The Experts below are selected from a list of 48 Experts worldwide ranked by ideXlab platform
Lee Homer - One of the best experts on this subject based on the ideXlab platform.
-
deviations from the flux recurrence time relationship in gs 1826 238 potential transient spectral changes
The Astrophysical Journal, 2008Co-Authors: Thomas W. J. Thompson, Duncan K. Galloway, Richard E. Rothschild, Lee HomerAbstract:The low-mass X-ray binary GS 1826� 238 is presently unique for its consistently regular bursting behavior. In previousRossiX-RayTimingExplorer(RXTE)measurementsbetween1997Novemberand2002July,thissource exhibited (nearly) limit-cycle Bursts with recurrence times that decreased proportionately as the persistent flux increased.HerewereportadditionalmeasurementsoftheburstrecurrencetimebyRXTE,Chandra,andXMM-Newton, as well as observations of Optical Bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier fluxYrecurrence time relationship, and most of these Bursts occurred earlier than would bepredictedbasedontheX-rayfluxlevel.TheepochswithearlyBurstswerealsoaccompaniedbyunusualbroadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (� 30% of the total) to account for the burst recurrence time discrepancy. A selfconsistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that the accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level. Subject headingg X-rays: binaries — X-rays: Bursts — X-rays: individual (Ginga 1826� 238)
-
Deviations from the Flux-Recurrence Time Relationship in GS 1826-238: Potential Transient Spectral Changes
The Astrophysical Journal, 2008Co-Authors: Thomas W. J. Thompson, Duncan K. Galloway, Richard E. Rothschild, Lee HomerAbstract:The low-mass X-ray binary GS 1826-238 is presently unique for its consistently regular bursting behavior. In previous Rossi X-Ray Timing Explorer (RXTE) measurements between 1997 November and 2002 July, this source exhibited (nearly) limit-cycle Bursts with recurrence times that decreased proportionately as the persistent flux increased. Here we report additional measurements of the burst recurrence time by RXTE, Chandra, and XMM-Newton, as well as observations of Optical Bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier flux-recurrence time relationship, and most of these Bursts occurred earlier than would be predicted based on the X-ray flux level. The epochs with early Bursts were also accompanied by unusual broadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (30% of the total) to account for the burst recurrence time discrepancy. A self-consistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level.
B C Lee - One of the best experts on this subject based on the ideXlab platform.
-
an untriggered search for Optical Bursts
The Astrophysical Journal, 2002Co-Authors: Robert Kehoe, C Akerlof, R Balsano, Jeffrey J Bloch, Don Casperson, S Fletcher, G Gisler, B C LeeAbstract:We present an untriggered search for Optical Bursts with the Robotic Optical Transient Search Experiment (ROTSE-I) telephoto array. Observations were taken that monitor an effective 256 deg2 field continuously over 125 hr to mROTSE = 15.7. The uniquely large field, moderate limiting magnitude, and fast cadence of ~10 minutes permit transient searches in a new region of sensitivity. Our search reveals no candidate events. To quantify this result, we simulate potential Optical Bursts with peak magnitude mp at t = 10 s, which fade as f = , where αt < 0. Simple estimates based on observational evidence indicate that a search of this sensitivity begins to probe the possible region occupied by gamma-ray burst (GRB) orphan afterglows. Our observing protocol and image sensitivity result in a broad region of high detection efficiency for light curves to the bright and slowly varying side of a boundary running from [αt,mp] = [-2.0, 6.0] to [-0.3, 13.2]. Within this region, the integrated rate of brief Optical Bursts is less than 1.1 × 10-8 s-1 deg-2. At ~22 times the observed GRB rate from BATSE, this suggests a limit on θopt/θγ 5, where θopt and θγ are the Optical and gamma-ray collimation angles, respectively. Several effects might explain the absence of Optical Bursts, and a search of the kind described here but more sensitive by about 4 mag should offer a more definitive probe.
-
an untriggered search for Optical Bursts
arXiv: Astrophysics, 2002Co-Authors: Robert Kehoe, C Akerlof, R Balsano, Jeffrey J Bloch, Don Casperson, S Fletcher, G Gisler, B C LeeAbstract:We present an untriggered search for Optical Bursts with the ROTSE-I telephoto array. Observations were taken which monitor an effective 256 square degree field continuously over 125 hours to m_{ROTSE}=15.7. The uniquely large field, moderate limiting magnitude and fast cadence of $\sim$10 minutes permits transient searches in a new region of sensitivity. Our search reveals no candidate events. To quantify this result, we simulate potential Optical Bursts with peak magnitude, m_{p}, at t=10 s, which fade as f=(\frac{t}{t_{0}}) ^{\alpha_{t}}, where \alpha_t < 0. Simple estimates based on observational evidence indicate that a search of this sensitivity begins to probe the possible region occupied by GRB orphan afterglows. Our observing protocol and image sensitivity result in a broad region of high detection efficiency for light curves to the bright and slowly varying side of a boundary running from [\alpha_{t},m_{p}]=[-2.0,6.0] to [-0.3,13.2]. Within this region, the integrated rate of brief Optical Bursts is less than 1.1\times 10^{-8} {\rm s}^{-1} {\rm deg}^{-2}. At $\sim$22 times the observed GRB rate from BATSE, this suggests a limit on \frac{\theta_{opt}}{\theta_{\gamma}}\lesssim 5 where \theta_{opt} and \theta_{\gamma} are the Optical and gamma-ray collimation angles, respectively. Several effects might explain the absence of Optical Bursts, and a search of the kind described here but more sensitive by about 4 magnitudes should offer a more definitive probe.
Thomas W. J. Thompson - One of the best experts on this subject based on the ideXlab platform.
-
deviations from the flux recurrence time relationship in gs 1826 238 potential transient spectral changes
The Astrophysical Journal, 2008Co-Authors: Thomas W. J. Thompson, Duncan K. Galloway, Richard E. Rothschild, Lee HomerAbstract:The low-mass X-ray binary GS 1826� 238 is presently unique for its consistently regular bursting behavior. In previousRossiX-RayTimingExplorer(RXTE)measurementsbetween1997Novemberand2002July,thissource exhibited (nearly) limit-cycle Bursts with recurrence times that decreased proportionately as the persistent flux increased.HerewereportadditionalmeasurementsoftheburstrecurrencetimebyRXTE,Chandra,andXMM-Newton, as well as observations of Optical Bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier fluxYrecurrence time relationship, and most of these Bursts occurred earlier than would bepredictedbasedontheX-rayfluxlevel.TheepochswithearlyBurstswerealsoaccompaniedbyunusualbroadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (� 30% of the total) to account for the burst recurrence time discrepancy. A selfconsistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that the accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level. Subject headingg X-rays: binaries — X-rays: Bursts — X-rays: individual (Ginga 1826� 238)
-
Deviations from the Flux-Recurrence Time Relationship in GS 1826-238: Potential Transient Spectral Changes
The Astrophysical Journal, 2008Co-Authors: Thomas W. J. Thompson, Duncan K. Galloway, Richard E. Rothschild, Lee HomerAbstract:The low-mass X-ray binary GS 1826-238 is presently unique for its consistently regular bursting behavior. In previous Rossi X-Ray Timing Explorer (RXTE) measurements between 1997 November and 2002 July, this source exhibited (nearly) limit-cycle Bursts with recurrence times that decreased proportionately as the persistent flux increased. Here we report additional measurements of the burst recurrence time by RXTE, Chandra, and XMM-Newton, as well as observations of Optical Bursts. On a few occasions we measured burst recurrence times which deviated significantly from the earlier flux-recurrence time relationship, and most of these Bursts occurred earlier than would be predicted based on the X-ray flux level. The epochs with early Bursts were also accompanied by unusual broadband timing signatures, with the entire power spectrum shifting to higher frequencies. Concurrent XMM-Newton observations during one of these occasions, in 2003 April, indicate that an additional soft component may be present in the spectrum containing enough flux (30% of the total) to account for the burst recurrence time discrepancy. A self-consistent interpretation for the increase in soft flux and accompanying timing changes during 2003 April is that accretion disk extends down to smaller radial distances from the source than during the other observing epochs. The RXTE observations since 2003 April show that the spectral and timing properties have nearly returned to the previously established level.
Luisa Rebull - One of the best experts on this subject based on the ideXlab platform.
-
csi 2264 simultaneous Optical and x ray variability in pre main sequence stars i time resolved x ray spectral analysis during Optical dips and accretion Bursts in stars with disks
Astronomy and Astrophysics, 2017Co-Authors: M G Guarcello, E Flaccomio, G Micela, C Argiroffi, S Sciortino, L Venuti, J R Stauffer, Luisa RebullAbstract:Context. Pre-main sequence stars are variable sources. The main mechanisms responsible for their variability are variable extinction, unsteady accretion, and rotational modulation of both hot and dark photospheric spots and X-ray-active regions. In stars with disks, this variability is related to the morphology of the inner circumstellar region (≤0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present-day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264, a set of simultaneous observations of NGC 2264 with 15 different telescopes. Aims. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during Optical Bursts and dips in order to unveil the nature of these phenomena. Stars without disks are studied in a companion paper. Methods. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent Optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of Optical extinction and X-ray absorption during the Optical dips; in stars with accretion Bursts, we search for soft X-ray emission and increasing X-ray absorption during the Bursts. Results. We find evidence for coherent Optical and X-ray flux variability among the stars with variable extinction. In 9 of the 24 stars with Optical dips, we observe a simultaneous increase of X-ray absorption and Optical extinction. In seven dips, it is possible to calculate the N_H/A_V ratio in order to infer the composition of the obscuring material. In 5 of the 20 stars with Optical accretion Bursts, we observe increasing soft X-ray emission during the Bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and Bursts, since favorable geometric configurations are required. Conclusions. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion Bursts, we observe, on average, a larger soft X-ray spectral component not observed in non-accreting stars.
-
csi 2264 simultaneous Optical and x ray variability in pre main sequence stars i time resolved x ray spectral analysis during Optical dips and accretion Bursts in stars with disks
arXiv: Solar and Stellar Astrophysics, 2017Co-Authors: M G Guarcello, E Flaccomio, G Micela, C Argiroffi, S Sciortino, L Venuti, J R Stauffer, Luisa RebullAbstract:Pre-main sequence stars are variable sources. In stars with disks, this variability is related to the morphology of the inner circumstellar region (<0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during Optical Bursts and dips in order to unveil the nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent Optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of Optical extinction and X-ray absorption during the Optical dips; in stars with accretion Bursts, we search for soft X-ray emission and increasing X-ray absorption during the Bursts. Results. We find evidence for coherent Optical and X-ray flux variability among the stars with variable extinction. In 9/24 stars with Optical dips, we observe a simultaneous increase of X-ray absorption and Optical extinction. In seven dips, it is possible to calculate the NH/AV ratio in order to infer the composition of the obscuring material. In 5/20 stars with Optical accretion Bursts, we observe increasing soft X-ray emission during the Bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and Bursts, since favorable geometric configurations are required. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion Bursts, we observe on average a larger soft X-ray spectral component not observed in non accreting stars.
L Venuti - One of the best experts on this subject based on the ideXlab platform.
-
csi 2264 simultaneous Optical and x ray variability in pre main sequence stars i time resolved x ray spectral analysis during Optical dips and accretion Bursts in stars with disks
Astronomy and Astrophysics, 2017Co-Authors: M G Guarcello, E Flaccomio, G Micela, C Argiroffi, S Sciortino, L Venuti, J R Stauffer, Luisa RebullAbstract:Context. Pre-main sequence stars are variable sources. The main mechanisms responsible for their variability are variable extinction, unsteady accretion, and rotational modulation of both hot and dark photospheric spots and X-ray-active regions. In stars with disks, this variability is related to the morphology of the inner circumstellar region (≤0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present-day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264, a set of simultaneous observations of NGC 2264 with 15 different telescopes. Aims. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during Optical Bursts and dips in order to unveil the nature of these phenomena. Stars without disks are studied in a companion paper. Methods. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent Optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of Optical extinction and X-ray absorption during the Optical dips; in stars with accretion Bursts, we search for soft X-ray emission and increasing X-ray absorption during the Bursts. Results. We find evidence for coherent Optical and X-ray flux variability among the stars with variable extinction. In 9 of the 24 stars with Optical dips, we observe a simultaneous increase of X-ray absorption and Optical extinction. In seven dips, it is possible to calculate the N_H/A_V ratio in order to infer the composition of the obscuring material. In 5 of the 20 stars with Optical accretion Bursts, we observe increasing soft X-ray emission during the Bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and Bursts, since favorable geometric configurations are required. Conclusions. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion Bursts, we observe, on average, a larger soft X-ray spectral component not observed in non-accreting stars.
-
csi 2264 simultaneous Optical and x ray variability in pre main sequence stars i time resolved x ray spectral analysis during Optical dips and accretion Bursts in stars with disks
arXiv: Solar and Stellar Astrophysics, 2017Co-Authors: M G Guarcello, E Flaccomio, G Micela, C Argiroffi, S Sciortino, L Venuti, J R Stauffer, Luisa RebullAbstract:Pre-main sequence stars are variable sources. In stars with disks, this variability is related to the morphology of the inner circumstellar region (<0.1 AU) and that of the photosphere and corona, all impossible to be spatially resolved with present day techniques. This has been the main motivation for the Coordinated Synoptic Investigation of NGC 2264. In this paper, we focus on the stars with disks. We analyze the X-ray spectral properties extracted during Optical Bursts and dips in order to unveil the nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I observations to search for coherent Optical and X-ray flux variability in stars with disks. Then, stars are analyzed in two different samples. In stars with variable extinction, we look for a simultaneous increase of Optical extinction and X-ray absorption during the Optical dips; in stars with accretion Bursts, we search for soft X-ray emission and increasing X-ray absorption during the Bursts. Results. We find evidence for coherent Optical and X-ray flux variability among the stars with variable extinction. In 9/24 stars with Optical dips, we observe a simultaneous increase of X-ray absorption and Optical extinction. In seven dips, it is possible to calculate the NH/AV ratio in order to infer the composition of the obscuring material. In 5/20 stars with Optical accretion Bursts, we observe increasing soft X-ray emission during the Bursts that we associate to the emission of accreting gas. It is not surprising that these properties are not observed in all the stars with dips and Bursts, since favorable geometric configurations are required. The observed variable absorption during the dips is mainly due to dust-free material in accretion streams. In stars with accretion Bursts, we observe on average a larger soft X-ray spectral component not observed in non accreting stars.