Engineering topics

Wilson, Colleen A.

Publications and source records attributed to Wilson, Colleen A..

At least 37 records · Page 2

NASA NTRS2001

Discovery of Weak EXO 2030+375 Outbursts with BATSE

EXO 2030+375 is a 42-second X-ray pulsar orbiting a Be star every 46 days. Previous work using epoch-folded frequency searches over one-day intervals of BATSE data indicated that EXO 2030+375 underwent a 2.5 year period of quiescence from 1993 Sept-1996 March. Improvements in the search method that reduced systematic errors due to aperiodic noise from the nearby black hole candidate Cyg X-1 and sources undergoing Earth occultation, have allowed longer time intervals to be searched, hence increasing BATSE's sensitivity. Using the improved method with four-day intervals, we detect EXO 2030+375 near most of its periastron passages during 1993 Sept-1996 March for most periastron passages during BATSE's nine years in orbit. Earth occultation measurements in the 20-100 keV band, selected with Cyg X-1 was below the Earth's horizon and epoch-folded at EXO 2030+375's orbital period of 46 days also indicate that EXO 2030+375 was active for most of the mission. We will present histories of EXO 2030+375's pulse frequency, pulse flux, and total flux. In addition, we will present results of pulse timing analyses and evidence that EXO 2030+375's outbursts shift in orbital phase.

Wilson, Colleen A.↗

NASA NTRS2001

Discovery of Weak EXO 2030+375 Outbursts With BATSE

EXO 2030+375 is a 42-second X-ray pulsar orbiting a Be star every 46 days. Previous work using epoch-folded frequency searches over 1-day intervals of BATSE data [1] indicated that EXO 2030+375 underwent a 2.5 year period of quiescence from 1993 September-1996 March. Improvements in the search method that reduced systematic errors due to (1) aperiodic noise from the nearby black hole candidate Cyg X-1 and (2) sources undergoing Earth occultation, have allowed longer time intervals to be searched, hence increasing BATSE's sensitivity. Using the improved method with 4-day intervals, we detect EXO 2030+375 near most of its periastron passages during 1993 September-1996 March and for most periastron passages during BATSE's 9 years in orbit. Earth occultation measurements in the 20-100 keV band, selected when Cyg X-1 was below the Earth's horizon and epoch-folded at EXO 2030+375's orbital period of 46 days, also indicate that EXO 2030+375 was active for most of the mission. We will present histories of EXO 2030+375's pulse frequency and pulsed flux. In addition, we will present results of a pulse timing analysis and evidence that EXO 2030+375's outbursts shift in orbital phase.

Wilson, Colleen A.↗

NASA NTRS2001

A Decade in the Life of EXO 2030+375: A Multiwavelength Study of an Accreting X-ray Pulsar

Using BATSE and RXTE observations from 1991 April to 2001 August we have detected 71 outbursts from 82 periastron passages of EXO 2030+375, a 42-second transient X-ray pulsar with a Be star companion, including several outbursts from 1993 August to 1996 April when the source was previously believed to be quiescent. Combining BATSE, RXTE, and EXOSAT data we have derived an improved orbital solution. Applying this solution results in a smooth profile for the spin-up rate during the giant outbursts and results in evidence for a correlation between the spin-up rate and observed flux in the brighter BATSE outbursts. Infrared and H(alpha) measurements show a decline in the density of the circumstellar disk around the Be star. This decline is followed by a sudden drop in the X-ray flux and a turn-over from a spin-up trend to spin-down in the frequency history. This is the first Be/X-ray binary which shows an extended interval, about 2.5 years, where the global trend is spin-down, but the outbursts continue. In 1995 the orbital phase of EXO 2030+375's outbursts shifted from peaking about 6 days after periastron to peaking before periastron. The outburst phase slowly recovered to peaking at about 2.5 days after periastron. We interpret this shift in orbital phase followed by a slow recovery as evidence for a global one-armed oscillation propagating in the Be disk. This is further supported by changes in the shape of the H(alpha) profile which are commonly believed to be produced by a reconfiguration of the Be disk. The truncated viscous decretion disk model provides an explanation for the long series of normal outbursts and the evidence for an accretion disk in the brighter normal outbursts. Long-term multi-wavelength observations such as these clearly add considerably to our knowledge of Be/X-ray binaries and the relationship between optical, infrared and X-ray observations.

Wilson, Colleen A.↗

NASA NTRS2001

Serendipitous Detections of XTE J1906+09 with the Rossi X-ray Timing Explorer

In 1996 during Rossi X-ray Timing Explorer (RXTE) observations of SCR 1900+14, the 89-second X-ray pulsar XTE J1906+09 was discovered. As a result of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A frequency grid search of these observations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. The non-detection of XTE J1906+09 in most observations indicates that it is a transient X-ray pulsar and therefore likely has a Be star companion, Pulse timing analysis of the second outburst revealed a sinusoidal signature in the pulse frequencies that is likely produced by periastron passage in a long period, eccentric, orbit. During the second outburst, the pulse shape changed with increasing intensity, Low intensity pulse profiles from the second outburst are consistent with profiles from the fainter first outburst. Energy dependent pulse shape variations were seen in both outbursts. Pulse phase spectroscopy was used to quantify these variations. The phase averaged spectrum, using the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the galactic ridge, was well fitted by an absorbed power law with a high energy cutoff with column density N(sub H) = (6 +/- 1) x 10(exp 22) cm(sup -2), photon index = 1.01 +/- 0.08, cutoff energy E(sub cut) = 11 +/- 1 keV, and folding energy E(sub fold) = 19 +/- 4 keV.

Wilson, Colleen A.↗

NASA NTRS2000

XTE J1946+274 = GRO J1944+26 Observations with RXTE and BATSE

XTE J1946+274 = GRO J1944+26 is a 15.8 second transient X-ray pulsar discovered simultaneously with the Rossi X-ray Timing Explorer (RXTE) and Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO) during an outburst in September 1998. Since its discovery, XTE J1946+274 has undergone 7 regularly spaced outbursts, that were observed with BATSE and the RXTE All-Sky Monitor (ASM). The pulse frequency and pulsed flux measurements with BATSE suggest that XTE J1946+274 is in an about 170 day orbit and is outbursting twice per orbit. The first outburst, which was brighter and longer than subsequent outbursts, was also observed with the RXTE Proportional Counter Array (PCA). We present histories of pulse frequency, pulsed flux, and total flux measured in the 20-50 keV band with BATSE and a history of the 2-10 keV total flux measured with the RXTE ASM. From the first outburst, we present energy and power spectra and pulse profiles from RXTE PCA observations.

Wilson, Colleen A.↗

NASA NTRS2000

XTE J1906+09 Observations with RXTE

The 89 s pulsar XTE J1906+09 was serendipitously discovered in a 1996 observation of the soft gamma repeater SGR 1900+14. Two outbursts, in 1996 September and 1998 September have been reported to date. Since this system appears to be transient, it is likely in a Be/X-ray binary, accreting material from a circumstellar disk around its companion. Preliminary pulse timing analyses of RXTE data from the 1998 September outburst revealed an orbital signature consistent with either (i) a low-eccentricity, short period orbit, similar to OAO 1657-415 or (ii) periastron passage in a longer orbital period, higher eccentricity Be/X-ray binary system. No companion has been identified to date due to the large 2 feet error radius of the current best position. SGR 1900+14 has been regularly monitored with RXTE since 1998. Because XTE J1906+09 is only 33 feet away, it often shares the RXTE/PCA field of view with SGR 1900+14, giving us a large number of archival PCA observations. We will present results of a search of RXTE data for outbursts of XTE 1906+09, including histories of pulse frequency, 2-30 keV intensity, pulse profiles, and energy spectra. Results of a pulse timing analysis of the detected outbursts will also be presented.

Wilson, Colleen A.↗

NASA NTRS1999

The Outbursts and Orbit of the Accreting Pulsar GS 1843 - 02 = 2S 1845 - 024

We present observations of a series of 10 outbursts of pulsed hard X-ray flux from the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory. These outbursts occurred regularly every 242 days, coincident with the ephemeris of the periodic transient GRO J1849-03, which has recently been identified with the SAS 3 source 2S 1845-024. Our pulsed detection provides the first clear identification of GS 1843-02 with 2S 1845-024. We present a pulse timing analysis that shows that the 2S 1845-024 outbursts occur near the periastron passage of the neutron star's highly eccentric (e = 0.88 +/- 0.01) 242.18 +/- 0.01 day period binary orbit about a high-mass (M(sub c) greater than 7 solar mass) companion. The orbit and transient outburst pattern strongly suggest that the pulsar is in a binary system with a Be star. Our observations show a long-term spin-up trend, with most of the spin-up occurring during the outbursts. From the measured spin-up rates and inferred luminosities we conclude that an accretion disk is present during the outbursts.

Finger, Mark H.↗

NASA NTRS1999

Rossi X-Ray Timing Explorer Observations of the Anomalous Pulsar 4U 0142+61

We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional Counter Array on board the Rossi X-Ray Timing Explorer in 1996 March. The pulse frequency was measured as v = 0.11510039(3) Hz, with an upper limit of absolute value of the first derivative of v <= 4 x 10(exp -13) Hz/s on the short-term change in frequency over the 4.6 day span of the observations. A compilation of all historical measurements showed an overall spin-down trend with slope first derivative of v = -3.0 +/- 0.1 x 10(exp -14) Hz/s. Searches for orbital modulations in pulse arrival times yielded an upper limit of a(sub x) sin i approximately < 0.26 lt-s (99% confidence) for the period range 70 s to 2.5 days. These limits combined with previous optical limits and evolutionary arguments suggest that 4U 0142+61 is probably not a member of a binary system.

Wilson, Colleen A.↗

NASA NTRS1999

Recent Outbursts from the Transient X-Ray Pulsar Cepheus X-4 (GS 2138 + 56)

We report on X-ray observations of the 66 s period transient X-ray pulsar Cep X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory and with the Rossi X-Ray Timing Explorer (RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July and in 1997 July. Pulse frequencies of v = 15.0941 + or - 0.0002 mHz on 1993 June 25 (MJD 49,163) and v = 15.0882 + or - 0.0002 mHz on 1997 July 12 (MJD 50,641) were each measured from 2-day spans of BATSE data near each outburst's peak. Cep X-4 showed an average spin down rate of dot-v = (-4.14 + or - 0.08) x 10(exp -14) Hz s(exp -1) between the 1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public observations were performed on 1997 July 18 and July 25 with the Proportional Counter Array (PCA) on RXTE. A pulse frequency of v = 15.088 + or - 0.004 mHz was measured from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise, with an rms variance of about 18% in the frequency range 0.01 - 1.0 Hz, was observed on both days. Energy- and intensity-dependent pulse shape variations were also seen in these data. Recently published optical observations associate Cep X-4 with a Be companion star. If all four outbursts observed from Cep X-4 are assumed to occur at the same orbital phase, we find that the orbital period is between 23 and 147.3 days.

Wilson, Colleen A.↗

NASA NTRS1999

GRO J2058+42 Observations with BATSE and RXTE

GRO J2058+422 is a 196 second pulsar discovered with BATSE during a giant outburst in 1995. It underwent a series of 9 weaker outbursts from 1995 to 1997 which alternated in intensity, with a 110 day cycle in the 20-50 keV band. These outbursts did not show the same, intensity variations in the 2-10 keV observations with the RXTE ASM. Additional outbursts after this series were observed with BATSE, using a more sensitive search method which accounts for excess aperiodic noise from Cygnus X-1, and with the RXTE PCA and ASM. A set of two outbursts, one "periastron" and one "apastron" outburst (assuming a 110 day orbital period) were observed with the RXTE PCA. Pulse shape differences were found between the two outbursts. Histories of pulse frequency, pulsed flux. and total flux are presented. Pulse profiles and spectra from PCA observations are also presented.

Wilson, Colleen A.↗

NASA NTRS1998

Discovery of the 198 Second X-Ray Pulsar GRO J2058+42

GRO J2058+42, a transient 198 s X-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mcrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 to 196 s during the 46 day outburst. The pulse shape evolved over the course of the outburst and exhibited energy-dependent variations. BATSE observed five additional weak outbursts from GRO J2058 + 42, each with a 2 week duration and a peak-pulsed flux of about 15 mcrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-Ray Timing Explorer (RXTE) proportional counter array (PCA) localized the source to within a 4 s radius error circle (90% confidence) centered on R.A. = 20h 59m.0, decl. = 41 deg 43 s (J2000). Additional shorter outbursts with peak-pulsed fluxes of about 8 mcrab were detected by BATSE halfway between the first four 15 mcrab outbursts. The RXTE All-Sky Monitor detected all eight weak outbursts with approximately equal durations and intensities. GRO J2058 + 42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron, No optical counterpart has been identified to date, and no X-ray source was present in the error circle in archival ROSAT observations.

Wilson, Colleen A.↗

NASA NTRS1998

Recent Outbursts from the Transient X-Ray Pulsar Cep X-4 (GS 2138+56)

We report on X-ray observations of the 66 s period transient X-ray pulsar Cep X-4 (GS 2138+56) with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO) and with the Rossi X-ray Timing Explorer (RXTE). Two outbursts from Cep X-4 were observed with BATSE in 1993 June-July and 1997 July. Spin periods of P(sub spin) = 66.2510 +/- 0.0009 s on 1993 June 25 (MJD 49,163) and P(sub spin) = 66.27 68 +/- 0.0007 s on 1997- July 12 (MJD 50,641) were each measured from 2 day spans of BATSE data near each outburst's peak. Cep X-4 showed an average spin down rate of P/P = (8.7 +/- 0.2) x 10(exp -5)/ yr between the 1993 and 1997 outbursts. After BATSE could no longer detect Cep X-4, public observations were performed on 1997 July 18 & 25 with the Proportional Counter Array (PCA) on RXTE. A spin period Of P(sub spin) = 66.28 +/- 0.02 s was measured from observations on 1997 July 18 (MJD 50,647). Significant aperiodic noise, with an rms variance of approx. 18% in the frequency range 0.01 - 1.0 Hz was observed on both days. Energy and intensity dependent pulse shape variations were also seen in these data. Recently published optical observations associate Cep X-4 with a Be companion star.

Wilson, Colleen A.↗

NASA NTRS1998

The Outbursts and Orbit of the Accreting Pulsar GS 1843-02=2S 1845-024

We present observations of a series of 10 transient outbursts of pulsed hard x-ray flux from the 10.6 mHz accreting pulsar GS 1843-02 using the Burst and Transient Source Experiment (BATSE) on the Compton Observatory. These outbursts occurred regularly every 242 days, coincident with the ephemeris of the periodic transient GRO J1849-03 (Zhang et al. 1996)), which has recently been identified with the the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Our pulsed detection provides the first clear identification of GS 1843-02 with 2S 1845-024. We present a pulse timing analysis which shows that the 2S 1845-024 outbursts occur near the periastron passage of a highly eccentric (e = 0.88 q 0.01) 242.18 +/- 0.01 day period binary orbit about a high mass (M(sub opt) > 7 solar mass companion. The orbit and transient outburst pattern suggest a Be star/X-ray pulsar system, however the optical companion is unknown. Our observations show a long-term spin-up trend, with most of the spin-up occurring during the outbursts. From the measured spin-up rates and inferred mass accretion rates we conclude that an accretion disk is present during the outbursts.

Finger, Mark H.↗

NASA NTRS1998

RXTE Observations of the Anomalous Pulsar 4U 0142+61

We observed the anomalous X-ray pulsar 4U 0142+6l using the Proportional Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE) in March 1996. The pulse frequency was measured as epsilon = 0.11510039(3) Hz with an upper limit of |epsilon| </= 4 x 10(exp -13) Hz s(exp -1) upon the short term change in frequency over the 4.6 day span of the observations. A compilation of all historical measurements showed an overall spin-down trend with slope epsilon = -3.0 +/- 0.1 x 10(exp -14) Hz s(exp -1). Searches for orbital modulations in pulse arrival times yielded an upper limit of a(sub x) sin i approximately less than 0.26 It-s (99% confidence) for the period range 70 s to 2.5 days. These limits combined with previous optical limits and evolutionary arguments suggest that 4U 0142+61 is not a member of a binary system.

Wilson, Colleen A.↗

NASA NTRS1997

Observations of Accreting Pulsars

We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A0535+26, GRO J2058+42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital param- eters were also known. We have also continuously measured the pulsed flux and spin frequency for eiaht persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the con- ventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti- correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.

Bildsten, Lars↗

NASA NTRS1997

A Sequence of Outbursts from the Transient X-Ray Pulsar GS 0834-430

GS 0834-430, a 12.3 s accretion-powered pulsar, has been observed in seven outbursts with the BATSE large-area detectors on the Compton Gamma Ray Observatory. The first five outbursts observed by BATSE occurred at intervals of about 107 days, while the final two outbursts were separated by about 140 days. The photon energy spectrum, measured by Earth occultation in the 20 100 keV band, can be fitted by a power law with photon index alpha approximately equals -3.7 or by an exponential spectrum with temperature kT approximately equals 15 keV, with some variations within outbursts. The source has a low pulse fraction, less than or equal to 0.15 in the 20-50 keV band. We have observed significant temporal and energy-dependent variations in epoch folded pulse profiles. Because the intrinsic torque effects for this system are at least comparable to orbital effects, pulse timing analysis did not produce a unique orbital solution. However, confidence regions for the orbital elements yielded the following 1 sigma limits: orbital period P(sub orb) = 105.8 +/- 0.4 days and eccentricity 0.10 less than or approximately equals epsilon less than or approximately equals 0.17. GS 0834-430 is most likely a Be/X-ray binary.

Wilson, Colleen A.↗

NASA NTRS1997

Observations of Accreting Pulsars

We summarize five years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered 5 new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A 0535+26, GRO J2058+42, 4U 1145-619 and A 1118-616), and also measured the accretion torque history of during outbursts of 6 of those transients whose orbital parameters were also known. We have also continuously measured the pulsed flux and spin frequency for eight persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long time scales, blurring the conventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars, but uncorrelated, or even anticorrelated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA-Goddard Space Flight Center.

Bildsten, Lars↗

NASA NTRS1997

Outbursts from the Transient X-Ray Pulsar Cep X-4 (GS 2138+56)

Cep X-4 was discovered with the Orbiting Solar Observatory (OSO) 7 Satellite in 1973 June-July, but no pulsations were detected. In March 1988, an additional outburst was observed with Gingaq. Pulsations at a period of 66.2490 +/- .0001 s were detected during a month long outburst which peaked at about 100 mCrab (1-20 keV) in early April 1988. The source apparently did not appear again until June 1993 when it was detected by Roentgen Satellite (ROSAT) and Burst and Transient Source Experiment (BATSE). Pulsations at a period of 66.2499 +/- .0007 s were detected by BATSE. The outburst lasted about two weeks and had a peak pulsed flux of 15-20 mCrab (20-50 keV). In July 1997, BATSE and the All-Sky Monitor (ASM) on Rossi X-ray Timing Explorer (RXTE) observed a new outburst from Cep X-4. Pulsations at a period of 66.2743 +/- 0.0005 s were detected by BATSE. This outburst lasted about 2 weeks and peaked at a pulsed flux of about 10-15 mCrab (20-50 keV). Results of a search of BATSE data for additional outbursts will be presented. Pulse frequency and flux histories will be presented and compared to the flux history from the RXTE ASM. Implications of the apparent spin-down between outbursts will be discussed.

Wilson, Colleen A.↗