Ankur Nath. Co-authors: Biplob Sarkar, Jayashree Roy, Ranjeev Misra

This poster summarizes the analytical results obtained for an observation of an low mass X-ray binary star system, GX 3+1 with the compact object a neutron star (NS). This source, is 6.1 kiloparsec distance away, as noted in literature. Briefly I would like to introduce such binaries for a better comprehension to dissolve the poster matter; X-ray Binaries are often detected as having an accretion disk , formed due to the spiral falling in of stellar matter into the high gravity compact source, either a black hole or a neutron star. GX 3 + 1 has a compact neutron star, which is evident from its burst phenomena. Rapid flash type bursts occur at such binaries, mostly accepted in literature as to originate from the upper layer of the Neutron star surface, and GX 3+1 is a famous burster. Though this source has been well studied globally with multiple X-ray mission telescopes, it was never reported from the Indian multi-wavelength observatory AstroSat, which has high time resolution proportional counters named Large Area X-ray Proportional counter or LAXPC (three of these are there, numbered 10, 20, 30) and a Soft X-ray telescope or SXT, manufactured to observe the soft X-ray photons energies from 0.3 - 8 keV. Our prime focus is to extract data from these two payloads of AstroSat, analyze the spectra and the light curves for this source and determine the necessary physical parameters such as inner accretion disk temperature and the Neutron Star's radius via statistical modelling. The reason we do this is because of the aforementioned high time resolution ( 10 microseconds) and the ability to detect X-ray photons in a wide range of energies: LAXPC undertakes a capability to observe in 3-80 keV range. Mostly GX 3+1 has been reported to display a soft spectrum in literature, but for a better energy resolved analysis of the burst, we exploit the LAXPC abilities. Type-1 bursts are those which can be modeled by a single blackbody, with temperatures ~ 2 keV. As we fitted the spectra of the source during the burst occurrence, the temperature is found to be 1.6 keV, which makes it clearly declare as having Type-1 status, though apart from that the burst is exponentially decaying within some 15 s, gives us the primitive intuition as the status.

Below I briefly explain the diagrams one by one.

Diagram (a) shows the simultaneous fitting of light curves from SXT and LAXPC 20.

Diagram (b) shows the energy resolved burst light curves. We can clearly see the burst primarily lies in the energy range of 5-8 keV, as it is having a higher photon count rate ( y-axis). Also, this brightest burst light curves decays within 15 seconds.

Both of the above diagrams are plotted with observational times in x-axis.

For the third diagram (c), this actually determines the state of the source. We can see, the ratio (15-25 keV)/ (3-5 keV) is much less than 1 (average 0.08), which indicates the source data has a larger proportion of soft x-ray photons. (Soft X-rays are having energies less than 10 keV). The photon intensity is along the x-axis. Also , low mass X-ray binaries states are grouped into two categories based on their Hardness Intensity diagram patterns: Island state and the banana state. We find that the source is in banana state and also to note here, GX 3+1 has been never reported showing an island state.

The last three diagrams are the spectral model fits .The first fit fig.(d) shows the modeling for the spectra obtained from both SXT and LAXPC 20, before the occurrence of the burst (840 ks of data) to understand the state parameters of the source without the burst. The statistical models taken are nthcomp which counts for the inversely compotonised disk photons (they get comptonised due to the NS corona) and a disk black body (multiple blackbody for the inner accretion disk, which is mainly bright in X-ray sky). We could succesfully fit them and obtain the required values for eg. the inner disk temperature (2.0 keV from our modelling) and the Normalisation constant of the diskbb. The Normalisation allows us to calculate the inner disk distance from the compact star as the center, in kms, which explains the relative closeness of the disk to the NS surface. We find the inner disk radius to be at 7.5 km.

During the burst, however we had to remove the diskbb in order to fit the spectra, or else it was overly fitting, destroying the acceptability from a statistical point of view. So then we fitted the spectra with a blackbody added to a comptonised component like previous case and achieved a burst blackbody temp near about 1.6 keV. To further confirm, we took a second model, but this time we chose a cutoff powerlaw of energy instead of nthcomp and this time we saw that there was very little alterations to the blackbody parameters. This clearly suggested how bright and prominent the spectra was with this blackbody emission, and any powerlaw of energy to account for the disk emissions was sufficient enough. Fig (e) depicts the cutoffpowerlaw and blackbody case and Fig (f) depicts the comptonised model fitting for the burst.

Finally we did some literature survey, to actually see how far the analysis that we have done, was familiar or matching with the reports. We found the normalisation of disk blackbody was with agreement with a recently published paper Ludlam et al. 2019, which comprises an analysis for GX 3+1 data from mission NuStar, which is also a multi-wavelength observatory. Also the neutron star radius was found around 6.8 km, and this is less than the upper bound theory supplies (10 km). The radius value is in agreement with a report by Kuulkers and van der Klis (2000).




Thank you.