GRO J1008-57

Also known as: PBC J1009.7-5816 Simbad

Monitoring data: RXTE/ASM Swift/BAT MAXI

RA 10h 09' 44", DEC -58° 17' 42"
RA 152.4333deg, DEC -58.2833deg
l 282.996, b -01.827

Binary system

X-ray Be transient
Distance: 5.8±0.5 kpc ()
Other distance estimates: 9.7±0.8 kpc (, alternative solution); ~5 kpc (); 2 kpc (), often found 'inherited' in the literature
mean orbital phase of outburst maxima (): -0.0323(17)

Orbit

Parameter	Value	Unit	Reference
Porb	249.48(4)	days
tau	54424.71(20)	MJD
a sin i	530(60)	lt-sec
e	0.68(2)
omega	-26(8)	deg
i*	36	deg

^* inclination between Be disk and orbital plane

Optical Companion

Spectral Class : B0e showing H_alpha emission line ()

Parameter	Value	Unit	Reference
Mopt	15	M⊙
Ropt	7	R⊙
Rdisk	72	R⊙
B	16.95
V	15.27
R	14.15
I	12.88
J	11.25(5)
H	10.49(3)
K	9.94(3)

Available data

• CGRO: regular outburst monitorings between 1993 and 1996
• INTEGRAL: outburst monitorings in 2004, 2005, and 2014
• RXTE: full orbit pointings in 1996, outburst monitorings in 2005, 2007 and 2011
• Swift: one pointing during 2007 outburst, outburst monitoring in 2011 and during the giant 2012/13
• Suzaku: one long exposure during 2007 outburst, one pointing during the 2012/13 giant outburst and during 2014
• NuSTAR: one pointing during the 2012 giant outburst

Description

The transient HMXB GRO J1008-57 was discovered by CGRO during an outburst in 1993 (, ). It consists of a neutron star on a wide, eccentric orbit around a Be type companion. Each time the neutron star passes its periastron, mass is accreted from the circumstellar disk of the companion onto the neutron star magnetic poles. In contrast to other systems like XTE J1946+274, this mechanism seems to be very stable in GRO J1008-57. Thus outbursts can be predicted on a high level, making this system an ideal target for scheduled observations ().

The mean outburst luminosity derived by RXTE/ASM data is around 20 mCrab between 2 and 10 keV. There were, however, two strong outbursts in December 2007 and December 2011 with luminosities above 60 mCrab.
In December 2012, the source featured the first "giant" type II outburst in the RXTE-ASM and Swift-BAT era, reaching nearly one Crab.

Spectrum

The spectrum of GRO J1008-57 was successfully modelled during its discovery outburst in 1993 by thermal bremsstrahlung with a temperature of 25 keV (). There was also an indication for a high energy cyclotron line at 88 keV (, ). If interpreted as fundamental line, this neutron star would have one of the strongest magnetic fields known.

Recent and ongoing analysis of the spectrum of the giant 2007 outburst suggest a cutoff powerlaw with an additional black body to model the spectrum of GRO J1008-57. The bremsstahlung model is no longer able to describe the spectrum for energies below 10 keV (). There are also hints for the claimed high energy cyclotron line in the RXTE/HEXTE and Suzaku/GSO spectra, the signal quality, however, does not yet allow a clear confirmation.

Pulse Period & Profile

The ~93.8 s spin period of the neutron star is clearly seen in the X-ray lightcurve. Since its discovery in 1993 the pulse period increased slightly from 93.59 s to 93.74 s during the giant outburst in 2007. Overlayed to this long-term spin-down, a spin-up of around ~1e-9 s/s during maximum luminosity of the 2007 outburst was detected (). During the weaker outburst in 2005, however, no spin-up or -down could be measured.

The pulse profile shows a double peaked structure, where the strength of the second one is strongly energy dependent. For energies above 20 keV, the profile is nearly single peaked (). Similar energy dependencies are also seen in, e.g., XTE J1946+274 and 4U 0115+634.

References