Swift and RXTE observe an extended low state of 4U 1820-30

ATel #2071; H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (CRESST/GSFC/UMD), M. D. Still (MSSL), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Palmer (LANL), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMD), J. Tueller (GSFC), T. Ukwatta (GSFC/GWU) (MSSL), S. D. Barthelmy (GSFC), W. Baumgartner (CRESST/GSFC/UMBC), J. Cummings (CRESST/GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Palmer (LANL), T. Sakamoto (CRESTT/GSFC/UMBC), G. Skinner (CRESST/GSFC/UMBC), J. Tueller (GSFC), T. Ukwatta (GSFC/GWU)
on 8 Jun 2009; 22:36 UT
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The low-mass X-ray binary neutron star system 4U 1820-30 (also known as H 1820-303) has been in a soft X-ray low-hard state since late April 2009 as seen in the X-ray monitors on Swift and RXTE. The source has been steadily brightening in the Swift/Burst Alert Telescope (BAT) 15-50 keV band since about 24 April 2009 (MJD 54945) and its most recent daily average (7 June 2009) is 0.032 ± 0.002 cts/cm^2/s (145 mCrab). This is in contrast to an average rate of 0.14 cts/cm^2/s and represents the highest sustained rate for this source during the Swift mission,

The RXTE All-Sky Monitor (ASM) and Proportional Counter Array (PCA) light curves shows that 4U 1820-30 has been in an unusually long-lived low state since late April 2009. The ASM count rate took a sharp drop at the same time (MJD 534944) that the flux began rising in the BAT monitor. The ASM count rate has averaged approximately 6.0 ± 0.5 cts/sec since this time, compared to a long-term average count rate of ~20 cts/sec. The RXTE PCA shows a similar decline from ~4000 cts/sec/5PCU to ~1000 cts/sec/5PCU over the same time period.

This type of behavior is atypical for 4U 1820-30 over the past 10-15 years. Usually, soft X-ray low states last one to two weeks.

The source was brought to our attention by a recent BAT trigger by a thermonuclear X-ray burst at 09:28:18.74 on 6 June 2009 (MJD 54988; Swift trigger 354224). Swift automatically slewed to the source location and began observations at T+85 sec with the Swift X-Ray Telescope (XRT) and UltraViolet-Optical Telescope (UVOT).

Swift/BAT event data was collected for the time interval from T-240 to T+502 seconds, when a pre-planned slew took the source out of the field of view. Except for the 1.024-second trigger interval, the mask-weighted light curve shows no significant variations in source intensity during this period. The spectrum for this total interval (persistent emission) is best fit by a simple power-law model. The power law photon index is 2.83 ± 0.27. The flux in the 15-150 keV band is 2.3 ± 0.3 X 10^-9 erg/cm^2/sec. The trigger interval (thermonuclear X-ray burst; T+0.0 to T+1.0 sec) is best fit to a black body model with kT = 2.71 ± 0.65 keV and a flux of 1.2 ± 0.4 X 10^-8 erg/cm^2/sec.

Swift/XRT data was collected for a total of 1.3 ksec, beginning at T+85 seconds and ending at T+17360 sec. The source flux did not vary appreciably over this interval. Fitting an absorbed power law to the combined XRT and BAT data yields nH = 2.7 ± 0.6 X 10^21 cm^2, alpha = 1.69 ± 0.05 and 0.3-10 keV flux = 2.6 ± 0.2 X 10^9 erg/cm^2/s. There is no thermal component required statistically within the fit.

The Swift-UVOT does not resolve 4U 1820-30 within the globular cluster NGC 6624. Five arcsec aperture photometry centered upon the transient position reveals no long-term photometric variation through the uvw2, u, b or v filters relative to Swift data obtained on MJD 53175.

Swift/BAT Hard X-ray Monitor light curves for 4U 1820-30 ( = H 1820-303)