Swift Detects Likely X-ray Burst from XTE J1701-407

ATel #1616; C. B. Markwardt (CRESST/GSFC/UMD), J. Cummings (CRESST/GSFC/UMBC), H. Krimm (CRESST/GSFC/USRA)
on 18 Jul 2008; 02:17 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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XTE J1701-407 was discovered in June 2008 by RXTE PCA scans (Markwardt et al., ATEL #1569) and Swift follow-up observations (Degenaar et al., ATEL #1572). Recently Swift BAT triggered on an X-ray flare from the same source (Barthelmy et al., GCN Circ. #7985). In this telegram we report on BAT results for the XTE J1701-407 flare, which we speculate is a thermonuclear X-ray burst from a neutron star.

The trigger occurred on 2008-07-17 at 13:31:36 UTC (trigger #317205), Swift slewed immediately to the location, and the transient was identified as XTE J1701-407 (GCN Circ. #7985). The mask-weighted light curve profile is approximately gaussian. The T90 duration measure is T90 (15-350 keV) is 66 +- 13 sec (estimated error including systematics).

The time-averaged spectrum from T-32 to T+56 sec is consistent with a black body with temperature 2.7 ± 0.25 keV. The total fluence in the 15-150 keV band is (8.7 ± 0.8) x 10^-7 erg cm^-2, and a peak flux of about 1.5 x 10^-8 erg cm^-2 s^-1. By dividing the burst into two halves, we found no evidence for black body cooling during the burst (nor "radius expansion" signatures).

In data processed to 16:47 UT by the Swift BAT transient monitor, the BAT continues to detect XTE J1701-407 in persistent emission, at its pre-burst level. Longer term, both the RXTE PCA and Swift BAT monitoring efforts detect a persistent source varying between 10-20 mCrab (in the 2-10 and 15-50 keV bands respectively).

The X-ray spectrum of the source is consistent with those seen for thermonuclear X-ray bursts from the surfaces of neutron stars. If the burst is indeed thermonuclear in origin, then XTE J1701-407 can be classified as a neutron star. However, the duration of ~60 sec is somewhat longer than a typical X-ray burst. While the duration might indicate unstable burning of helium, such bursts are considered to have sharp rises (e.g. Woosley et al 2004, ApJS 151 75), which is not the case here. If we assume a peak luminosity for helium burning of 1 x 10³⁸ erg s^-1 (ibid), we can set a distance upper limit of about 30 kpc.