GRO J1744-28

Other names : 2EG J1746-2852 ([1])

Monitoring data: Swift/BAT

Type

Transient Low-mass X-ray Binary exhibiting Type I and II X-ray bursts and pulsations. Next to the Rapid Burster this is one of a few sources where Type II bursts are observed. Discovered on 1995 December 2 with the Burst And Transient Source Experiment (BATSE) on-board the Compton Gamma Ray Observatory (Kouveliotou et al. 1996)

RA 17h 44‘ 33.09“ DEC -28° 44‘ 27.0“

Binary system

• Distance: 7.5-8.5 kpc (, )
• Optical companion: G4 III star (, )

Orbit

The orbital parameters were approximated to Porb = 11.836 days, T π/2 = 2456696.19880 (JED), ax sin(i) = 2.637 light-sec on the basis of the 2014 outburst with no constrains on the longitude of periastron or eccentricity (). See NSSTC Gamma Ray Astrophysics.

Pulsations & Magnetic field

GRO J1744-28 is special because it exhibits X-ray bursts and pulsations at the same time. Sources which show X-ray bursts are generally believed to have surface conditions (low B-fields) which do not allow pulsations. Pulse period: 2.14Hz

The magnetic field strength deduced from disk reflection models lies in the 2–6×10^10 G range (Degenaar et al. 2014), mismatching the values deduced from the CRSF measurements (5.27±0.06 × 10^11 G ) by one order of magnitude.

Outbursts

• 1995 December: Discovery and first report of Type II X-ray bursts ()
• 1996 December: Similar burst characteristics (), CRSF report at 5keV in BeppoSAX data (not yet proven) ()
• 2014 February: Outburst after 18 years of quiescence (, no CRSF), CRSF report at 5keV, 10keV and 15keV in XMM-Newton/INTEGRAL data, still under debate ()
• 2017 February: Fourth outburst with ~two orders of magnitude lower luminosity (Koenig et al. in prep.)

X-ray Spectrum

Cyclotron Features

D’Aì et al. (2015) reported a fundamental CRSF line at 4.68±0.05 keV, with the indication of a second and third harmonic at 10.4 ± 0.1 keV and 15.8+1.3−0.7 keV in XMM-Newton/INTEGRAL data (using gabs). Shortly later, Doroshenko et al. (2015) claim to have found a fundamental line at ∼4.5 keV in BeppoSAX data taken during the 1997 outburst (also using gabs). (, ) This makes GRO J1744−28 one of the few LMXBs where a CRSF has been reported below 10 keV. (Other candidates are X1822−371 with a claimed cyclotron line energy of 0.7 keV (Iaria et al. 2015) and SWIFT J0051.8−7320 at 5 keV (Maitra et al. 2018)). ) The cyclotron line in this source is under debate.

References