Broadband X-ray observations of GRO J1744-28 during outburst
ATel #5997; N. Masetti, M. Orlandini (INAF/IASF, Bologna), P. Parisi, M. Fiocchi (INAF/IAPS, Rome), C. Sanchez-Fernandez and E. Kuulkers (ESAC/ESA), on behalf of a large collaboration
on 21 Mar 2014; 16:37 UT
Credential Certification: Nicola Masetti (masetti@iasfbo.inaf.it)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
On March 10, 2014 the "Bursting Pulsar" GRO J1744-28 was simultaneously observed with Swift and INTEGRAL, during its ongoing outburst which started on January 18, 2014 (ATels #5790, #5810, #5845, #5858, #5883). These Target-of Opportunity observations were acquired under a multiwavelength effort to monitor the source during its active phase (see ATel #5904).
INTEGRAL data were acquired for 37.8 ks starting at 03:41 UT, and included publicly available Galactic Bulge monitoring observations (e.g., ATel #438), whereas the Swift pointing began at 07:53 UT and lasted 4.5 ks.
Using the light curves of the source generated with JEM-X onboard INTEGRAL and with Swift/XRT we selected the time intervals during which GRO J1744-28 does not display X-ray bursts, so to focus on the persistent emission. We however kept in our data selection the post-burst dip part of
the source emission (see e.g. Nishiuchi et al. 1999; ApJ, 517, 436).
A broadband spectral fit in the 2-80 keV band, covered with the XRT and IBIS instruments, is best achieved (reduced chi squared = 1.17 for 557 degrees of freedom) with an absorbed blackbody plus cutoff power law. The (preliminary) best-fit parameters are kTbb = 2.27+-0.05 keV, Gamma =
3.1+-0.2 and E_cut = 25 [+10, -6] keV, with a column density N_H = (8.0+-0.7)e22 cm-2. This latter is consistent with previous estimates (e.g., ATel #5858).
With this description, the average unabsorbed X-ray fluxes of GRO J1744-28 are ~1.4e-8 erg cm-2 s-1 (2-10 keV) and ~5.7e-9 erg cm-2 s-1 (20-100 keV), corresponding to luminosities of ~1.2e38 erg s-1 and ~9.6e37 erg s-1, respectively, assuming a distance of 8.5 kpc. This would confirm the
near-Eddington regime for this source as suggested in ATel #5963.
We thank the INTEGRAL Science Operations Center and Neil Gehrels and the Swift team, respectively, for scheduling and performing the INTEGRAL and Swift Target-of-opportunity pointings we requested.