MAXI News

According to the large flux increase observed with MAXI/GSC during the giant outburst in 2012 November,  the MAXI team requested Suzaku ToO observation. It was performed at the outburst peak from 2012-11-20 (MJD 56251) 14:44 to 2012-11-22 5:21 with an exposure of 50 ks. The MAXI/GSC flux was 400 +- 11 mCrab in 4-10 keV. The flux with Suzaku was 6.8 x 10^-9 erg cm^-2 s^-1 in 2-10 keV, and 2.65 x 10^-8 erg cm^-2 s^-1 in 1-100 keV. The luminosity was 8.0 x 10³⁷ erg s^-1 in 1-100 keV at the 5 kpc distance (Coe et al. 1994).

Our preliminary analysis of the Suzaku HXD/PIN and GSO data revealed that the spectrum can be reproduced by an NPEX with a cyclabs model with a reduced chi squared (dof) = 0.94 (127). The derived parameters of the cyclotron-line feature are E_cyc = 75.5 (+2.5, -1.5) keV, W = 4.0 (+5.4, -4.0) keV, and D = 1.6 (+9.4, -0.6) with errors of 90% confidence limits. The feature existed throughout the entire phase of the apparent 93.6257 s spin period. The E_cyc were 77.7, 74.7, 75.5, and 76.2 keV for the devided spin phases of rise, peak, fall, and valley of the pulse shape, respectively.

Suzaku spectrum does not show any signature of cyclotron-line feature in 20-50 keV range. Thus, the feature at 76 keV is considered as the fundamental cyclotron line. It corresponds to a magnetic field strength of 6.6 x 10¹² G, which is the highest among binary X-ray pulsars ever observed.

The spectra with Suzaku PIN and GSO fitted with a NPEX and cyclabs model. The residuals in the bottom panel are without cyclabs model.

The pulse profile of GRO J1008-57. The pulse phases are divided into four phases : rise, peak, fall, and valley.

The Be/X-ray binary pulsar, GX 304-1, started brightening on September 17, suggesting that the source now entered a new outburst. The X-ray flux reached 40 +- 5 mCrab (4-10 keV) on September 23. It was 15 days earlier than that expected from the orbital period of 132.5 days and the phase of the outburst onset of the previous 11 outbursts.
The figure shows folded MAXI/GSC X-ray light curves by the orbital period for GX 304-1 (in red) and A 0535+25 in 2009 (in gray). The Swift/BAT curves are also overlaid. The shapes of double peaks and a precursor agree very well between the two pulsars. Then, a giant outburst may follow in GX 304-1 in mid-October.
(by Nakajima and Mihara)

Folded light curves of GX 304-1 (red) and A 0535+26 (gray) by their orbital periods. Double-peaked outbursts of the two are similar, and now GX 304-1 is tracing the precursor in prior to the giant outburst.

MAXI/GSC detected a new activity from transient pulsar GX 304-1. The figure shows the lightcurve folded with 132.5-day orbital period. As shown in below figure, the current flux increase is about 1-week earlier than that of the previous outburst within the orbital cycle of 132.5-day period. Thus, the current activity may be a precursor, or an onset of an anomalous giant outburst that occurs out of regular recurrent phase. We have reported this news to Astronomer’s Telegram #3075.

We reported in Atel#2970 (Mihara et al.) that MAXI/GSC detected a new activity from X-ray pulsar A0535+262. The figure shows the recent four light curves folded with 115-day outburst period. The current consecutive activities exhibit the outbursts and precursor events at phases ~0.2 and ~0.95, respectively. The phase zero is taken at MJD 55153.5, which is the periastron passage prior to the previous giant outburst.