arXiv subjects

Philip Kaaret

Publications and source records attributed to Philip Kaaret.

At least 199 records · Page 11Linked to original sources

arXiv2003

Chandra ACIS and Newton EPIC Observations of the X-ray Luminous SN1978K in NGC 1313

We describe an observation of the X-ray luminous SN1978K in NGC 1313 using the ACIS detector on Chandra and an archival Newton EPIC observation. The models that provided good fits to the ASCA SIS and GIS and the ROSAT PSPC spectra no longer do so for the ACIS or EPIC spectra. The best-fit model to the ACIS and EPIC spectra are dual hot plasma model (vmekal); one component is soft (T = 0.61+0.04-0.05 keV, 90% errors) and the other is harder (T=3.16+0.44-0.40 keV). For the varying abundances permitted within the model, only the Si abundance of the soft component differs from solar, with a value n(Si)(SN78K)/n(Si)(solar) = 3.20+1.80-1.90 (90% errors). From a ratio of the low- and high-T model fits to the Chandra and XMM-Newton spectra, we infer an exponent n of the ejecta density distribution, rho_ejecta ~ r^(-n), of ~5.2, adopting a circumstellar matter distribution exponent of s = 2 (rho_cs ~ r^(-s)). The 0.5-2 keV light curve shows essentially no decline; the 2-10 keV light curve, constructed only of the ASCA, XMM-Newton, and Chandra observations, shows a drop of 1.5 from the ASCA epoch. The hard band decline, together with the apparently enhanced Si emission, signal the start of the X-ray decline of SN1978K.

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arXiv2003

Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities

Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near the end of its 2001-2002 outburst after its transition to the low-hard state at X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source distance of 4 kpc). Our results include a characterization of the spectral and timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra observation to measure the power spectrum at low frequencies. For the 3 epochs at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum is consistent with a spectral model consisting of a power-law with interstellar absorption. We detect evolution in the power-law photon index from 1.66 +/- 0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source softens at low luminosities. The power spectra are characterized by strong (20-35% fractional rms) band-limited noise, which we model as a zero-centered Lorentzian. Including results from an RXTE study of XTE J1650-500 near the transition to the low-hard state by Kalemci et al. (2003), the half-width of the zero-centered Lorentzian (roughly where the band-limited noise cuts off) drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the spectral and timing parameters evolve with luminosity, it is notable that the general shapes of the energy and power spectra remain the same, indicating that the source stays in the low-hard state. This implies that the X-ray emitting region of the system likely keeps the same overall structure, while the luminosity changes by a factor of 470. We discuss how these results may constrain theoretical black hole accretion models.

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arXiv2003

Chandra Detections of Two Quiescent Black Hole X-Ray Transients

Using the Chandra X-ray Observatory, we have detected the black hole transients V4641 Sgr and XTE J1859+226 in their low luminosity, quiescent states. The 0.3-8 keV luminosities are (4.0^(+3.3)_(-2.4))E31 (d/7 kpc)^2 erg/s and (4.2^(+4.8)_(-2.2))E31 (d/11 kpc)^2 erg/s for V4641 Sgr and XTE J1859+226, respectively. With the addition of these 2 systems, 14 out of the 15 transients with confirmed black holes (via compact object mass measurements) now have measured quiescent luminosities or sensitive upper limits. The only exception is GRS 1915+105, which has not been in quiescence since its discovery in 1992. The luminosities for V4641 Sgr and XTE J1859+226 are consistent with the median luminosity of 2E31 erg/s for the systems with previous detections. Our analysis suggests that the quiescent X-ray spectrum of V4641 Sgr is harder than for the other systems in this group, but, due to the low statistical quality of the spectrum, it is not clear if V4641 Sgr is intrinsically hard or if the column density is higher than the interstellar value. Focusing on V4641 Sgr, we compare our results to theoretical models for X-ray emission from black holes in quiescence. Also, we obtain precise X-ray positions for V4641 Sgr and XTE J1859+226 via cross-correlation of the X-ray sources detected near our targets with IR sources in the 2 Micron All-Sky Survey catalog.

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arXiv2003

X-Ray Flares and Oscillations from the Black Hole Candidate X-Ray Transient XTE J1650-500 at Low Luminosity

We report on X-ray observations made with the Rossi X-ray Timing Explorer of the black hole candidate (BHC) transient XTE J1650-500 at the end of its first, and currently only, outburst. By monitoring the source at low luminosities over several months, we found 6 bright ~100 second X-ray flares and long time scale oscillations of the X-ray flux. The oscillations are aperiodic with a characteristic time scale of 14.2 days and an order of magnitude variation in the 2.8-20 keV flux. The oscillations may be related to optical "mini-outbursts" that have been observed at the ends of outbursts for other short orbital period BHC transients. The X-ray flares have durations between 62 and 215 seconds and peak fluxes that are 5-24 times higher than the persistent flux. The flares have non-thermal energy spectra and occur when the persistent luminosity is near 3E34 (d/4 kpc)^2 erg/s (2.8-20 keV). The rise time for the brightest flare demonstrates that physical models for BHC systems must be able to account for the situation where the X-ray flux increases by a factor of up to 24 on a time scale of seconds. We discuss the flares in the context of observations and theory of Galactic BHCs and compare the flares to those detected from Sgr A*, the super-massive black hole at the Galactic center. We also compare the flares to X-ray bursts that are seen in neutron star systems. While some of the flare light curves are similar to those of neutron star bursts, the flares have non-thermal energy spectra in contrast to the blackbody spectra exhibited in bursts. This indicates that X-ray bursts should not be taken as evidence that a given system contains a neutron star unless the presence of a blackbody component in the burst spectrum can be demonstrated.

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arXiv2003

Radio Emission from an Ultraluminous X-Ray Source

The physical nature of ultraluminous x-ray sources is uncertain. Stellar mass black holes with beamed radiation and intermediate mass black holes with isotropic radiation are two plausible explanations. We discovered radio emission from an ultraluminous x-ray source in the dwarf irregular galaxy NGC 5408. The x-ray, radio and optical fluxes as well as the x-ray spectral shape are consistent with beamed relativistic jet emission from an accreting stellar black hole. If confirmed, this would suggest that the ultraluminous x-ray sources may be stellar-mass rather than intermediate mass black holes. However, interpretation of the source as a jet-producing intermediate-mass black hole cannot be ruled out at this time.

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arXiv2002

X-Ray Jet Emission from the Black Hole X-Ray Binary XTE J1550-564 with Chandra in 2000

We have discovered an X-ray jet due to material ejected from the black hole X-ray transient XTE J1550-564. The discovery was first reported by Corbel et al. (Science, 298, 196 and astro-ph/0210224), and here, we present an analysis of the three Chandra observations made between 2000 June and 2000 September. For these observations, a source is present that moves in an eastward direction away from the point source associated with the compact object. The separation between the new source and the compact object changes from 21.3 arcseconds in June to 23.4 arcseconds in September, implying a proper motion of 21.2 +/- 7.2 mas/day, a projected separation of 0.31-0.85 pc and an apparent jet velocity between 0.34 +/- 0.12 and 0.93 +/- 0.32 times the speed of light for a source distance range of d = 2.8-7.6 kpc. These observations represent the first time that an X-ray jet proper motion measurement has been obtained for any accretion powered Galactic or extra-galactic source. While this work deals with the jet to the east of the compact object, the western jet has also been detected in the X-ray and radio bands. The most likely scenario is that the eastern jet is the approaching jet and that the jet material was ejected from the black hole in 1998. Along with a 1998 VLBI proper motion measurement, the Chandra proper motion indicates that the eastern jet decelerated between 1998 and 2000. We present results on the morphology and energy spectrum of the jet. We cannot definitively determine the X-ray emission mechanism, but a synchrotron origin is viable and may provide the simplest explanation for the observations.

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arXiv2002

Rotational Broadening Measurement for the Neutron Star X-Ray Transient XTE J2123-058

We used optical spectroscopy of the neutron star X-ray transient XTE J2123-058 to measure the rotational broadening of the binary companion's stellar absorption lines and determined that the companion's projected rotational velocity is vsini = 121 (+21)(-29) km/s. This value is considerably larger than the measurements of vsini obtained previously for three other neutron star systems where the values are between 34 and 65 km/s. The larger value is likely due to the combination of high binary inclination and short (6 hr) orbital period for XTE J2123-058. Along with previously measured parameters, the rotational broadening measurement allowed us to determine the binary parameters, including the ratio of the neutron star mass to the companion mass, q = M_1/M_2 = 2.7 (+1.3)(-0.9), the neutron star mass, M_1 = 1.46 (+0.30)(-0.39) solar masses, and the companion mass, M_2 = 0.53 (+0.28)(-0.39) solar masses, assuming a Roche lobe filling companion synchronously rotating with the binary orbit. These values are consistent with a previous measurement where the H alpha emission line was used to determine the semiamplitude of the neutron star's radial velocity curve (K_1). It is a significant result that the two methods give consistent values. We also report the first measurement of the XTE J2123-058 companion's radius. Assuming synchronous rotation, we obtain R_2 = 0.62 (+0.11)(-0.15) solar radii, which is in-line with that expected for a late K-type star on or close to the main sequence. Finally, we report the first precise spectroscopic determination of the binary orbital period (P_orb = 21442.3 +/- 1.8 seconds).

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arXiv2002

A Chandra High-Resolution Camera Observation of X-Ray Point Sources in M31

We present results from a 47 ks observation of the Andromeda galaxy, M31, using the High-Resolution Camera of the Chandra X-Ray Observatory. We detect 142 point sources spanning three orders of magnitude in luminosity, from $L_X = 2\times 10^{35} \rm erg s^{-1}$ to $L_X = 2\times 10^{38} \rm erg s^{-1}$ in the 0.1-10 keV band. The X-ray source location accuracy is better than $1\arcsec$ in the central regions of the galaxy. One source lies within $1.3\arcsec$ of SN 1885 but does not coincide with the UV absorption feature identified as the supernova remnant. However, there is an optical transient, which is likely an optical nova, at the location of the X-ray source. There is a weak source, $L_X \sim 4 \times 10^{36} \rm erg cm^{2} s^{-1}$, coincident with the nucleus of M31, and 14 sources coincident with globular clusters. Our observation has very high efficiency down to luminosities of $1.5\times 10^{36} \rm erg s^{-1}$ for sources within $5\arcmin$ of the nucleus. Comparing with a ROSAT observation made 11 years earlier, we find that $0.46 \pm 0.26$ of the sources with $L_X > 5 \times 10^{36} \rm erg s^{-1}$ are variable. We find no evidence for X-ray pulsars in this region, indicating that the population is likely dominated by low-mass X-ray binaries. The source density radial profile follows a powerlaw distribution with an exponent of $1.25 \pm 0.10$ and is inconsistent with the optical surface brightness profile. The x-ray point source luminosity function is well fitted by a differential broken powerlaw with a break at a luminosity of $(4.5^{+1.1}_{-2.2}) \times 10^{37} \rm erg s^{-1}$. The luminosity function is consistent with a model of an aging population of X-ray binaries.

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arXiv2001

Keck Measurement of the XTE J2123-058 Radial Velocity Curve

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123-058. Its semi-amplitude (K_2) of 298.5 +/- 6.9 km/s is the highest value that has been measured for any neutron star LMXB. The high value for K_2 is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f_2) of 0.684 +/- 0.047 solar masses, along with our constraints on the companion's spectral type (K5V-K9V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 solar masses. We also derive a source distance of 8.5 +/- 2.5 kpc, indicating that XTE J2123-058 is unusually far, 5.0 +/- 1.5 kpc, from the Galactic plane. Our measurement of the systemic radial velocity is -94.5 +/- 5.5 km/s, which is significantly different from what would be observed if this object corotates with the disk of the Galaxy.

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arXiv2001

X-Ray Observations of XTE J1550-564 During the Decay of the 2000 Outburst - II. Timing

We investigate the timing behavior of XTE J1550-564 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the hard state during the decay of the 2000 outburst. We detect a high frequency QPO at 65 Hz in one observation with a fractional rms amplitude of 4.9%. This is the first time that a high frequency QPO has been detected in a transition to the hard state during outburst decay. We also observe low frequency QPOs in the 0.36-4.1 Hz range and rich aperiodic variability. The changes in the temporal properties during the decay are very similar to the state transitions observed for other sources. We find a correlation between the QPO frequency and the break frequency in the continuum power spectra. We investigate the energy dependence of rms amplitudes of the QPOs. We compare these timing properties with those previously observed for XTE J1550-564 and other Galactic black hole candidates and discuss the implications for QPO models and black hole accretion.

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arXiv2001

X-Ray Observations of XTE J1550-564 During the Decay of the 2000 Outburst - I. Chandra and RXTE Energy Spectra

We report on the evolution of the X-ray energy spectrum for the black hole candidate (BHC) X-ray transient XTE J1550-564 during the decay of the 2000 outburst. The Rossi X-ray Timing Explorer (RXTE) and Chandra observations span nearly five orders of magnitude in luminosity. The RXTE spectra are dominated by a power-law component and a comparatively weak soft component was also detected for the first two observations. The source made a transition to the canonical hard state near a luminosity of 9E36 erg/s over several observations as evidenced by a drop in the flux of the soft component in the RXTE energy band and a hardening of the power-law component to a photon index near 1.6. The power-law did not exhibit this behavior for the previous XTE J1550-564 outburst. For some observations, we detect a high energy cutoff and find that the cutoff is more significant and at lower energy during the transition than in the hard state. The cutoff in the hard state is at higher energy than has been seen for most previous accreting BHCs. The Chandra spectrum provides evidence for spectral evolution after the hard state transition. It is well, but not uniquely, described by a power-law with a photon index of 2.30^{+0.41}_{-0.48} (90% confidence) and interstellar absorption. Advection-dominated accretion flow models predict gradual spectral softening as the luminosity drops, but our observations do not allow us to determine if the spectral evolution is gradual or sudden. The lowest luminosity we measure for XTE J1550-564 with Chandra is 5E32 erg/s (0.5-7 keV, for a distance of 4 kpc). Although this is probably not the true quiescent luminosity, it represents a useful upper limit on this quantity.

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arXiv2001

Chandra X-Ray Point Sources, including Supernova 1979C, in the Spiral Galaxy M100

Six x-ray point sources, with luminosities of $4 \times 10^{38} - 2 \times 10^{39} \rm ergs s^{-1}$ in the 0.4--7 keV band, were detected in Chandra observations of the spiral galaxy M100. One source is identified with supernova SN 1979C and appears to have roughly constant x-ray flux for the period 16--20 years after the outburst. The x-ray spectrum is soft, as would be expected if the x-ray emission is due to the interaction of supernova ejecta with circumstellar matter. Most of the other sources are variable either within the Chandra observation or when compared to archival data. None are coincident with the peak of the radio emission at the nucleus. These sources have harder spectra than the supernova and are likely x-ray binaries. M100 has more bright x-ray sources than typical for spiral galaxies of its size. This is likely related to active star formation occurring in the galaxy.

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arXiv2000

The X-ray Properties of Low-Frequency Quasi-Periodic Oscillations from GRS 1915+105 up to 120 keV

We present a study of the properties of strong 0.8-3.0 Hz quasi-periodic oscillations (QPOs) that occurred during 1997 RXTE observations of the microquasar GRS 1915+105 in the low-hard state. The high count rates allow us to track individual QPO peaks, and we exploit this to develop a QPO folding technique. In contrast to previous QPO studies with RXTE, we emphasize the high energy QPO properties and report the detection of a QPO in the 60-124 keV energy band. Our technique allows us, for the first time, to measure the phase of the QPO harmonics relative to the fundamental. Variation in this phase difference leads to changes in the shape of the QPO profile with energy and over time. The strength of the QPO fundamental increases up to 19 keV, but the data do not suggest that the strength continues to increase above this energy. In some cases, the QPO amplitudes in the 30-60 keV and 60-124 keV energy bands are significantly less than in the 13-19 keV and 19-29 keV energy bands. We also use our technique to measure the phase lag of the QPO fundamental and harmonics. In the case where negative phase lags are detected for the fundamental, positive phase lags are detected for the first harmonic.

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arXiv2000

Spectral Signatures of KiloHertz Quasi-Periodic Oscillations from Accreting Neutron Stars

Correlations discovered between millisecond timing properties and spectral properties in neutron star x-ray binaries are described and then interpreted in relation to accretion flows in the systems. Use of joint timing and spectral observations to test for the existence of the marginally stable orbit, a key prediction of strong field general relativity, is described and observations of the neutron star x-ray binary 4U1820-303 which suggest that the signature of the marginally stable orbit has been detected are presented.

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arXiv2000

X-Ray Spectral and Timing Evolution During the Decay of the 1998 Outburst from the Recurrent X-Ray Transient 4U 1630-47

We report on the X-ray spectral and timing behavior of the recurrent X-ray transient 4U 1630-47 for 51 RXTE observations made during the decay of its 1998 outburst. The observations began when the source was still relatively bright, and, during one of the early observations, a QPO with a non-Lorentzian profile occurred near 6 Hz. As the source decayed, the X-ray flux dropped exponentially with an e-folding time of 14.4 d. The exponential decay was interrupted by an increase in the X-ray flux, and a secondary maximum occurred 89 d after the onset of the outburst. A transition marked by significant changes in the timing and spectral properties of the source occurred 104 d after the start of the outburst. The transition is similar to soft-to-hard state transitions observed in other black hole candidate X-ray binaries. Most of the changes associated with the transition occurred in less than 2 d. The timing changes include an increase in the continuum noise level from less than 4% RMS to greater than 10% RMS and the appearance of a quasi-periodic oscillation (QPO) at 3.4 Hz with an RMS amplitude of 7.3% in the 2-21 keV energy band. At the transition, the energy spectrum also changed with an abrupt drop in the soft component flux in the RXTE band pass. A change in the power-law photon index from 2.3 to 1.8, also associated with the transition, occurred over a time period of 8 d. After the transition, the source flux continued to decrease, and the QPO frequency decayed gradually from 3.4 Hz to about 0.2 Hz.

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arXiv2000

Relations Between Timing Features and Colors in the X-Ray Binary 4U 0614+09

We study the correlations between timing and X-ray spectral properties in the low mass X-ray binary 4U 0614+09 using a large (265-ks) data set obtained with the Rossi X-ray Timing Explorer. We find strong quasi-periodic oscillations (QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418 Hz and, perhaps, as low as 153 Hz. We report the highest frequency QPO yet from any low mass X-ray binary at 1329+-4 Hz, which has implications for neutron star structure. This QPO has a 3.5-sigma single-trial significance, for an estimated 40 trials the significance is 2.4-sigma. Besides the kilohertz QPOs, the Fourier power spectra show four additional components: high frequency noise (HFN), described by a broken power-law with a break frequency between 0.7 and 45 Hz, very low frequency noise (VLFN), which is fitted as a power-law below 1 Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz and 97 to 158 Hz, respectively. We find strong correlations between the frequencies of the kilohertz QPOs, the frequency of the 6 to 38 Hz broad Lorentzian, the break frequency of the HFN, the strength of both the HFN and the VLFN and the position of the source in the hard X-ray color vs. intensity diagram. The frequency of the 97 to 158 Hz Lorentzian does not correlate with these parameters. We also find that the relation between power density and break frequency of the HFN is similar to that established for black hole candidates in the low state. We suggest that the changing mass accretion rate is responsible for the correlated changes in all these parameters.

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arXiv1999

Discovery of a Hard X-Ray Source, SAX J0635+0533, in the Error Box of the Gamma-Ray Source 2EG 0635+0521

We have discovered an x-ray source, SAX J0635+0533, with a hard spectrum within the error box of the GeV gamma-ray source in Monoceros, 2EG J0635+0521. The unabsorbed x-ray flux is 1.2*10^-11 erg cm^-2 s^-1 in the 2-10 keV band. The x-ray spectrum is consistent with a simple powerlaw model with absorption. The photon index is 1.50 +/- 0.08 and we detect emission out to 40 keV. Optical observations identify a counterpart with a V-magnitude of 12.8. The counterpart has broad emission lines and the colors of an early B type star. If the identification of the x-ray/optical source with the gamma-ray source is correct, then the source would be a gamma-ray emitting x-ray binary.

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arXiv1999

Pulsar Radiation and Quantum Gravity

Quantum gravity may lead to an energy dependence in the speed of light. The high energy radiation from gamma-ray pulsars can be used to place limits on such effects. We find that emission from the Crab pulsar at energies above 2 GeV trails that at 70-100 MeV by no more than 0.35 ms (95% confidence) and place a lower bound on the energy scale of quantum gravitational effects on the speed of light of 1.8 x 10^15 GeV. This bound might be improved by two orders of magnitude by observation of pulsations from the Crab at higher energies, 50-100 GeV, in the near future.

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