IGR J16493-4348 - a radiopulsar or a new X-ray binary

ATel #457; S. A. Grebenev (IKI, Moscow), A. J. Bird (Southampton University), S. V. Molkov (IKI, Moscow), S. Soldi (ISDC, Versoix), P. Kretschmar (ESA/ESAC, Vilspa), R. Diehl (MPE, Garhing), C. Budz-Joergensen (DSRI, Copenhagen), B. McBreen (UCD, Dublin)
on 12 Apr 2005; 14:52 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: S.A.Grebenev (sergei@hea.iki.rssi.ru)

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During the Galactic plane survey carried out with INTEGRAL in 2004 a weak X-ray source was marginally (S/N=6.3) detected with IBIS/ISGRI at the position coincident with that of the 0.87 s radiopulsar PSR J1649-4349 (Bird et al., ApJ, 607, L33, 2004; Bassani et al. ATEL #232).

This source was recently detected again during the INTEGRAL deep survey of the Norma Arm region at the S/N ratio of 9.1 in the 18-45 keV band. The best fit position, R.A.=16h49m21s, Decl.=-43d48m36s (equinox 2000.0, uncertainty 2'), was only 45 arcsec off that of PSR J1649-4349. For the first time it became possible to measure with confidence the photon flux from this source 5.6+/-0.6 mCrab (averaged over the interval April 6.239-9.198, 2005 UT). Some signs of the variability were detected with a short (~1 h) increase in flux to 15-20 mCrab but this requires further analysis and observations. Assuming that the spectrum has a Crab-like shape we derive the source's 18-45 keV luminosity to be 2.2 x 10^{35} erg/s at a 5.6 kpc distance of PSR J1649-4349 (Manchester et al. MNRAS, 328, 17, 2001). This value exceeds by 5 orders of magnitude the rate of energy loss (2.6 x 10^{30} erg/s) due to the spin-down (Pdot = 4.4 x 10^{-17}) of a neutron star in this pulsar.

We conclude that either this pulsar is a member of a currently unrecognized binary system with a notable level of episodic accretion (accretion should be irregular to save a radiopulsar; taking into account the pulsar's large age ~3 x 10^8 years and the weakness of its magnetic field B~2 x10^{11} G the system could be at the stage of transition to LMXB) or, that is more probable, the pulsar is serendipitous for the detected source and we can report on a previously unknown X-ray binary IGR J16493-4348 discovered with INTEGRAL.

We encourage further observations of this interesting object at all wavelengths.