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The spin-down rate, energetics and spectrum of the 70.5 ms pulsar in AX J1838.0-0655

ATEL # 1405; L. Kuiper (SRON), W. Hermsen (SRON, UvA), M. Klein-Wolt, R. Wijnands (UvA)
on 3 Mar 2008; 17:10 UT
Password Certification: Lucien Kuiper (L.M.Kuiper@sron.nl)

Subjects: X-ray, Gamma Ray, Neutron Stars, Pulsars

Monitoring observations with the PCA instrument (2-60 keV) aboard the Rossi X-ray timing Explorer performed between 2008, February 17 and 26 (43.5 ks exposure in total) revealed the spin-down rate of the newly discovered 70.5 ms pulsar in AX J1838.0-0655 (Gotthelf et al.; ATEL #1392). We obtained a phase coherent timing solution yielding a greatly improved pulse period of 70.4982056(2) ms and a period derivative of 4.94(5)E-14 s/s evaluated at the epoch MJD 54513.0 (TDB). From these numbers we derive a characteristic age of 22.6 ky and a spin-down luminosity of 5.6E36 erg/s and thus proving that this object is indeed a young and energetic pulsar. The pulse profile is broad (FWHM is ~0.5), structured with two distinct maxima. The timing signal is detected up to 30 keV in the PCA data and its spectrum is very hard and consistent with a power-law with photon index 1.12(2). This value is similar to that derived for the pulsed spectra of other young pulsars detected with RXTE and/or INTEGRAL: PSR J1846-0258 (in Kes 75), PSR J1811-1925 (in G11.2-0.3), PSR J1930+1852 (in G54.1+0.3) and PSR J1617-5055. INTEGRAL IBIS ISGRI (20-300 keV) imaging data collected over 2003, March 10 and 2006, November 19 showed emission from AX J1838.0-0655 up to 150 keV combining more than 6.3 Ms of exposure time. Its total spectrum (pulsed plus constant (DC) from the pulsar wind nebula (PWN)) is power-law like with a photon index of 1.72(7) consistent with the value reported by Malizia et al. 2005, ApJ 630, L157. The pulsed RXTE PCA and total INTEGRAL ISGRI spectra cross near 40 keV, beyond which the pulsed fraction becomes 100% and the contribution of the PWN negligible. We had a 1.2 ks Chandra HRC-I observation targeted at AX J1838.0-0655 performed on 2007 May 28. Unfortunately, the 31 counts detected from the candidate source at RA,DEC = (18 38 3.216,-06 55 33.85) (Epoch 2000) were insufficient to obtain a statistically significant timing signal.


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