those reported from Fermi data. In addition, the analysis of the BATSE GRBs with the new prompt emission model results in a relation between the time-resolved energy flux of the non-thermal component, Fi nTh, and its corresponding νFν spectral peak energy, Epeak, i nTh (i.e., Fi nTh - Epeak, i nTh), which has a similar index - when fitted to a PL - as the one initially derived from Fermi data. For GRBs with known redshifts (z) this results in a possible universal relation between the luminosity of the non-thermal component, Li nTh, and its corresponding νFv spectral peak energy in the rest frame, Epeak,i NT,rest(i.e.,Li nTh - Epeak,i NT,rest). We estimated the redshifts of GRBs 941017 and 970111 using GRB 990123 - with z = 1.61 - as a reference. The estimated redshift for GRB 941017 is typical for long GRBs and the estimated redshift for GRB 970111 is right in the range of the expected values for this burst. © 2016. The American Astronomical Society. All rights reserved..

Guiriec S.,NASA | Guiriec S.,University of Maryland University College | Guiriec S.,Center for Research and Exploration in Space Science and Technology | Mochkovitch R.,University Pierre and Marie Curie | And 6 more authors.
Astrophysical Journal | Year: 2015

Over the past few years, evidence has been accumulated in support of the existence of a thermal-like component during the prompt phase of gamma-ray bursts (GRBs). However, this component, which is often associated with the GRB jet's photosphere, is usually subdominant compared to a much stronger non-thermal one. The prompt emission of GRB 131014A - detected by the Fermi Gamma-ray Space Telescope (hereafter Fermi) - provides a unique opportunity to trace the history of this thermal-like component. Indeed, the thermal emission in GRB 131014A is much more intense than in other GRBs and a pure thermal episode is observed during the initial 0.16 s. The thermal-like component cools monotonically during the first second while the non-thermal emission kicks off. The intensity of the non-thermal component progressively increases until being energetically dominant at late time, similar to what is typically observed. This is a perfect scenario to disentangle the thermal component from the non-thermal component. The initial decaying and cooling phase of the thermal-like component is followed by a strong re-brightening and a re-heating episode; however, despite a much brighter second emission phase, the temperature of the thermal component does not reach its initial value. This re-brightening episode is followed by a global constant cooling until the end of the burst. We note that there is a shallower low-energy spectral slope than the typical index value +1, corresponding to a pure Planck function, which better matches with the thermal-like spectral shape; a spectral index around +0.6 seems to be in better agreement with the data. The non-thermal component is adequately fitted with a Band function whose low- and high-energy power-law indices are ∼-0.7 and <∼-3, respectively; this is also statistically globally equivalent to a cutoff power law with a ∼-0.7 index. This is in agreement with our previous results. Finally, a strong correlation is observed between the time-resolved energy flux, Fi nTh, and the corresponding spectral peak energy, Epeak,i nTh, of the non-thermal component with a slope similar to the one reported in our previous articles. Assuming a universal relation between the time-resolved luminosity of the non-thermal component, Li nTh, and its rest frame Epeak,i nTh, Epeak,i rest,nTh, which we derived from a limited sample of GRBs detected by Fermi, we estimate a redshift of ∼1.55 for GRB 131014A, which is a typical value for long GRBs. These observational results are consistent with the models in which the non-thermal emission is produced well above the GRB jet photosphere but they may also be compatible with other scenarios (e.g., dissipative photosphere) that are not discussed in this article. © 2015. The American Astronomical Society. All rights reserved.

Swenson C.A.,Pennsylvania State University | Maxham A.,University of Nevada, Las Vegas | Roming P.W.A.,Pennsylvania State University | Roming P.W.A.,Southwest Research Institute | And 11 more authors.
Astrophysical Journal Letters | Year: 2010

GRB 090926A was detected by both the Gamma-ray Burst Monitor and Large Area Telescope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. Swift follow-up observations began ∼13 hr after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long-lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+105s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 gamma-ray bursts; nine of these bursts, including GRB 090926A, also have been observed by Swift. Of the nine Swift-observed LAT bursts, six were detected by UVOT, with five of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for five years and has detected nearly 500 bursts, but has only seen ∼ 30% of bursts with optical afterglows that live longer than 105 s. We have calculated the predicted gamma-ray fluence, as would have been seen by the Burst Alert Telescope (BAT) on board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate that the LAT bursts are more energetic than 88% of all Swift bursts and also have brighter than average X-ray and optical afterglows. © 2010. The American Astronomical Society. All rights reserved.

Holland S.T.,Goddard Space Flight Center | Holland S.T.,Universities Space Research Association | Holland S.T.,Center for Research and Exploration in Space Science and Technology | De Pasquale M.,University College London | And 20 more authors.
Astrophysical Journal | Year: 2012

We present an analysis of the unusual optical light curve of the gamma-ray burst GRB081029, a long-soft burst with a redshift of z = 3.8479. We combine X-ray and optical observations from the Swift X-Ray Telescope and the Swift UltraViolet/Optical Telescope with ground-based optical and infrared data obtained using the REM, ROTSE, and CTIO 1.3m telescopes to construct a detailed data set extending from 86s to 100000s after the BAT trigger. Our data cover a wide energy range from 10keV to 0.77 eV (1.24 -16000 ). The X-ray afterglow shows a shallow initial decay followed by a rapid decay starting at about 18000s. The optical and infrared afterglow, however, shows an uncharacteristic rise at about 3000s that does not correspond to any feature in the X-ray light curve. Our data are not consistent with synchrotron radiation from a jet interacting with an external medium, a two-component jet, or continuous energy injection from the central engine. We find that the optical light curves can be broadly explained by a collision between two ejecta shells within a two-component jet. A growing number of gamma-ray-burst afterglows are consistent with complex jets, which suggests that some (or all) gamma-ray-burst jets are complex and will require detailed modeling to fully understand them. © 2012. The American Astronomical Society. All rights reserved.

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