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

Abstract

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, $|\dot{P}|<3.6\times 10^{-13}{ss}^{-1}$ (3-$\sigma$). 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 $\dot{P}\sim 10^{-13}{ss}^{-1}$ similar to PSR J1814-1744, or a neutron star born with an initial period of $\approx 8.3$ s and $\dot{P}\sim 10^{-15}{ss}^{-1}$. 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.

5 pages, 4 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted by ApJ Letters. Minor changes to Table 1 since version 1

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Cited by in corpus (28)

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• The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125
• Isolated neutron stars: Magnetic fields, distances, and spectra
• A broad absorption feature in the X-ray spectrum of the isolated neutron star RBS1223 (1RXS J130848.6+212708)
• A Strong, Broad Absorption Feature in the X-ray Spectrum of the Nearby Neutron Star RX J1605.3+3249
• Enhanced cooling of neutron stars via Cooper-pairing neutrino emission
• Constraining the Spin-down of the Nearby Isolated Neutron Star RX J2143.0+0654
• An X-ray Search for Compact Central Sources in Supernova Remnants I: SNRs G093.3+6.9, G315.4-2.3, G084.2+0.8, & G127.1+0.5
• Constraining the Spin-down of the Nearby Isolated Neutron Star RX J0806.4-4123, and Implications for the Population of Nearby Neutron Stars
• The Nearby Neutron Star RX J0720.4-3125 from Radio to X-rays
• A Coherent Timing Solution for the Nearby Isolated Neutron Star RX J0720.4-3125
• Detection of pulsations and a spectral feature in the X-ray emission of the isolated neutron star 1RXS J214303.7+065419/RBS 1774
• A Coherent Timing Solution for the Nearby Isolated Neutron Star RX J1308.6+2127/RBS 1223
• Long-term variability in the X-ray emission of RX J0720.4-3125
• AXPs and X-ray dim neutron stars: Recent XMM-Newton and Chandra results
• The Distance to the Isolated Neutron Star RX J0720.4-3125
• Spectral Features in the Thermal Emission from Isolated Neutron Stars: Dependence on Magnetic Field Strengths
• Constraining Superfluidity in Dense Matter from the Cooling of Isolated Neutron Stars
• A Probable Optical Counterpart for the Isolated Neutron Star RX J1308.6+2127
• Timing Analysis of the Isolated Neutron Star RX J0720.4-3125 Revisited
• Spectral and temporal variations of the isolated neutron star RX J0720.4-3125: new XMM-Newton observations
• Radio Observations of Two Isolated Neutron Stars, RXJ0720.4-3125 and RX J0806.4-4132
• Symmetry of the Neutron and Proton Superfluidity Effects in Cooling Neutron Stars
• Updated phase coherent timing solution of the isolated neutron star RX J0720.4-3125 using recent XMM-Newton and Chandra observations
• Statistical ages and the cooling rate of X-ray dim isolated neutron stars