arXiv subjects

D. L. Kaplan

Publications and source records attributed to D. L. Kaplan.

171 records · Page 10Linked to original sources

arXiv2003

The Nearby Neutron Star RX J0720.4-3125 from Radio to X-rays

We present radio, optical, ultraviolet, and X-ray observations of the isolated, thermally-emitting neutron star RX J0720.4-3125 using the Parkes radio telescope, the Very Large Array, the Hubble Space Telescope, and the Chandra X-ray Observatory. From these data we show that the optical/UV spectrum of RX J0720.4-3125 is not well fit by a Rayleigh-Jeans tail as previously thought, but is instead best fit by either a single non-thermal power-law or a combination of a Rayleigh-Jeans tail and a non-thermal power-law. Taken together with the X-ray spectrum, we find the best model for RX J0720.4-3125 to be two blackbodies plus a power-law, with the cool blackbody implying a radius of 11-13 km at an assumed distance of 300 pc. This is similar to many middle aged (10^{5-6} yr) radio pulsars such as PSR B0656+14, evidence supporting the hypothesis that RX J0720.4-3125 is likely to be an off-beam radio pulsar. The radio data limit the flux at 1.4 GHz to be <0.24 mJy, or a luminosity limit of 4*pi*d^2*F < 3e25*d_300^2 ergs/s, and we see no sign of extended nebulosity, consistent with expectations for a pulsar like RX J0720.4-3125.

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arXiv2002

The Quiescent Counterpart of the Soft Gamma-ray Repeater SGR 0526-66

It is now commonly believed that Soft gamma-ray repeaters (SGRs) and Anomalous X-ray pulsars (AXPs) are magnetars -- neutron stars powered by their magnetic fields. However, what differentiates these two seemingly dissimilar objects is, at present, unknown. We present Chandra observations of RX J052600.3-660433, the quiescent X-ray counterpart of SGR 0526-66. Restricting to a period range around 8-s, the period noted in the afterglow of the burst of 5 March 1979, we find evidence for a similar periodicity in two epochs of data, obtained 20 months apart, implying a period derivative of $6.6(5)e-11$ s/s which is similar to the period derivatives of the magnetars. The spectrum is best fitted by a combination of a black body and a power law. However, the photon index of the power law component, $Γ\sim 3$ -- intermediate to those of AXPs and SGRs. This continuum of $Γ$ leads us to suggest that the underlying physical parameter which differentiates SGRs from AXPs is manifested in the power law component. Two decades ago, SGR 0526-66 was a classical SGR whereas now it behaves like an AXP. Thus it is possible that the same object cycles between SGR and AXP state. We speculate that the main difference between AXPs and SGRs is the geometry of the $B$-fields and this geometry is time dependent. [Abstract truncated]

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arXiv2002

X-ray Timing of the Enigmatic Neutron Star RX J0720.4-3125

RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8-s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, and BeppoSAX we are able to place an upper limit to the period derivative, $|\dot{P}| < 3.6\times10^{-13}{s s}^{-1}$ (3-$σ$). While our upper limit on $\dot P$ allows for the accretion model, this model is increasingly untenable for another similar but better studied neutron star, RX J1856.5-3754, and we therefore consider the accretion model unlikely for RX J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be $\lsim 10^{14}$ G based on cooling models, suggesting that it is not and never was a magnetar, but is instead middle-aged neutron star. We propose that it is either a long-period high-magnetic field pulsar with $\dot P\sim 10^{-13}{s s}^{-1}$ similar to PSR J1814-1744, or a neutron star born with an initial period of $\approx 8.3$ s and $\dot P\sim 10^{-15}{s s}^{-1}$. The proximity of RX J0720.4-3125 is strongly suggestive of a large population of such objects; if so, radio pulsar surveys must have missed many of these sources.

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arXiv2002

The Parallax and Proper Motion of RX J1856.5-3754 Revisited

RX J1856.5-3754, a bright soft X-ray source believed to be the nearest thermally emitting neutron star, has commanded and continues to command intense interest from X-ray missions. One of the main goals is to determine the radius of this neutron star. An integral part of the determination is an accurate parallax. Walter (2001) analyzed Hubble Space Telescope (HST) data and derived a parallax, pi=16.5+/-2.3 mas. Combining this distance with the angular radius derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT, EUVE, HST, Pons et al. (2001) derived an observed radius ("radiation radius"), R_{infty}= 7 km. This value is smaller than the radii calculated from all proposed equations-of-state (EOS) of dense baryonic matter (Haensel 2001). Here, we have analyzed the same HST data and find pi=7+/-2 mas. We have verified our result using a number of different, independent techniques, and find the result to be robust. Adopting our parallax, the radius of RX J1856.5-3754 is R_{infty}=15+/-6 km. This radius falls squarely in the range of radii, 12--16 km, expected from calculations of neutron star structure for different equations of state. With additional HST observations, the parallax estimate can be improved to the point by which the inferred radius can constrain the choice of EOS.

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arXiv2001

Deep Radio, Optical, and Infrared Observations of SGR 1900+14

We present HST/STIS, Keck J- and K_s-band, and VLA 332-MHz and 1400-MHz images of the region around the Soft Gamma-ray Repeater SGR 1900+14. No non-stellar sources were detected at the position of SGR 1900+14, giving 3-sigma limits of S_{332 MHz} < 6 mJy, S_{1.4 GHz} < 1.5 mJy, m_{50CCD} > 29.0 mag, J > 22.8 mag, K_s > 20.8 mag (point sources), and S_{332 MHz} < 6.1 mJy arcmin^{-2}, S_{1.4 GHz} < 6.2 mJy arcmin^{-2} (extended emission). Given the very high extinction, the STIS and J-band non-detections are not constraining, but the K_s-band limit rules out basic accretion disk models for SGR 1900+14, and is almost comparable in depth to the K_s-band detection for the anomalous X-ray pulsar 4U 0142+61. Finally, we report the detection in this field of three new candidate supernova remnants, SNRs G043.5+00.6, G042.0-00.1, and G041.5+00.4.

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arXiv2001

Precise Chandra Localization of the Soft Gamma-ray Repeater SGR 1806-20

We present observations of the Soft Gamma-ray Repeater SGR 1806-20 taken with the Chandra X-ray Observatory. We identify the X-ray counterpart of SGR 1806-20 based on detection of 7.5-s pulsations. Using three unrelated X-ray sources (and USNO stars) as position references, we are able to determine that the SGR is at alpha_{2000}=18h08m39.32s and delta_{2000}=-20o24'39.5", with rms uncertainty of 0.3" in each coordinate. We find that SGR 1806-20 is located within the 1-sigma error region determined by Interplanetary Network data and is 14+/-0.5 arcsec distant from the non-thermal core of SNR G10.0-0.3, excluding SGR 1806-20 as the origin of the core. We see evidence for a significant deviation of the spin-down of SGR 1806-20 from its long-term trend, demonstrating erratic spin-down behavior in this source similar to that seen in other SGRs. Finally, we show that there is a broad X-ray halo surrounding SGR 1806-20 out to radii ~1' due to scattering in the interstellar medium.

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arXiv2001

High-Resolution X-ray and Radio Observations of SGR 1900+14 in the Immediate Aftermath of a Giant Flare

We present the results of Chandra, RXTE, and VLA observations of SGR 1900+14 in the immediate aftermath of its 2001 April 18 giant flare event. In the X-ray band we find the source in a pulsating and bursting state, with time-averaged 2--10 keV flux initially elevated by 20% above the source's previous quiescent periods. In the radio we establish upper limits on the strength of any persistent post-flare emission of 0.7 and 0.1 mJy at 1.4 GHz and 8.0 GHz, respectively. The position of the X-ray source is consistent, to approximately 1 arcsec precision, with the August 1998 VLA determination, and the one-dimensional X-ray profile is consistent with that of a point source. The X-ray spectrum is best-fit by a two component power-law plus blackbody model, with fitted blackbody temperature kT_{BB} ~ 0.5 keV and radius R_{BB} ~ 1.5 km for an assumed distance of 5 kpc. The spectral parameters of this thermal component are consistent with those reported for the source in quiescence, and the variations in the source flux we observe may be explained as variations in the power-law component alone, providing support for magnetar models of SGR 1900+14.

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arXiv2001

Search for an Near-IR Counterpart to the Cas A X-ray Point Source

We report deep near-infrared and optical observations of the X-ray point source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have identified a J=21.4 +/- 0.3 mag and Ks=20.5 +/- 0.3 mag source within the 1-sigma error circle, but we believe this source is a foreground Pop II star with Teff=2600-2800 K at a distance of ~2 kpc, which could not be the X-ray point source. We do not detect any sources in this direction at the distance of Cas A, and therefore place 3-sigma limits of R >~ 25 mag, F675W >~ 27.3 mag, J >~ 22.5 mag and Ks >~ 21.2 mag (and roughly H >~ 20 mag) on emission from the X-ray point source, corresponding to M_{R} >~ 8.2 mag, M_{F675W} >~ 10.7 mag, M_{J} >~ 8.5 mag, M_{H} >~ 6.5 mag, and M_{Ks} >~ 8.0 mag, assuming a distance of 3.4 kpc and an extinction A_{V}=5 mag.

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arXiv2001

HST Observations of SGR 0526-66: New Constraints on Accretion and Magnetar Models

Soft Gamma-ray Repeaters (SGRs) are among the most enigmatic sources known today. Exhibiting huge X- and Gamma-ray bursts and flares, as well as soft quiescent X-ray emission, their energy source remains a mystery. Just as mysterious are the Anomalous X-ray pulsars (AXPs), which share many of the same characteristics. Thanks to recent Chandra observations, SGR 0526-66, the first SGR, now appears to be a transition object bridging the two classes, and therefore observations of it have implications for both SGRs and AXPs. The two most popular current models for their persistent emission are accretion of a fossil disk or decay of an enormous (~10^15 G) magnetic field in a magnetar. We show how deep optical observations of SGR 0526-66, the only SGR with small enough optical extinction for meaningful observations, show no evidence of an optical counterpart. These observation place strong new constraints on both accretion disk and magnetar models, and suggest that the spectral energy distribution may peak in the hard-UV. Almost all accretion disks are excluded by the optical data, and a magnetar would require a ~10^15-10^16 G field.

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