RXTE and Swift Discovery of the Intermittent Source XTE J1704-445

ATel #1164; C. B. Markwardt (CRESST & NASA/GSFC), J. H. Swank (NASA/GSFC), M. Klein-Wolt (U. Amsterdam), D. M. Smith (U. C. Santa Cruz)
on 31 Jul 2007; 01:05 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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We report the detection of an intermittent source XTE J1704-445 (hereafter J1704). This source was first detected in RXTE PCA scans of the galactic bulge and ridge regions over approximately a month. Because J1704 was near another bright source 4U 1705-440, whose variability can sometimes mask other nearby positions, it took some time to recognize J1704 as "real." The best-fit RXTE PCA position is R.A. = 17h04.5m, Dec. = -44d32' (J2000), with a 95% confidence error radius of 6 arcmin. The 2-10 keV flux was in the 4-5 mCrab range.

Follow-up observations by Swift XRT on 27 Jul 2007, 15:05 UT, have detected a source at R.A. = 17h04m12.7s, Dec. = -44d31'36.7" (J2000) with a 90% error radius of 3 arcsec. The flux was approximately 6 mCrab. Because of the small offset between the PCA and XRT positions (3 arcmin) and the approximately equal fluxes, we propose that the Swift XRT source is identical to J1704.

The Swift XRT counterpart is 20 arcsec from the known ROSAT source 1RXS J170410.9-443131. The offset is less than the 23 arcsec error radius for the ROSAT source (90% confidence). If the two are associated, the flux has varied by a factor of ~12 since the ROSAT measurement was taken in 1990. According to Simbad, there are no publications on this source in the literature.

In retrospect, J1704 was detected in the PCA bulge scans since ~Feb 2004, and it has a 2-10 keV flux range of 0-12 mCrab. The long-term profile is highly intermittent with no obvious periodicities which might have indicated an orbit.

Swift XRT observations show a source with a relatively hard spectrum (power law photon index ~1.55, or thermal bremsstrahlung, kT > 10 keV) and mild absorption (N_H ~ 1.3e22 - 1.5e22 /cm^2). The light curve varies by about 50% over the 1 ks observation. No significant pulsations are detected.

The spectrum and long-term light curve are suggestive of an X-ray pulsar, but to date no periodicities have been detected.

The nearest 2MASS infrared source, 2MASS 17041207-4431311, is 8.7 arcsec away, and therefore presumably unrelated to the Swift XRT source. An O/B supergiant companion should be visible to 2MASS anywhere within the Galaxy. If the XTE and Swift sources are indeed the same, then the system is not a new member of the class of Supergiant Fast X-ray Transients (SFXTs), despite its very rapid variability.