High-energy characteristics of the schizophrenic pulsar PSR J1846-0258 in Kes 75

Multi-year RXTE and INTEGRAL observations crossing the magnetar-like outburst

¹ SRON-Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands e-mail: L.M.Kuiper@sron.nl
² Astronomical Institute “Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: W.Hermsen@sron.nl

Received: 23 December 2008
Accepted: 22 April 2009

Abstract

Aims. PSR J1846-0258 is a young rotation-powered pulsar with one of the highest surface magnetic field strengths, located in the centre of SN-remnant Kes-75. In June 2006 a magnetar-like outburst took place. Using multi-year RXTE and INTEGRAL observations covering the epoch of the outburst, we aim to study the temporal and spectral characteristics of PSR J1846-0258 over a broad ~3–300 keV energy range to derive constraints on theoretical scenarios aiming to explain this schizophrenic behaviour.

Methods. We explored all publically available RXTE observations of PSR J1846-0258 to generate accurate ephemerides over the period January 30, 2000–November 7, 2007. Phase-folding procedures yielded pulse profiles for RXTE PCA (~3–30 keV), RXTE HEXTE (~15–250 keV) and INTEGRAL ISGRI (~20–300 keV). The pulsed spectrum over the full ~3–300 keV energy range was derived, as well as the total spectrum (including the pulsar wind nebula) over the 20–300 keV band with the ISGRI. The timing, spatial, and spectral analyses were applied for epochs before, during, and after the magnetar-like outburst to study the evolution of the high-energy characteristics.

Results. ISGRI detected PSR J1846-0258/Kes-75 before outburst during 2003–2006 with a power-law-shape spectrum over the 20–300 keV energy range with photon index Γ = and energy flux (20–300 keV) of erg/cm² s. More than 90 days after the onset of the outburst, still during the decay phase, the same spectral shape was measured () with an indication for a 52% (2.3σ) enhanced total emission, while one year after the outburst the hard X-ray non-thermal emission of PSR J1846-0258/Kes-75 was found to be back to its pre-outburst values. PCA monitoring of PSR J1846-0258 before the outburst yielded phase-coherent ephemerides confirming the earlier derived breaking index of the spindown. During the outburst, incoherent solutions have been derived. We show that the radiative outburst was triggered by a major spin-up glitch near MJD with a glitch size in the range . Using all pre-outburst observations of ISGRI and HEXTE for the first time pulse profiles have been obtained up to 150 keV with a broad single asymmetric pulse. The pulse shape did not vary with energy over the 2.9–150 keV energy range, nor did it change during the magnetar-like outburst. The time-averaged pre-outburst ~3–300 keV pulsed spectrum measured with the PCA, HEXTE, and ISGRI was fitted with a power-law model with . A fit with a curved power-law model gives an improved fit. Around 150 keV the pulsed fraction approaches 100%. For the first 32 days of the magnetar-like outburst, the 3–30 keV pulsed spectrum can be represented with two power laws, a soft component with index and a hard component with the pre-outburst value . Above ~9 keV, all spectra during outburst are consistent with the latter single power-law shape with index ~1.2. The 2–10 keV flux increased by a factor ~5 and the 10–30 keV flux increased with only 35%. After ~120 days the soft outburst and the enhancement of the hard non-thermal component both vanish.

Conclusions. The varying temporal and spectral characteristics of PSR J1846-0258 can be explained in a scenario of a young high-B-field pulsar in which a major glitch triggered a sudden release of energy. Resonant cyclotron upscattering could subsequently generate the decaying/cooling soft pulsed component measured during outburst between 3 and 10 keV. The (variation in the) non-thermal hard X-ray component can be explained with synchrotron emission in a slot-gap or outer-gap pulsar model.